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. 2024 Feb 14;9(8):9577–9584. doi: 10.1021/acsomega.3c09453

A Method for Prediction of High-Water-Production Areas in Coalbed Methane Blocks Based on γ Logging Data: A Case Study of the Taiyuan Formation in Liulin Area, Eastern Ordos Basin, China

Yinan Liu †, Li Wang ‡, Yanyong Xu †, Peng Zong §,*, Yu Dong §
PMCID: PMC10905601  PMID: 38434838

Abstract

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The roof aquifer of the Carboniferous Taiyuan Formation coal beds in the Liulin area severely restricts the development and utilization of coalbed methane (CBM). A method for quantitatively predicting high-water-production areas was established by analyzing the relationship between the geophysical logging data and water production. The results showed that the logging profile of the limestone aquifers in high-water-production wells was unique, with high acoustic velocity (AC), high γ-ray values (GR), and low resistivity (Rd). The developed pores and fractures in the roof limestone increase the interval transit time. The formation water in the pores and fractures of the roof limestone decreases the resistivity. The clay filling in the pores and fractures of the roof limestone originated from the dissolution product of limestone and hydrodynamic transportation, which resulted in increased GR values. Furthermore, the representative natural GR log data were used to calculate the clay content in limestone, which indicated that the clay content in limestone had a positive correlation with the water yield of the CBM wells. The water-bearing characteristics of roof limestone showed that the water content was higher in the northern area and decreased gradually toward the south. The method for predicting the high-water-production area was helpful for the CBM exploration and production.

1. Introduction

Coalbed methane (CBM) has achieved commercial development in the Taiyuan Formation of the southern Qinshui and eastern Ordos Basins of China,1 but there are still CBM resources that have not been effectively developed and utilized. Previous investigations showed that the hydrogeological conditions were the key influence factors on the formation and development of CBM production of the Taiyuan Formation.2−5 The control of hydrogeological conditions on the law of CBM enrichment and formation mainly includes three aspects. First, the fugitive effect when CBM is transported with groundwater, resulting in the dispersion of CBM; second, the hydraulic closure or gas-control effect, which is conducive to the preservation of CBM; and third, the promotional effect of appropriately active hydrodynamic conditions on the generation of secondary biogenic gases from medium–low rank CBM reservoirs.3,6,7 In the direction of development, the study of hydrogeological conditions is of great significance to the ease of drainage depressurization, the law of depressurization propagation, and the treatment and application of produced water.8 Hydrology is not only an important factor in CBM accumulation but also a vital part of gas production.9

The limestone aquifer is the main source of water in the coal seam.9,10 Water production from CBM wells is an important indicator parameter, reflecting the water-richness characteristics of coal seams and their roofs. Production data show that the water production of the Taiyuan Formation coal seam in the Liulin area is high and highly variable. Vertically, the water production of the Taiyuan Formation coal seams in the Liulin area is generally higher than that of the Shanxi Formation coal seams. In plane, the water production in the northern part of the block is significantly higher than that in the central and southern parts. The strong water-richness of the reservoir will restrict the development and utilization of the CBM resources. Therefore, an effective prediction of the high-water-cut limestone is of great significance for determining CBM development in the Taiyuan Formation.

Currently, the prediction method for high-water-cut limestone in the CBM development area is mainly based on the hydrogeology background and CBM accumulation theory. It is difficult to accurately determine the distribution of high-water-cut limestone, which is required for highly efficient CBM extraction. The evaluation of the distribution of high-water-cut limestone aquifers using geophysical logging data is gradually considered to be the most convenient and accurate method. In the field of conventional oil and gas research, acoustic time difference, compensated density, and neutron logging are often used for joint calculation and prediction of reservoir porosity.11−13 The prediction of the water content can be evaluated and predicted on the basis of porosity prediction in conjunction with apparent resistivity logging or NMR logging data. However, at present, the use of logging to predict the watering characteristics of the roof limestone of coal beds is relatively rare, but it is a very important technical method for the development of CBM in the Taiyuan Formation.

Combined with the validation of water production data from existing drainage wells, this paper comprehensively analyzes the logging curves of a large number of CBM wells in the study area and provides a quantitative method for predicting the high-water-production areas based on the geophysical logging data. The water enrichment zones of the limestone aquifer in the Taiyuan Formation of the Liulin area were divided, which is helpful for CBM exploration and production.

2. Geologic Setting

The Liulin area is a trail area for CBM development in China,9 which is located in the south of the Lishi nose structure belt in the eastern Ordos basin, with an area of 194.42 km2. The tectonic features are simple, and the stratigraphic strike is close to NW-SN, with a dip gently toward the southwest (Figure 1). The Jucaita fault zone developed in the north of this area and is composed of two normal faults and several small-scale faults. The main seams in the Taiyuan Formation are the No.8, No.9, and No.10 seams, which have abundant CBM resources.14−16

Figure 1.

Figure 1

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Location of the Liulin area in China and the thickness contour map of the roof limestone of the Taiyuan Formation coal seam.

There are three aquifers from top to bottom, the Lower Permian Shihezi Formation and Shiqianfeng Formation sandstone fracture aquifers, Lower Permian Shanxi Formation sandstone fracture aquifer, Upper Carboniferous limestone Karst fracture aquifer, and middle Ordovician limestone Karst fracture aquifer. Among them, the coal seam roof limestone aquifers of the Upper Carboniferous Taiyuan Formation are the main aquifer groups related to CBM development in the Liulin area (Figure 2). Influenced by the tectonic movement, limestones were lifted to the surface, dissolution occurred, and fractured. From this, many tectonic fractures, karstic fissures, and soluted pores and cavities developed in the roof limestone aquifers of the Taiyuan Formation. And the surface water recharge into these well-connected water storage spaces from the exposed Carboniferous carbonate area of the northeastern Liulin area (Figure 1).

Figure 2.

Figure 2

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Stratigraphic column of the Permo-Carboniferous coal-bearing strata in the Liulin area.4

3. Data and Methods

Geophysical logging and production data from 14 CBM wells were collected in the analysis of the methodology for predicting high-water-production areas in CBM. In order to guarantee the reliability of produced water statistical data and to avoid interference with early hydraulic fracturing injection water, the first 3 months were discarded, and the average daily water yield was calculated over 12 months. The whole workflow of this work is shown as follows (Figure 3)

  • (1)

    Obtain the water production data from actual production wells and then divide the data into different water yield types. The high-water-production wells have more than 50 m3 average daily water yield, and the low-water production wells have less than 20 m3 average daily water yield.17

  • (2)

    Complete depth correction of logging data by comparing drilled coal seam information with density logging curves, select some logging curves as mark curves, which have obvious indications for roof limestone aquifers, and analyze the characteristics of the mark logging curves in high- and low-water production wells according to the results from step (1).

  • (3)
    Choose the representative natural γ (GR) log data to obtain the parameters that can be used to calculate the development degree of the pores and fractures in the roof limestone. Based on the law of clay filling in limestone, the formula to calculate the clay content in limestone is18
    graphic file with name ao3c09453_m001.jpg 1
    where Vsh represents the clay content, fraction; C represents the hilchie index, dimensionless; and SH represents the clay content index, dimensionless, which can be calculated by
    graphic file with name ao3c09453_m002.jpg 2
    where GR represents the GR log value of the target layer, API; GRmin represents the GR log value of a clean formation, API; and GRmax represents the GR log value of clean mudstone, API.
  • (4)

    Analyze the distribution of pores and fractures in the roof limestone and based on these results, estimate the water content of the roof limestone, and predict the high-water-yield areas in the Liulin area.

Figure 3.

Figure 3

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Workflow was used to predict the high-water-yield areas in the Liulin area.

4. Characteristics of Water Production and the Logging Response in the Taiyuan Formation

The production data from 14 CBM wells showed that the daily water production of the coal seams in the Taiyuan Formation varied considerably from 0.89 to 142.6 m3/d (Table 1). The production practice indicated that when the water output exceeds 50 m3/d, it is difficult to decrease the coal reservoir pressure. However, the frequent system malfunction of the CBM wells increases. This has become a significant challenge for CBM development in this area. In reality, the roof limestone of the coal seam is very tight and has poor aquosity under the original conditions. However, due to the effect of the hydrogeology conditions and later tectonic movements, the pores and fractures developed in some areas and enhanced the water abundance of the limestone. CBM reservoirs are usually hydraulically fractured before they are developed.19 The fractured fractures in the study area connected the coal seam to the roof limestone. The abnormally high-water production of the CBM wells mainly comes from the transgressive recharge of the highly porous, permeable surrounding rock aquifer. Therefore, the water production of CBM wells in the Taiyuan Formation can indirectly reflect the water content of the roof limestone of the coal seam. As a result of water enrichment, some of the physical properties of the limestone also changed and some of the logging parameters showed abnormal values.20−22 The water enrichment zones of the limestone aquifer in the Taiyuan Formation of the Liulin area can be identified by extracting these logging parameters, which reflect the development degree of the fractures in the roof limestone.

Table 1. Calculation of the Clay Content by the Representative Natural γ (GR) Log Data.

well clay content SH GR GRmin GRmax average daily water yield (m3)
P4 0.448 0.615 131.349 26.1 197.31 129.4
P5 0.457 0.622 105 44 142 133.7
L1 0.556 0.708 169.93 19.89 231.86 100.2
P1 0.631 0.766 148.055 33 183.147 142.6
D2 0.555 0.707 115.69 20.58 155.07 131.4
P2 0.494 0.656 101.409 18.3 145 128.5
C7 0.268 0.425 103.698 21.01 215.36 25.7
C2 0.227 0.374 92.046 19.97 212.67 4.9
D3 0.133 0.242 75.333 28.9 220.69 2.2
P7 0.245 0.397 116.026 35.71 238.04 6.2
P8 0.166 0.533 127 29 213 0.86
C1 0.271 0.429 124.086 25.16 255.56 16.8
G2 0.211 0.354 102.887 22.95 248.55 0.89
Y18 0.153 0.273 99.647 26.34 295.27 3.3
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According to the analysis of the logging curves, the logging curves of the roof limestone aquifers in high-water-production wells (average daily water production: 139 m3) are unique, with high acoustic velocity (AC), high γ-ray values (GR), and low resistivity (Rd) (Figure 4). Meanwhile, the logging curves of the roof limestone aquifers in the low-water production wells (average daily water production <10 m3) lack these characteristics (Figure 5).

Figure 4.

Figure 4

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Logging curve characteristics of a high-water-yield well (P1).

Figure 5.

Figure 5

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Logging curve characteristics of a low-water-yield well (P7).

The main reasons are as follows:

  • (1)

    The developed pores and fractures in the roof limestone increase the interval transit time.23,24 High AC can indicate the existence of fractures in the roof limestone of high-water-producing coal seams. The development of fractures destroys the denseness and homogeneity of the formation, and AC curve is prone to circumferential jumps, presenting high AC.

  • (2)

    The formation water in the pores and fractures of the roof limestone decrease the resistivity.25

  • (3)

    The clay filling in the pores and fractures of the roof limestone increase the γ-ray values.26 It has been shown that there will be higher clay content within some fracture zones or in areas of high fracture development.18 Clay-filled roof limestone fractures are caused by two main reasons. On the one hand, it is by the dissolution products of limestone. The roof limestone of the coal seam of the high-yield water well is a fracture-developed layer section with active water activity, and the dissolved oxygen and carbon dioxide in the water will react with the carbonate minerals of the limestone (eq 3).27 At the same time, the limestone is gradually dissolved by dissolution, and the impurities in the limestone eventually lead to an increase in clay content.28,29 On the other hand, it is due to the hydrodynamic transportation: the clay from other formations is transported into the pores and fractures of the roof limestone and deposited during the formation water flow.30Figure 6 shows a thin section of limestone that has been subjected to dissolution in the study area. As a result of dissolution, cavities and fractures are formed, and increased clay content is observed in the fractures.

Figure 6.

Figure 6

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Thin section of limestone that has been subjected to dissolution.

Therefore, these logging curve characteristics can be used to predict the distribution of water enrichment zones of the limestone aquifers in the Taiyuan Formation in the Liulin area.

4. 3

5. Prediction of High-Water-Production Areas Based on the Geophysical Logging Data

The roof limestone of the coal seam is very tight and has a poor aquosity under the original conditions. However, the fractures developed in some areas and enhanced the water abundance of the limestone. As a result of water enrichment, the limestone is gradually dissolved by dissolution, and the impurities in the limestone eventually lead to an increase in the clay content. Meanwhile, the clay from other formations is transported into the pores and fractures of the roof limestone and deposited during formation water flow. According to eqs 1 and 2, the representative natural γ (GR) log data were used to calculate the clay content in limestone to reflect the development degree of pores and fractures in the roof limestone based on the law of clay filling in limestone (Table 1). The calculation results indicate that the clay content in limestone has a positive correlation with the water yield of CBM wells (Figure 7). With the increase of the clay content, the water production of CBM wells increases. The water production of CBM wells in the Taiyuan Formation can indirectly reflect the water content of the roof limestone of the coal seam. Therefore, clay content is used to analyze the development degree of the pores, fractures, and water abundance in the roof limestone.

Figure 7.

Figure 7

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Relationship between the clay content in limestone and the average daily water yield of CBM wells in the Liulin area.

The water-bearing characteristics of the roof limestone indicate that the water content is higher in the northern area, the maximum value occurs near well P1, and the water content decreases gradually toward the south (Figure 8). This suggests that the roof limestone develops dissolved pores and fractures in the central and northern areas. In the southern area, pores and fractures are undeveloped, and the hydrodynamic conditions are relatively weak. This prediction is consistent with the actual geological conditions. As the only fault zone, the Jucaita fault zone is composed of two normal faults and several small derived faults in the north boundary of the Liulin area. Under the influence of tectonic movement, the limestone was uplifted to the surface. Many tectonic fractures, karstic fissures, and soluted pores and cavities developed in the roof limestone aquifers of the Taiyuan Formation. The surface water recharge into these well-connected water storage spaces from the exposed Carboniferous carbonate area of the northeastern Liulin area. Meanwhile, the thickness of limestone at the top of the coal beds of the Taiyuan Formation thins from north/northwest to southeast. The smallest thickness is less than 4 m in the southeastern area (Figure 1). Therefore, the northern part of the study area has a higher water content.

Figure 8.

Figure 8

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Contour map of the clay content of the roof limestone in the Liulin area.

6. Conclusions

  • (1)

    The daily water yields of the coal seams in the Taiyuan Formation varied substantially. When the average water output of single wells is more than 50 m3/d, it will lead to the difficulty in decreasing the coal reservoir pressure and increasing the frequent system malfunctions of the CBM wells.

  • (2)

    The developed pores and fractures in the roof limestone increased the interval transit time, the formation water in the pores and fractures of roof limestone decreased the resistivity, and the clay filling in the pores and fractures of the roof limestone increased the γ-ray values. Therefore, the logs of the roof limestone aquifers of the high-water-production wells have high acoustic velocity (AC), high γ-ray values (GR), and low resistivity (Rd).

  • (3)

    The roof limestone of the coal seam is very tight and has poor aquosity under the original conditions. However, the fractures developed in some areas and enhanced the water abundance of the limestone. As a result of water enrichment, the limestone is gradually dissolved by dissolution, and the impurities in the limestone eventually lead to an increase in clay content. Meanwhile, the clay from other formations is transported into the pores and fractures of the roof limestone and deposited during the formation water flow. Therefore, the clay content can be calculated to analyze the development degree of the pores, fractures, and water abundance in the roof limestone. The distribution of the water enrichment zones was determined in the Taiyuan Formation of the Liulin area, which is beneficial for CBM exploration and determining the production of the Taiyuan coal-bearing strata.

Acknowledgments

This study was financially supported by the National Natural Science Foundation of China (42172188).

The authors declare no competing financial interest.

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