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. 2024 Feb 17;9(8):9269–9285. doi: 10.1021/acsomega.3c08439

Influence of Mineral Composition on Initiation Pressure of Waterflood-Induced Fractures in Tight Sandstone Reservoir

Zezhong Su †,‡,*, Tiantai Li †,‡,*, Mingxing Bai §, Zhenjiang Zhou †,‡
PMCID: PMC10905734  PMID: 38434837

Abstract

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Revealing the impact of core mineral composition on the initiation pressure of waterflood-induced fractures (WIFs) in tight sandstone reservoirs is a crucial aspect of studying the initiation mechanism of WIFs. In this paper, through quantitative characterization of the core mineral composition from six samples of the Chang 6 reservoir in the Wuqi oilfield, western Ordos Basin, and modified experimental cores and displacement equipment for WIF experiments, the influence of the core mineral composition on the initiation pressure of WIFs in tight oil reservoirs is investigated. The conclusions are as follows. (1) The rock mineral composition of the Chang 6 reservoir in the Wuqi oilfield, western Ordos Basin, includes quartz, feldspar calcite, and clay, characterizing it as a typical feldspar sandstone reservoir. Quartz and calcite are considered as brittle minerals, while feldspar and clay are categorized as lithologic minerals. (2) For feldspar sandstone reservoirs, including quartz, feldspar, calcite, and clay minerals, when the combined content of quartz and feldspar exceeds 600% of the total mineral content, the changes of quartz and feldspar content will affect the initiation pressure of WIFs. As the ratio of the quartz content to feldspar content Rqf increases, the initiation pressure of WIFs exhibits a logarithmic function decrease. (3) Considering the contribution of diagenetic minerals to rock brittleness, the calculation method for the brittleness index of feldspar tight sandstone reservoirs is improved. (4) The relationships between Rqf, brittleness index, and initiation pressure of induced fractures suggest that an increase in Rqf leads to a power-law increase in the brittleness index, while the initiation pressure of WIFs relative to the brittleness index shows a power-law decrease. This phenomenon indicates an increased likelihood of WIFs occurring during the long-term water injection process in feldspar sandstone reservoirs. This work contributes to understanding how core minerals affect the initiation pressure of WIFs in tight sandstone reservoirs.

1. Introduction

Tight sandstone reservoirs are worldwide important for exploration and development and have become the majority for increasing oil and gas production in various countries. The waterflooding technique has been most widely applied in tight sandstone reservoirs. However, due to the small pore-throat radius and complex pore-throat structures, the waterflood-induced fractures (WIFs) will be generated during the long-term water-flooding process in tight reservoirs.1−11 WIFs are the strongest heterogeneous characteristics in tight sandstone reservoirs during the middle and high water cut stage, essentially categorized as dynamic fractures.12 Meanwhile, the WIFs have two side effects during the water-flooding development in tight oil reservoirs, i.e., the WIFs can significantly improve the formation permeability and water injection capacity; however, it can also exacerbate the reservoir heterogeneity as the dominant seepage channel.13 The main manifestations of WIFs in the oilfield are11 (1) the significant decrease in daily oil production and increase in water cut when the daily fluid production increases during the production process; (2) injection well shows prominent linear flow characteristics without hydraulic fracturing; (3) the water absorption profile test shows the spike-like absorption; and (4) the tracer testing in injection Wells has the obvious directionality. Currently, the research on WIFs mainly focuses on four aspects: the production mechanism of WIFs, numerical simulation of WIFs, identification and inversion of WIFs, and experimental study on WIFs.

Based on the field experiences of water injection development, the production mechanism of WIFs can be summarized as follows: (1) the WIFs are initiated when the injection pressure exceeds the reservoir rock breakdown pressure, with poor reservoir permeability and weak water absorption capacity, (2) the WIFs are produced by activating the closed natural fractures when the formation pressure reaches the natural microfracture initiation pressure in the process of waterflooding, and (3) the WIFs are initiated through the natural fracture opening and extension pressure changes caused by the continuous changes of mechanical parameters and in situ stress field during the waterflooding development.13−17

The numerical simulation models of WIFs can be summarized into five types. By evaluating the strength factor at the fracture tips, the dynamic characterization of fractures can help determine the static fracture conditions (static, closed, or extended) at different time steps.18−21 The second model uses the increment of fracture length to present and simulate the extension process of WIFs for simplification. During the fracture initiation simulation, the current formation pressure of blocks needs to be evaluated if it reaches the critical pressure with the preassumed fracture extension direction, and the evolution process of WIFs is simulated by establishing the equations for dynamic change of fracture permeability with both increasing and decreasing formation pressure.22−24 The third model divides the simulation area into homogeneous reservoir and fracture equivalent areas. This homogeneous reservoir is a low permeability formation with constant permeability, and the conventional oil–water relative permeability curve is applied for the porous flow. The equivalent fracture area contains WIFs with the opening and closing rules, where the directional pressure sensitivity effect characterizes permeability change characteristics and the seepage law is characterized by directional relative permeability.25,26 The fourth one constructs the unstructured grid attribution model based on the attribute mapping method. According to the dynamic fracture extension evolution process, this model redivides the grid with a new fracture as the inner boundary condition; the grid is then automatically and adaptively generated and updates the discrete fracture network information.27−29 The fifth model establishes an evolutionarily dynamic model of WIFs considering fracture initiation, propagation, and closure, which is iteratively coupled to the reservoir numerical device with embedded discrete fracture model modeling function to simulate the dynamic change process of initiation, propagation, and closure for WIFs.30

The identification and monitoring methods of WIFs include Hall plot analysis and its derivatives,31−34 skin factor,35 injectivity index tests,36 water injection indication curves,37 water absorption profile tests,38 water-flooding front tests,39 production dynamic analysis,40 well testing,41−43 interwell connectivity,44,45 and tracer test,11 as shown below in Table 1.

Table 1. Identification and Monitoring Methods of WIFs.

name principles and mathematical expressions judgment basis
Hall plot analysis and its derivatives
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 (1) Hall curves overlap with the derivatives, and no blockage or fracture occurs
 
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 (2) derivative becomes larger, and the formation with blockage
    (3) derivative becomes smaller, and the formation occurs with WIFs
skin factor
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 (1) skin factor becomes larger, and the formation with blockage
    (2) skin factor becomes smaller, and the formation occurs in the reservoir
water absorption index test
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 (1) water absorption index becomes larger, and the formation with blockage
    (2) water absorption index becomes smaller, and the formation occurs with WIFs
water injection indication curves the relationship curves between injection and wellhead pressure (1) water injection indicator curve is upward, the water absorption capacity is weakened, and the formation with blockage
    (2) water injection indicator curve is folded down, the water absorption capacity is increasing, and the formation occurs WIFs
water absorption section of injection well the relative water absorption capacity of each single layer and the continuous changes of water absorption of each layer under a certain pressure the WIFs can be determined by multiple tests of the same well gradually showing an unimodal, bimodal, or multipeak water absorption profile
waterflood front test the waterflood front and pore pressure will change during the injection process, and the pore pressure changes and WIF formation process will propagate the microseismic wave to the surrounding area by determining the waterflood front position to observe whether the WIFs are produced
production dynamic analysis the interwell connectivity relationship is determined by analyzing the injection rate and production rate in injection and rate well groups (1) fracture water breakthrough. The water breakthrough time is generally within half a year, and the water cut rises quickly after the water breakthrough
    (2) pore-fracture water breakthrough. The production time in low water cut at the initial water injection stage is more than 2 years, and the production well will be flooded quickly after initiating the WIFs with the injection time increase
analysis of the interwell connectivity the interwell connectivity can be evaluated by the connectivity coefficient, calculated by the quantitative characterization equation established between the injection rate and production rate of corresponding production wells the greater the well connectivity coefficient, the better the interwell connectivity, and the greater the possibility of WIFs
well testing to conduct the pressure drop test of the injection well and to monitor whether the water injection induction joint occurs through the double log test curve the injection well shows obvious linear seepage characteristics without fracturing
tracer test the interwell connectivity is determined by monitoring the tracer detects in the production well the tracer test results have obvious directionality, generally consistent with the direction of maximum horizontal in situ stress
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The method mentioned in Table 1 has a high error rate when identifying and monitoring reservoir WIFs independently, and the identification and monitoring results are accidental and uncertain. Therefore, multiple methods are generally adopted to identify and monitor reservoir WIFs and their expansion comprehensively. Step-rate tests combined with the reservoir numerical simulation technology to identify WIFs produced in the process of seawater injection development in the Prudhoe Bay oilfield were the earliest reports of comprehensive identification and monitoring of WIFs by multiple methods.46 After that, the water absorption index, production dynamic analysis, and Hall curve were introduced and combined with the step-rate tests to comprehensively monitor the growth process of WIFs.47 Hustedt and Snippe introduced a falloff test into the WIFs monitoring with production dynamic analysis, step rate test, and reservoir numerical simulation comprehensively identified the WIFs and combined the monitoring results and the FRAC-IT model to analyze the water-flooding development status in Pierce oilfield of North Sea.48 However, the falloff test considered that the fracture length and conductivity were fixed values, which obviously did not conform to the actual situation of WIFs. Wang et al. proposed a semianalytical well-test interpretation model that considered fracture closure, variable fracture length, reduced fracture conductivity to describe the changes in bottom hole pressure of injection wells under the influence of WIFs, and revised the well-testing interpretation method of WIFs.49 The above research is mainly aimed at the comprehensive identification and monitoring method of WIFs by a single well. Therefore, the researchers further optimized the above method and introduced the study of interwell connectivity to form the workflow of identification and monitoring of WIFs at the scale of injection-production well groups and reservoirs. For the identification and monitoring of WIFs in injection-production well groups, the fracturing and expansion of WIFs must first be determined. Almarri et al.50 adopted a comprehensive method of the modified Hall curve, fracturing index, injection index, and water absorption index. Wang et al.51 monitored the fracture initiation and expansion of WIFs by modifying the Hall curve, skin factor changes, injection index, step-rate test, tracer test, and passive seismic method. Then, the interwell connectivity changes caused by WIFs were determined by interwell connectivity studies. Almarri et al.50 adopted the capacitance resistance model, while Wang et al.51 adopted the multiple linear regression model. For the identification and monitoring of WIFs at the reservoir scale, the overall water-flooding efficiency of the reservoir is first determined according to the water-flooding utilization coefficient, then the distribution of water-flooding and residual oil was defined through the areal and vertical sweep characteristics, and finally, the influence of sand body connectivity, WIFs, and injection and production relationships on the water-flooding effect was evaluated to realize the identification and monitoring of WIFs at the reservoir scale.52

In addition, some scholars have carried out experimental research studies on WIFs in tight sandstone reservoirs. According to the experimental study of WIFs in tight sandstone reservoirs, the relevant results show that effective pressure was the necessary condition to produce WIFs, that is to say, when the injection rate was constant, the injection pressure was greater than the confining pressure to produce WIFs.53 When the injection pressure gradually increased to the fracture propagation pressure, new fractures would be generated along the original fracture direction. The fracture propagation could be divided into three stages: initial fracture, continuous expansion, and stable extension.54 When the injection pressure is lower than the minimum principal stress, the existing fractures in the rock will expand twice, resulting in the formation of a complex fracture network.55 In addition, with the increase of rock water saturation, the fracturing pressure would decrease, which resulted in the rock being easier to produce fractures and forming a complex fracture network.56

In conclusion, there are few laboratory experiments on WIFs, especially the study on the influence of the mineral composition on initiation pressure of WIFs in tight sandstone reservoirs has not been reported. Studying the fracture initiation mechanism is essential in revealing the propagation mechanism of WIFs in tight sandstone reservoirs. For this purpose, six tight sandstone core samples of Chang 6 reservoir in Wuqi Oilfield located in the western Ordos Basin are characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), and cast thin section, to determine the mineral composition and surface shape. The experimental core samples and the displacement equipment are then modified to conduct the WIF experiment, and finally, the influence of mineral composition on the initiation pressure of WIFs in tight sandstone reservoirs is analyzed. The result can help understand how core minerals affect the initiation pressure, which can provide a theoretical basis for the initiation and propagation mechanism of WIFs in tight sandstone reservoirs.

2. Experimental Section

2.1. Overview of the Study Area

The Ordos Basin is a multicycle superimposed basin developed on the North China Craton. The basin is generally a rectangular basin with a near north–south trend of the asymmetric syncline, with a broad and slow east wing and a steep asymmetrical macro syncline on the west wing. The whole basin can be divided into six first-order tectonic units: Yimeng uplift in the north, Weibei Uplift in the south, western border overthrust belt, Tianhuan depression in the middle west, Jinxi flexural fold belt in the east, and Yishan slope in the middle. From top to bottom, the hydrocarbon series of the basin contains the Jurassic Yan’an Formation, Triassic Yanchang Formation, Permian Shihezi Formation, Shanxi Formation, Taiyuan Formation, Carboniferous Benxi Formation, and Ordovician Majiagou Formation, as shown in Figure 1. Wuqi Oilfield is located in the western Yishan Slope, and the main exploration and development formation is the Triassic Yanchang Formation. According to the lithology characteristics, the Yanchang formation can be divided into five lithologies from T3y1 to T3y5 and 10 reservoir groups, the T3y1 includes the Chang10 reservoir, the T3y2 includes the Chang9 reservoir and Chang8 reservoir, the T3y3 consists of Chang7 reservoir, Chang6 reservoir, and Chang4 + 5 reservoir, the T3y4 consists of Chang3 reservoir and Chang2 reservoir, and the T3y1 includes the Chang1 reservoir. The Chang6 reservoir in Wuqi Oilfield was formed in the contraction period of the lake basin, which is a delta front-predelta deposit. The lithology is an interlayer of gray feldspar fine sandstone and dark gray mudstone with varying thicknesses, and the formation thickness is 130–150 m.

Figure 1.

Figure 1

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Study area diagram in this paper.

2.2. Materials

Six core samples are obtained from the Chang 6 reservoir of the Wuqi oilfield in the western Ordos basin. Table 2 shows the information on six core samples, such as ID, formation layer, diameter, gas permeability, and porosity. The measured gas permeability of six core samples ranges from 0.0153 to 0.1468 mD, and the measured porosity ranges from 7.16 to 10.39%. The experimental liquid is the simulated formation water prepared in the laboratory, which is the CaCl2 water type with a total salinity of 60,349 mg/L. The particular ionic species and concentration of simulated formation water are shown in Table 3.

Table 2. Information of Core Samples.

ID formation layer diameter (cm) gas permeability (10–3 μm2) porosity (%)
1 Chang6 2.56 0.1281 8.79
2 Chang6 2.50 0.0334 8.04
3 Chang6 2.52 0.0212 10.39
4 Chang6 2.51 0.0153 7.16
5 Chang6 2.50 0.1468 8.25
6 Chang6 2.50 0.0457 8.55
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Table 3. Ionic Concentration of Simulated Formation Water.

ionic species Na+, K+ Ca2+ Mg2+ Cl– SO42– HCO3–
ionic concentration (mg/L) 19,142 3396 492 36,050 425 444
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2.3. Experimental Analysis Methods

XRD is first used to measure the mineral content of each core sample to characterize the mineral composition. In order to accurately characterize the mineral composition, 5.0 g of particles for each core sample is selected for the XRD test, which is repeated three times to ensure the accuracy of test results. Then, the SEM technology is used to characterize the surface morphology of each core sample. Before the SEM scanning, the core surface is physically polished with the argon ion, and a golden thin film is coated on the ion-polished core surface to enhance the conductivity.

2.4. Experimental Procedures

Based on the results of XRD, SEM, and cast thin section, six core samples are conducted to the WIF experiment. The key to this experiment is to modify the core sample and the Hassler core holder so that the core samples can produce WIFs successfully.

The first step is to modify the experimental core: (1) using a hand-held drilling rig to drill a water-inlet hole with a diameter of 3.1–3.2 mm at the center of the core inlet end face, and the depth of the hole is about 40% of the overall length of the core; (2) using heat-shrinkable tubing to wrap the core, and one end of a high-pressure line with a length of 23–25 cm and a diameter of 3 mm is inserted into the core water-inlet hole, and the other end is fitted with a threaded connector; (3) the epoxy resin, with a maximum pressure of 50 MPa, is used to seal the water injection hole, high-pressure pipeline, and the import end face of the core sample. The length of epoxy resin should be 2 cm to ensure the sealing effect and at room temperature for 48 h to ensure the full solidification of epoxy resin. The schematic diagram of the experimental core sample is shown in Figure 2.

Figure 2.

Figure 2

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Modified schematic diagram of the core sample.

The second step is to modify the core holder. The Hassler core holder is used for the WIF experiment. For the conventional displacement experiment, the injection fluid is injected into the core sample through the circular hole at the distribution plug of the entrance side for the Hassler core holder, and the fluid is displaced out of the core sample by the circular hole at the distribution plug of the exit side. In the displacement process, the injection pressure is a constant value and cannot reach the rock fracturing pressure to produce WIFs. For this reason, the distribution plug at the injection side of the Hassler core holder is removed so that the epoxy resin side of the modified core sample can directly contact the spacer, and confining pressure is applied to the core sample through the spacer. The schematic diagram of the modified core holder is shown in Figure 3.

Figure 3.

Figure 3

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Modified schematic diagram of the hassler core holder.

The third step is to conduct the WIF experiment: (1) loading the modified core into the core holder and connecting the experimental device according to Figure 4 to check the device sealability, (2) the confining pressure pump is used to slowly apply the confining pressure to the core until the confining pressure reaches the experimental requirement of 15 MPa, (3) after the confining pressure stabilized, the WIF experiment is conducted at an injection rate of 0.0167 mL/s and recorded the injection time tz and injection pressure Pz, and (4) the experiment is finished when the injection volume reaches the set value, then to study the influence of core mineral composition on initiation pressure of WIFs.

Figure 4.

Figure 4

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Experimental schematic of the conducting WIF experiment.

3. Results and Discussion

3.1. Core Sample Characterization

The test results of the mineral composition of six core samples are given in Table 4 and Figure 5. The core samples of Chang 6 reservoir in Wuqi oilfield are observed to have similar mineral compositions, mainly including quartz, feldspar, calcite, and clay minerals. Feldspar includes potash feldspar and plagioclase feldspar, and its content ranges from 27.5 to 53.8%, with an average content of 39.5%. The quartz content is between 27.4 and 45.6%, with an average content of 35.3%. The calcite content ranges from 0.9% to 10.9%, with an average of 4.3%. The clay content is between 13 and 32.9%, with an average content of 21.0%. The SEM test confirms that the clay includes hair-like illite and foliated chlorite, as shown in Figure 6.

Table 4. Mineral Compositions of the Six Core Samples in the Wuqi Oilfield.

ID quartz (%) feldspar (%)
 calcite (%) clay (%)
    potash feldspar plagioclase feldspar total    
1 27.4 12.7 41.1 53.8 0.9 17.9
2 41.3 7.7 26.9 34.6 10.9 13.2
3 45.6 9.0 24.1 33.1 1.5 19.8
4 33.7 11.1 29.7 40.8 3.9 21.6
5 29.2 14.3 32.9 47.2 3.2 20.4
6 34.3 6.8 20.7 27.5 5.3 32.9
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Figure 5.

Figure 5

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Mineral compositions of the six core samples.

Figure 6.

Figure 6

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Digital SEM images of core samples.

According to the results of the XRD analysis, the average content of feldspar in the Chang6 reservoir of Wuqi oilfield is 38.87%, and the sandstone type is mainly feldspar sandstone, with mostly linear contact between the sandstone clastic particles, and the local contact is point-line contact or concave–convex contact. The cementation type is mainly porous cementation, and the compaction between the particles is strong, which is usually accompanied by increasing filling of autogenous quartz, as shown in Figure 7a,b. The average content of calcite in the samples is 3.4%, and the results of the cast thin section show that the calcite accumulates in pores by filling to form the carbonate cementation, as shown in Figure 7c. The SEM results show that chlorite is mainly attached to the surface of feldspar and quartz, and the chlorite filling can be seen in the intergranular pores to form the chlorite membrane to produce authigenic clay mineral cementation, as shown in Figure 7d. Due to the change of the diagenetic environment, the feldspar corrodes to form intragranular solution pores, as shown in Figure 7e. In addition, the metasomatism of calcite to feldspar boundary changes the composition and structure of feldspar, as shown in Figure 7f.

Figure 7.

Figure 7

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Testing results of the cast shin section.

3.2. Injection Pressure and Initial Pressure

The pressurization rate is introduced to help analyze the change of injection pressure in this paper, and its physical significance indicates the increased degree of injection pressure per unit of time. According to the change in the pressurization rate curve, the injection pressure can be divided into three stages: energy storage, pressurization, and decline. The energy storage stage refers to the stage from the beginning of water injection to the pressurization rate drops to the lowest, and injection pressure changes to less than 0.5 MPa in this stage. Pressurization is defined as the stage when the pressure-increasing rate varies from its lowest to the highest. The core samples can be potentially fracked when reaching their initiation pressure. The decline stage refers to when the pressurization rate begins to decline after reaching the maximum value, while the injection pressure of the core sample with WIFs will plummet and eventually stabilize, and the injection pressure of the core without WIFs will reach the initiation pressure at this stage, and the injection pressure will drop abruptly and eventually stabilize after inducing the WIFs.

Figure 8 and Table 5 show the change in the injection pressure and the pressurization rate for six core samples. The results show that the injection pressure increases gradually with the injection time, and the injection pressure changes abruptly after the core occurs in the WIFs. In the energy storage stage, the injection pressure of six core samples is between 0.3 and 0.5 MPa, and the energy storage time is between 131 and 388 s. In the pressurization stage, the pressurization rate of the no.1 core sample reaches the maximum value of 0.0276 MPa/s, corresponding to an injection pressure of 25.4 MPa. The injection pressure and pressurization rate decrease synchronously, which may be caused by the microfractures produced inside the core. The pressurization rate of the no. 6 core sample reaches the maximum value of 0.0258 MPa/s, corresponding to an injection pressure of 17.3 MPa after the water breakthrough period, the pressurization rate begins to decrease, and the injection pressure continues to increase. The no. 2, no. 3, no. 4, and no. 5 core samples reach the initiation pressure when the injection pressure increases to 16.5, 20.9, 22.1, and 29.5 MPa, respectively, while the pressurization rates of four core samples reach the maximum value at this point, and the pressurization rates begin to decrease after generating the WIFs. In the decreasing stage, the pressurization rate of the no.1 core sample decreases from 0.0270 to 0.0240 MPa/s and then continues to increase, and the corresponding injection pressure also shows a trend of decreasing first and then increasing; the fracture is generated when the injection pressure increases to 27 MPa, and then the injection pressure and pressurization rate decreased rapidly. The no. 6 core sample generates the WIFs when the injection pressure sample increases by 19.9 MPa, and then the pressurization rate gradually decreases. With the pressurization rate decreasing, the injection pressure of six core samples drops sharply and gradually stabilizes.

Figure 8.

Figure 8

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Injection pressure curves and pressurization rate curves during the WIF experiment (① energy storage stage, ② pressurization stage, and ③ decline stage).

Table 5. Changes in Injection Pressure and Pressurization Rate during the WIF Experiment.

sample ID energy storage stage
 pressurization stage
 decline stage
 initial pressure (MPa)
  maximum injection pressure (MPa) maximum pressurization rate (MPa/s) maximum injection pressure (MPa) maximum pressurization rate (MPa/s) maximum injection pressure (MPa) maximum pressurization rate (MPa/s)  
1 0.5 0.0023 25.4 0.0276 27 0.0270 27
2 0.4 0.0013 16.5 0.0147 15.1 0.0134 16.5
3 0.4 0.0011 20.9 0.0162 19.8 0.0161 20.9
4 0.4 0.0010 22.1 0.0159 22.0 0.0157 22.1
5 0.4 0.0026 29.5 0.0269 29.4 0.0264 29.5
6 0.4 0.0020 17.3 0.0258 19.9 0.0257 19.9
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Yifei et al.57 pointed out that the initiation pressure decreased exponentially with the increase of rock permeability in the case of high filtration loss during the process of fracturing flooding. However, the gas permeability of the no. 1 and no. 5 core samples in this paper is 0.1281 × 10–3 and 0.1468 × 10–3 μm2, respectively, which are higher than that of the other four core samples, and the initiation pressure no. 1 and no. 5 core samples are also higher than that of the other four core samples, which is obviously inconsistent with the Zhang’s conclusion. This is because the six core samples in this paper belong to tight sandstone reservoirs with gas permeability ranging from 0.015 × 10–3 to 0.1468 × 10–3 μm2. In tight sandstone reservoirs, the pore-throat radius is small, the pores are mainly connected by small throats, and the seepage resistance is easily generated by the Jamin effect during water injection. As a result, when the WIF experiment is tested with a low injection rate, the fluid filtration loss of injection water into the core is very small, the injection pressure increases rapidly in the core, and WIFs occur when the rock initiation pressure is reached. This shows that the fracture initiation pressure of WIFs in tight sandstone has no strong correlation with permeability under the condition of the low injection rate and low filtration.

In addition, it should be pointed out that the simulated formation water does not affect the initiation pressure of WIFs. Existing studies have confirmed that the water–rock reaction usually requires a long reaction time and a high experimental temperature,58 but the experimental temperature in this paper is indoor temperature (15–20 °C), and the experimental time is only between 2000 and 3000 s, so the water–rock reaction would not occur in the experimental process and would not affect the experimental results.

3.3. Influence of the Mineral Composition on Initiation Pressure of WIFs

Table 6 shows the mineral composition and initiation pressure of WIFs for the six core samples, in which the highest feldspar content of the no. 1 core sample is 53.8%, the calcite content of the no. 2 core sample is 10.9%, the quartz content of no. 3 core sample is 45.6%, and clay mineral content of no. 6 core sample is 32.9%. The content of quartz and feldspar accounts for more than half of the total core minerals, and the two physical properties are quite different, Rqf which means the ratio of quartz content and feldspar content is introduced to study the relationship between the mineral composition and initiation pressure of WIFs.

Table 6. Mineral Composition and Initial Pressure of the Core Sample.

ID quartz (%) feldspar (%) calcite (%) clay (%) Rqf initial pressure (MPa)
1 27.4 53.8 0.9 17.9 0.51 27
2 41.3 34.6 10.9 13.2 1.19 16.5
3 45.6 33.1 1.5 19.8 1.38 20.9
4 33.7 40.8 3.9 21.6 0.83 22.1
5 29.2 47.2 3.2 20.4 0.62 29.5
6 34.3 27.5 5.3 32.9 1.25 19.9
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Compared to no. 1 and no. 3 samples, it can be seen that calcite and clay with similar mineral content in the two core samples, and quartz and feldspar content have a larger difference, Rqf value is 0.51 and 1.38, respectively, initiation pressure is 27 and 20.9 MPa, respectively, it indicates that the greater the Rqf, the smaller the initiation pressure of WIFs, where the Rqf characterizes the ratio of quartz content and feldspar content, the Rqf is large, indicating the more quartz content and less feldspar content in the core sample, the Rqf is small, suggesting the less quartz content and more feldspar content in the core sample. That is when calcite and clay mineral content is similar, the change of quartz and feldspar content in core samples will affect the initiation pressure of WIFs. Further analysis of the correlation between quartz and the initiation pressure of WIFs in six core samples shows that there is a negative correlation between the quartz content and initiation pressure (y = −0.5196x + 40.967, R2 = 0.5779), indicating that the higher the quartz content, the lower the initiation pressure of WIFs. This is because the reservoir rock is a mixture of cemented mineral components, and quartz is a brittle mineral. The higher the quartz content in the core sample, the larger Young’s modulus, and the smaller the Poisson’s ratio, the greater the contribution of quartz to the reservoir rock brittleness at the same time, which makes the rock brittleness becomes stronger. So it is easy to produce WIFs at the lower initiation pressure,59,60 As shown in Figure 9(1), Figure 9(2) indicates that the feldspar content and initiation pressure have a positive correlation in six core samples (total feldspar is y = 11.294e0.0171x + 9.483, R2 = 0.6261, plagioclase feldspar is y = 0.4728x + 8.7404, R2 = 0.5119, potash feldspar is y = 1.5151x + 7.0951, and R2 = 0.8632), indicating that the higher feldspar content, the higher the initiation pressure. However, there is no unified understanding of whether feldspar is a brittle mineral, and we will focus on this in the next subsection.

Figure 9.

Figure 9

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Relationship between mineral and the initiation pressure of WIFs.

Comparing samples no. 5 and no. 6, it can be seen that sample no. 5 has a high content of feldspar, sample no. 6 has a high content of quartz, and Rqf values are 0.65 and 1.25, respectively. The calcite content of the two samples is similar, while the difference in clay mineral content reaches 12.5%, but the initiation pressure of sample 5 is significantly greater than that of sample 6, indicating that the initiation pressure is affected by the quartz and feldspar content and has no obvious relationship with the clay mineral content. Figure 9(3) indicates that the clay content and initiation pressure have a poor correlation in six core samples (y = 19.822e0.0022x, and R2 = 0.0082), that is to say, there is no obvious correlation between clay content and initiation pressure. However, according to the existing research results,61 clay minerals belong to ductile minerals, and there may be a positive correlation between the clay content and initiation pressure, namely, the higher the clay mineral content, the higher the initiation pressure.

Comparing no. 2 and no. 3 samples, it can be seen that the quartz contents of the two cores are all higher than feldspar contents, with Rqf values of 1.38 and 1.19, respectively. According to the above analysis results, the greater the Rqf value, the smaller the initiation pressure. However, the initiation pressure of the no. 2 core sample is less than the no. 3 core sample, indicating that the calcite content affects the initiation pressure. This is because the difference between the calcite content of sample no. 2 and sample no. 3 reached 9.4%, indicating that the change in the calcite content will also affect the initiation pressure of WIFs. Figure 9(4) shows the correlation analysis results between calcite and initiation pressure of WIFs in six core samples. It can be seen that there is a positive correlation between calcite content and initiation pressure of WIFs (y = 19.469e0.0319x, R2 = 0.1771), but the correlation value is not strong. Calcite is a brittle mineral, and it is easy to generate fractures at low initiation pressure,62 which is obviously inconsistent with the above result. Therefore, the fundamental reason for this phenomenon is that the low calcite content and high feldspar content lead to higher initiation pressure of WIFs.

In conclusion, for feldspar sandstone reservoirs, including quartz, feldspar, calcite, and clay minerals, when the content of quartz and feldspar reaches more than 60% of the total mineral content, the change of the quartz and feldspar content will affect the initiation pressure of WIFs. There is a negative linear correlation between quartz content and initiation pressure, and a positive relationship between feldspar content and initiation pressure, the relationship between calcite content and initiation pressure depends on the size of the calcite content. For the actual reservoir, if the quartz content in a certain area is high, the initiation pressure of WIFs can be predicted according to the correlation between the quartz content, and the generation possibility of WIFs can be judged. Similarly, if the feldspar content of a certain area is high, the generation possibility of WIFs can also be judged according to the correlation between the feldspar content and the initiation pressure of WIFs.

3.4. Influence of Brittle Minerals and Ductile Minerals on Initiation Pressure of WIFs

3.4.1. Influence of Brittle Minerals on the Initiation Pressure of WIFs

The larger Young’s modulus, the greater the rock brittleness, the smaller the Poisson ratio, and the easier the rock is to break down.63,64 Therefore, the greater the Young’s modulus the lower the Poisson ratio, and the higher the rock brittleness index. Yan et al.65 analyzed the correlation of quartz and carbonate minerals (calcite and dolomite) with Young’s modulus and Poisson’s ratio, which belong to the same reservoir of different regions in Ordos Basin, to conclude that quartz and carbonate minerals increase with the increase of Young’s modulus and decrease with the decrease of Poisson’ ratio, which confirmed that quartz and carbonate minerals of Chang 6 reservoir in Ordos Basin are brittle minerals. Therefore, it is found that a favorable negative logarithmic function correlation exists between the brittle mineral contents and initiation pressure [y = −18.57ln(x) + 90.507, R2 = 0.7795]. That is to say, the higher the brittle minerals content, the lower the initiation pressure of WIFs, which is consistent with the current understanding, as shown in Figure 10.

Figure 10.

Figure 10

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Relationship between brittle mineral content and the initiation pressure of WIFs.

3.4.2. Influence of Ductile Minerals on the Initiation Pressure of WIFs

Feldspar brittle has not been a unified standard because feldspar is the general term for feldspar family minerals, different feldspar minerals have different properties. Lei et al.66 used a high-resolution field emission CMOS scanning electron microscope to confirm that the brittleness of calcium feldspar is stronger than the potash feldspar, and the soda feldspar is between the calcium feldspar and potash feldspar by crystallographic and mineralogy. This is because potash feldspar has stronger crystal symmetry and shorter axial length, and calcium feldspar has worse crystal symmetry and longer axial length. Under the same condition, potash feldspar has the least stress sensitivity and weakest brittleness, while calcium feldspar has the greatest stress sensitivity and strongest brittleness, and is most easily to break down along the cleavage or double crystal surface. According to the XRD test results in Table 4, the feldspar minerals of Chang6 reservoir are mainly potash feldspar and plagioclase feldspar, and combining the SEM characterization results can further identify plagioclase feldspar as soda feldspar, as shown in Figure 11. In summary, due to the weak brittleness of potash feldspar and soda feldspar, the feldspar minerals are classified as ductile mineral in this paper, which is also consistent with the experimental results in Figure 7(2), that is the higher the feldspar mineral content, the less brittleness the rock and the higher the fracture initiation pressure. The correlation analysis shows that there is a good positive linear correlation between the ductile mineral content and initiation pressure (y = 0.4717x – 5.8738, and R2 = 0.7739), which means the higher the ductile mineral content, the higher the initiation pressure, as shown in Figure 12.

Figure 11.

Figure 11

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Soda feldspar in typical core samples.

Figure 12.

Figure 12

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Relationship between ductile mineral content and the initiation pressure of WIFs.

3.5. Influence of the Brittleness Index on Initiation Pressure

The essence of WIFs is the fracturing effect during long-term water-flooding, and the quantification of reservoir susceptibility to WIFs can be achieved through the use of a brittleness index.67 Brittleness index calculation methods include strength parameter method, stress–strain curve method, elastic parameter method, mineral compositions method, and conventional logging curve method.68 The strength parameter method usually adopts uniaxial compressive strength (σc) and Brazilian tensile strength (σt) to calculate the rock brittleness index, primarily used for analyzing the rock fragmentation efficiency. The stress–strain curve method calculates the rock brittleness index by obtaining the deformation, stress, and energy through the uniaxial or triaxial stress–strain curve, which is mainly used to analyze the mechanical behavior and stability of deep hard rock tunnels and mines. The elastic parameter method mainly uses rock elastic parameters such as Young’s modulus and Poisson’s ratio to calculate the rock brittleness index. The mineral composition is obtained by the mineralogical logging and XRD test, categorizing minerals into brittle and ductile types, and calculating the brittleness index by the percentage of brittle minerals in the total mineral content. The conventional logging curve method is to calculate the brittleness index from the logging data such as natural gamma, sonic time difference, and neutron porosity. The elastic parameter method, the mineral component method, and the conventional logging curve method can be used to analyze the fracability of unconventional reservoirs.

Brittle and ductile mineral classification is crucial in calculating the brittleness index using the mineral composition method. Table 7 summarizes the main methods used to calculate the rock brittleness index by the mineral component method. Jarvie et al.59 considered only quartz as a brittle mineral and established the BI1 formula to calculate the brittleness index. Wang and Gale69 identified quartz and calcite as brittle minerals, establishing the BI2 formula to calculate the brittleness index of shale. Xiaochun et al.70 identified quartz, feldspar, mica, calcite, and dolomite as brittle minerals and established the BI3 formula to calculate the brittleness index. Lai et al.71 considered quartz and calcite as brittle minerals and feldspar as ductile minerals, establishing the BI4 formula for calculating the brittleness index of tight sandstone. Shi et al.72 considered calcite as a brittle mineral, assigning coefficients to each mineral content, and the BI5 formula was established to calculate the brittleness index of tight sandstone. Moghadam et al.73 established the BI6 formula for calculating the brittleness index by the volume fraction of quartz, feldspar, pyrite, and carbonate rock. Liu and Sun74 established the BI7 formula to calculate the brittleness index by the brittleness coefficient and volume fraction of different mineral components. Huo et al.75 also adopted the same method to calculate the rock brittleness, with the main difference between the two methods being calculating brittleness coefficients.

Table 7. Definition of Brittleness by the Mineral Composition Method.

definition of brittleness description references
graphic file with name ao3c08439_m006.jpg
 Wqtz, Wcarb, and Wclay are the weights of quartz, carbonate minerals, and clay Jarvie et al68
graphic file with name ao3c08439_m007.jpg
 Wqtz, Wdol, Wcarb, Wclay, and Wtoc are the weights of quartz, dolomite, carbonate minerals, clay, and total organic content Wang and Gale59
graphic file with name ao3c08439_m008.jpg
 Wqfm is the weight sum of quartz, feldspar, and mica, Wcal, Wdol, and Wtot are the weights of calcite, dolomite, and total minerals Jin et al69
graphic file with name ao3c08439_m009.jpg
 Wqtz, Wcal, Wfels, and Wclay are the weights of quartz, calcite, feldspar, and clay Lai et al70
graphic file with name ao3c08439_m010.jpg
 Wcal, Wtoc, Wclay, and Wfsp are the weights of calcite, total organic content, clay, and feldspar, a, b, c, d, and e are the coefficients Shi et al71
graphic file with name ao3c08439_m011.jpg
 Vqtz, Vfsp, Vpy, Vcarb, and Vtot are the volume fractions of quartz, feldspar, pyrite, carbonate, and total minerals, respectively Moghadam et al72
BI7 = ∑i=1n(αifi) αi and fi are the brittleness factor of each kind of mineral and volume fraction of various minerals, respectively Liu and Sun73
BI8 = ∑i=1M(AiMi) Ai and Mi are the normalized brittleness coefficients to quartz, mineral content Huo et al74
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As the core minerals include quartz, feldspar, calcite, and clay minerals, the BI4 formula can be used to calculate the brittleness index of core samples. However, the BI4 formula assumes equal contributions of each core mineral to rock brittleness, which is not realistic with the actual situation. Therefore, the normalized brittleness coefficient is introduced,75 representing the percentage of each core mineral’s brittleness coefficient relative to the quartz brittleness coefficient, as shown in Table 8. The BI9 formula is proposed to calculate the brittleness index of the core sample, and the results are shown in Table 9. By analyzing the correlation between the brittleness index and initiation pressure of WIFs, it can be seen that there is a strong negative correlation power-law relationship (y = 12.034x – 2.492, and R2 = 0.7702). In other words, the larger the brittleness index of the core, the smaller the initiation pressure of WIFs, as shown in Figure 13.

Table 8. Brittleness Coefficient of Each Mineral to the Quartz.

mineral Young’s modulus/E (GPa) Poisson’s ratio/ν brittleness coefficient brittleness coefficient of each mineral to the quartz references
quartz 96.00 0.06 1600.00 1.00 Schon76
calcite 83.70 0.32 261.56 0.16  
potash feldspar 91.78 0.28 327.78 0.20 Yang77
oblique feldspar 69.00 0.28 246.42 0.15 Hu et al78
chlorite 76.43 0.27 283.07 0.17 Mavko et al79
illite 66.56 0.32 208.00 0.13 Wang et al80
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Table 9. Brittleness Index of Core Sample.

ID quartz (%) potash feldspar oblique feldspar calcite (%) clay (%) BI9
1 27.4 12.7 41.1 0.9 17.9 0.71
2 41.3 7.7 26.9 10.9 13.2 0.85
3 45.6 9.0 24.1 1.5 19.8 0.85
4 33.7 11.1 29.7 3.9 21.6 0.78
5 29.2 14.3 32.9 3.2 20.4 0.73
6 34.3 6.8 20.7 5.3 32.9 0.79
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Figure 13.

Figure 13

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Relationship between the brittleness index and the initiation pressure of WIFs.

3.5. 1

Among the six samples, the content of quartz and feldspar in the total mineral content exceeds 70%, which belongs to typical feldspar sandstone reservoirs. By analyzing the relationship between the ratio of quartz content to feldspar content (Rqf) and brittleness index, it is found that as Rqf increases, the rock brittleness index increases exponentially, and there is a strong correlation between the two (R2 = 0.8704), as shown in Figure 14. Therefore, in the feldspar sandstone reservoir, when the quartz and feldspar mineral content is greater than 70%, the Rqf can be used to estimate the rock brittleness index. Figure 15 shows the relationship between the Rqf and the initiation pressure of WIFs. Combined with the analysis of the relationship between the brittleness index and the initiation pressure of WIFs, it is observed that as Rqf increases, the initiation pressure of WIFs decreases logarithmically. As the Rqf increases, the quartz content increases, and the feldspar content decreases, leading to an increase in rock brittleness, an increase in the brittleness index, and a decrease in the initiation pressure of WIFs, which will increase the possibility of WIFs in the long-term water injection process of feldspar sandstone reservoir.

Figure 14.

Figure 14

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Relationship between the Rqf and the brittleness index.

Figure 15.

Figure 15

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Relationship between the Rqf and the initiation pressure of WIFs.

4. Conclusions

Through the characterization of six core samples from the Chang 6 reservoir in the Wuqi oilfield, western Ordos Basin, the mineral composition of the Chang 6 reservoir is analyzed. Based on the characterization results, the six core samples are conducted to research the influence of mineral composition on the initiation pressure of WIFs. The novelty of this work is to elucidate the potential relationship between the core mineral composition and the initiation pressure of WIFs. The specific conclusions are as follows. (1) The rock mineral composition of the Chang 6 reservoir in the Wuqi oilfield, western Ordos Basin, includes quartz, potash feldspar, plagioclase feldspar, calcite, and clay minerals. The clay minerals consist of hair-like illite and foliated chlorite, which are typical feldspar sandstone reservoirs. Quartz and calcite belong to brittle minerals, while potash feldspar, plagioclase feldspar, and clay belong to lithological minerals. (2) For feldspar sandstone reservoirs, including quartz, feldspar, calcite, and clay minerals, when the combined content of quartz and feldspar exceeds 60% of the total mineral content, changes in the quartz and feldspar content can affect the initiation pressure of WIFs. As the Rqf increases, the initiation pressure of WIFs exhibits a logarithmic function decrease. The Rqf ratio can be used to estimate the rock brittleness index. (3) Considering the contribution of diagenetic minerals to rock brittleness, the calculation method for the brittleness index of feldspar tight sandstone reservoirs is improved, and the new brittleness index calculated by this method is more accurate. (4) Analyzing the relationships between the Rqf ratio, brittleness index, and initiation pressure of WIFs, it is found that as the Rqf increases, the rock brittleness index exhibits a power-law increase. Simultaneously, the initiation pressure of WIFs shows a power-law decrease. This suggests an increased possibility of WIFs occurring in feldspar sandstone reservoirs during long-term water injection processes.

This study provides a preliminary analysis of the influence of core mineral composition on the initiation pressure of WIFs in feldspar sandstone reservoirs. However, due to sample quantity limitations, further sample validation is needed in the future. The mineral composition of tight sandstone is complex and diverse, varying across different regions. In the future, it is necessary to continue to investigate the impact of the mineral composition of other tight sandstone reservoirs on the initiation pressure of WIFs, so as to enhance the understanding of the mechanism underlying the influence of core mineral composition on the initiation pressure of WIFs in tight sandstone reservoirs.

Acknowledgments

This work was supported by the National Natural Science Foundation of China (grant no. 52074223 and 52174020).

The authors declare no competing financial interest.

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