Hydrodynamic and Geostress Controls on CBM Enrichment in the Anze Block, Southern Qinshui Basin, North China

School of Energy Resources, China University of Geosciences, Beijing 100083, China Coal Reservoir Laboratory of Natural Engineering Research Center of CBM Development & Utilization, China University of Geosciences, Beijing 100083, China Beijing Furuibao Energy Technology Company, Beijing 100176, China School of Engineering, Fraser Noble Building, King’s College, University of Aberdeen, AB24 3UE Aberdeen, UK


Introduction
Identifying the distribution characteristics of coalbed methane (CBM) is one of the main objectives of CBM resource exploration and an essential guarantee for efficient CBM development [1]. The study of the effect of the main geological factors on the CBM enrichment is the basis for evaluating the distribution characteristics of CBM resources. Based on the analysis of a large amount of exploration and development data, tectonic conditions, coal seam burial depth, hydrogeological conditions, depositional environment, and coal seam physical properties were considered as the main geological factors that affect CBM resource enrichment [2][3][4]. Among them, the influence of hydrodynamic field on CBM enrichment was investigated by groundwater geochemical characterization in coal seams [5] and fluid energy calculations based on specific reservoir parameters [6]. It was concluded that the distribution of methane carbon isotopes in coalbed water is related to the CBM formation [5,7,8]. Weak runoff areas with stagnant groundwater have higher salinity and desulfurization coefficients [9], which are easy to form hydrodynamic seal [10] and favorable for CBM enrichment [11,12]. On the contrary, strong hydrodynamic conditions result in hydrodynamic scour and cause gas loss [13].
Previously in the literature, the geostress distribution were directly tested by arranging borehole stress testing [14] or predicted with geostress models based on logging data, well test data, and hydraulic fracturing data [15][16][17]. Then, formulate correlation between geostress and reservoir gas content [18]. It is concluded that high tectonic stress determines the presence of high reservoir pressure and high gas content [19]. A higher vertical stress guarantees a lower possibility of CBM transport and dissipation to the surface, and thus, it is easier to store gas [20].
Anze Block is one of the blocks with high gas content in Qinshui Basin, and there is a significant amount of work focused on geological structure, reservoir permeability, and gas content prediction [21]. However, no systematic research on combined effect of multifield, for example, geostress field and hydrodynamic field, on CBM enrichment in Anze Block has been carried out. In this work, the geostress field, hydrodynamic field, and gas content distributions were evaluated based on geological exploration data and laboratory test data in Anze Block, and the effects of the geostress field, hydrodynamic field, and their combined effect on CBM enrichment was investigated thoughtfully (Figure 1). The research is expected to provide a geological basis for further exploration and development of CBM resources in the Anze Block.

Geological Background
2.1. Tectonic Evolutionary Features. The Anze Block is located in the southern part of the Qinshui Basin, Shanxi Province, China, with regional stratigraphic trends to the north-east and gentle production, and stratigraphic tendencies to the south-east, with stratigraphic dips generally in the range of 2°to 12°, to the west by the Huoshan backslope and to the east by the Qinshui complex oblique ( Figure 2).
The Anze Block experienced four stages of tectonic evolution. The first stage refers to the north-south extrusive stresses from the Late Hercynian period to Indo-Chinese periods forming WE-oriented folds and increasing the burial depth of coal seams to the maximum burial depth. In the early and middle Yanshan period, the block was mainly dominated by extrusion uplift and folding on account of the Yanshan movement, which formed NE-SW and NNE-SSW trending backward and forward slope. In the late Yanshan period, the crust was in a state of tectonic uplift and uplift, which result in strong denudation in the Anze Block, and the burial depth of coal seam decreased gradually. In the early and late Himalayan period, the structural differentiation was stronger due to the different ground stress character, which is leading to most areas keep stable except some parts of areas were subsided or subsiding slightly.

Characteristics of the Coal Seam.
The main coal-bearing strata consists of Carboniferous Taiyuan Formation and the Permian Shanxi Formation in the Qinshui Basin, which include nos. 3 and 15 coal seams [22][23][24], and here in this work, we focus on the no. 3 coal seam. The thickness of no.3 coal seam in Anze Block increases along the W-E direction gradually, and the thickest part of the coal seam located in the south and southeast of the study area. No. 3 coal seam has a large thickness in a single layer, and the thickness variation range from 4.5 m to 7.7 m, with an average thickness of 6.77 m. Part of coal seam is divided into upper and lower layers by 1 or 2 gangue layers (as shown in Figure 3). The porosity of coal seam in the Anze Block varies from 1.91% to 5.44%, with an average porosity of 3.98%. The permeability variation ranges from 0.011 mD to 0.21 mD, with an average permeability of 0.068 mD. The structure of coal seam in the study area is mainly fracture structure. The minimum gas content of no. 3 coal seam is 5.5 cm 3 /g, the maximum is 25.2 cm 3 /g, and the average gas content is 12.23 cm 3 /g.
The roof and floor of coal reservoir are important for affecting the preservation of CBM in coal seams. In the study area, the roof with has strong stability and great thickness is mostly consist of dark gray mudstone, dark gray tight sandstone, and argillaceous sandstone. Meanwhile, the floor lithology is dominated by dark gray sandy mudstone and mudstone. During geological evolution process, the mudstone and argillaceous sandstone normally have low porosity and permeability due to the compaction by diagenesis, which indicates that the overlying strata has good sealing and preservation performance for CBM in the no. 3 coal seam.

The Chemical Characteristic of Groundwater in Coal
Seams. The chemistry characteristics of coal reservoir groundwater include the total dissolved solids (TDS), ion type, hydrogen and oxygen isotopes of water, and microelement and rare-earth elements, which could be used to clarify the groundwater quality, water source, hydrodynamic intensity, and runoff characteristics [5,25]. In this work, 23 water samples were collected for chemistry characteristics analysis based on the SY/T 5523-2006 standard; the concentration of ions in groundwater was measured with manual titration. All experiments in this work were performed at 25°C, 0.1 MPa, and 30% in relative air humidity. The results of chemistry characteristics in Anze Block are summarized in Table 1. As shown in this table, the primary compositions of water produced from coal reservoir are Ca 2+ , Mg 2+ , Na + +K + , SO 4 2-, Cl -, and HCO 3 -. The contents of Na + +K + , Cl -, and HCO 3 are the major components, which vary greatly from well to well (as shown in Table 1) and play an important role in identifying the type, TDS, and intensity of groundwater activity of recovered water from CBM. Na + +K + concentration of the highest of up to 2455.95 mg/ L in well A1-2, the minimum concentration is 356.32 mg/L in well A29, and average value is 707.31 mg/L in Anze Block. The maximum concentration value is 3129.80 mg/L for Cl -, the minimum of it appears in well A21 is 106.09 mg/L, and the average value is 443.10 mg/L. In addition, the highest and lowest concentration of HCO 3 is 2349.21 mg/L and 630.28 mg/L, respectively, and the average concentration was 1183.03 mg/L. Compared with Na + +K + , Cl -, and HCO 3 -, the concentration of Ca 2+ , Mg 2+ , and SO 4 2fluctuated less excluding a handful of outliers in some wells; these outliers may be related to formation structure or lithology around the well. Figure 4 presents TDS as a function of Na + +K + concentration for the formation water samples. As shown in this figure, the TDS increases linearly with the Na + +K + concentration. TDS of water in CBM wells indicate the total amount of various ions, molecules, and compounds which dissolved in 2 Geofluids water. A wide range of TDS was measured ranging from 1209.93 to 6816.71 mg/L in study area, with an average value of 3064.64 mg/L, which is much lower than that of seawater TDS of 35000 mg/L. Based on the ion concentration, TDS of water, trilinear diagram ( Figure 5(a)), and stiff diagram ( Figure 5(b)), there are two types of formation water in the Anze Block, NaCltype and NaHCO 3 -type, respectively. According to the previous study, CBM always enriched in stagnant areas where the water is static or moves slowly and is dominated by Na + and Cl − [5,12], which indicates that the Anze Block is favorable for CBM preservation.

Groundwater Geochemical Characteristics in Coal
Seams. The formation water geochemical characteristics can be evaluated by different parameters, including metamorphic coefficient (Cl-Na coefficient), desulfurization coefficient, Cl-Mg coefficient, Ca-Mg coefficient, and carbonate equilibrium coefficient [5].
The groundwater geochemical parameters in coal seams, obtained from 19 wells in the Anze Block ( Figure 6), show that the metamorphic coefficient ranges from 1.23 to 8.01, with a median of 3.35; the desulfurization coefficient ranges from 0.36 to 100.45, with a median of 2.76; the carbonate equilibrium coefficient is in the range of 8.74 to 195.49, with a median of 61.98; the Cl-Mg coefficient ranges from 7.61 to 106.45, with a median of 20.29; and the Ca-Mg coefficient ranges from 0.40 to 3.01, with a median of 1.00. The favorable aeras with rich gas content are characterized by high TDS, Cl-Mg coefficient, low metamorphic coefficient and desulfurization coefficient, low Ca-Mg coefficient, and low carbonate equilibrium coefficient. Meanwhile, TDS, which have a good linear relationship with ion concentration (Figure 4), is also a common geochemical parameter used to evaluate hydrodynamic strength. The strong hydrody-namic field with TDS more than 2000 mg/L always has a good reservoir preservation condition, while the weak hydrodynamic field with TDS less than 2000 mg/L has a bad reservoir preservation condition.

Hydrodynamics of Groundwater.
The TDS of groundwater in coal seams, which is the result of long-term evolution of rock strata and groundwater, reflects the concentration characteristics of major ionic components and is also an important indicator for researching the hydrochemistry of the formation. Thus, the TDS could be used to characterize the hydrodynamics of groundwater [3]. Previous study shows that the Cl -, Na + , and K + contents in the study area are low, and the water type is dominated by SO 4 -HCO 3 -Ca-Mg-type. In the strong runoff area, the water type is dominated by Ca-SO 4 or Mg-SO 4 . For stagnant area, the water type is NaHCO 3 -type or NaCl-type [5].
The TDS of groundwater in Anze Block is relatively low with range from 1209.93 mg/L to 6816.71 mg/L, and the overall trend shows that the western part is lower than the eastern part, the southern and northern parts are relatively higher, and the central region is lower in general (see Figure 7(b)). According to the TDS distribution characteristics, the study area could be divided into three parts, including runoff area, stagnant area, and transition area (weak runoff area). Runoff area is in the western part of the study area, stagnant area is located in the northeastern and parts of the southern, and transition area is in the central region. Meanwhile, the flow direction of groundwater in the study area is roughly from the west to the east, as presented in Figure 7(b). Runoff area would be formed due to the flow from the both sides in the north and south directions and then further formed stagnant area, that is, the flow direction of groundwater is from the runoff area to stagnant area.

Geostress Characteristics
4.1. Calculation of Geostress. The Anderson in situ stress model is widely used to study geostress; in this model, tectonic stress generated horizontally is proportional to the pressure of the overlying rock layer [26,27], and the model is chosen for this study to investigate the geostress distribution in the Anze Block. Which could be written as follows: where σ h and σ H are the minimum and maximum horizontal 4 Geofluids geostresses, respectively, MPa. σ v is the vertical geostress, MPa. μ is the Poisson's ratio of the rock. α is the biot factor. p p is the pressure of reservoir, MPa. β h and β H are the minimum and maximum horizontal geostress tectonic factor, respectively. The vertical stress is usually calculated from the gravity of the overlying strata, and in the study area, its estimation equation can be expressed as where h is the thickness of the overlying strata (or burial depth), m.
Combining the geostress parameters of some known wells in the study area and the Anderson in situ stress model, the minimum and maximum horizontal geostress tectonic factor can be inferred (Table 2), and the results show that β h varies from -0.1395 to 0.4791, with the average of 0.1482, and the β H varies from 0.1966 to 1.8286, with the average of 0.7599. The model for calculating geostress in this study area could be expressed as: The vertical geostress in Anze Block shows a trend of low in the west and high in the east as presented in Figure 8(a). The distribution of the minimum and maximum horizontal principal stresses is similar to that of vertical geostress (see

Geofluids
horizontal stress in the central part is relatively low. The overall trend of geostress in the study area is low in the west and high in the east, and the south and north are higher than the central area.
The minimum horizontal stresses, maximum horizontal stresses, and vertical stresses vs. the burial depth of the coal seam is presented in Figure 9(a). As shown in this subfigure, positive correlation is observed within the margin of  6 Geofluids experimental error, indicating that the burial depth of the coal seam has great effect on the stress distribution and value. The lateral pressure coefficient k, which is used to describe the state of ground stress at a point, is the ratio of the average of the minimum and maximum horizontal principal stress to the vertical stress and is calculated as follows: As shown in Figure 9(b), the lateral pressure coefficient is opposite to the extension direction of the burial depth contour of coal seam, that is, it decreasing with the increase of the coal seam depth. The lateral pressure coefficients of the Anze Block were fitted with reference to Hoek-Brown model [28], and the linear equations were obtained as follows: The geostress in the area is mainly dominated by σ H > σ v > σ h type based on the distribution of lateral pressure coefficients in the Anze Block as shown in Figure 9(a). The coal reservoir is in compression state, which is favorable to CBM production. A part of areas where the coal seam is buried at a shallow depth (650-750 m) develop horizontal fractures, which has a negative effect on the escape of CBM from the coal reservoir.
The lateral pressure coefficient of the Anze Block (Fitting envelope) is between the Hoek-Brown envelope and Chinese envelope and is generally distributed close to the Chinese inner envelope (see Figure 9(b)). The lateral pressure coefficient shows a general trend of decreasing with deeper coal seam burial, but this trend is not very obvious. The individual discrete points that appear are likely due to the shallow coal seams being affected by fault fractures and tectonic activity.

Hydrodynamics and Geostress Control on CBM Enrichment
5.1. The Distribution of Gas Content. According to the distribution range of gas content in the study area, it can be divided into three types: low gas content areas with the gas content less than 10 cm 3 /g; middle gas content areas with the gas content between 10 and 20 cm 3 /g; high gas content areas with the gas content greater than 20 cm 3 /g. Meanwhile, the hydrodynamic field of the coal reservoir in the study area could be divided into two types based on the effect of TDS on the reservoir preservation conditions. Strong dynamic field, where the groundwater activity of the coal reservoir is active and has a scouring effect on the CBM, is classified by the criterion of TDS < 2000 mg/L. Weak dynamic field, where the groundwater activity of the coal seam is inactive, is classified by the criterion of TDS < 2000 mg/L. The flow direction of groundwater is from the shallow part of coal seam with low burial depth to the high burial depth or from the strong dynamic field to the weak dynamic field as shown in Figure 10, and the values of TDS are affected due to the flow and accumulation of groundwater. High TDS of groundwater characterizes for whole study area especially around wells A1, A4, and A40; the TDS value is above 3000 mg/L. Meanwhile, the groundwater of coal reservoir is mainly dominated by NaHCO 3 -type water, representing a relatively close hydrodynamic environment, and in a reducing environment, which is favorable for preservation and enrichment of CBM. The gas content of these high  7 Geofluids TDS areas is also relatively high (see Figure 10). The gas content of well A1 is 15.08 cm 3 /g and that of well A4 is 17.48 cm 3 /g, both of which are in the medium gas content area, while the gas content of well A40 is 25.25 cm 3 /g, indicating that there is a weak dynamic field and a reducing environment in the high TDS area, and the high TDS area is beneficial to CBM preservation.
The horizontal stress difference in the study area could be divided into two different areas based on the effect of the horizontal stress difference on the gas content. The low stress area with horizontal stress difference is lower than 13 MPa and the high stress area with horizontal stress difference is higher than 13 MPa. According to the distribution of gas content and geostress shown in Figure 11, the horizontal stress difference decreases gradually along the west to the east direction, which is consistent with burial depth of coal seam. The central part of the study area is tectonically complicated, and the fault is developed well, which leads to frequently changed and ununi-formed distribution of horizontal stress difference in the central part of the study area. There is a similar changing rule of the horizontal stress difference and the gas content of coal reservoir in the study area, and the two show a positive correlation, that is, as the value of horizontal stress difference of coal reservoir increases, the gas content increases, which may result from a good adsorption capacity for CBM improved due to the reservoir stress increment.

The Combined Effect of Hydrodynamics and Geostress.
The hydrodynamic field has an important influence on the fluid pressure, composition of CBM, and groundwater type in the coal seam; it controls the distribution of gas content through the "recharge-runoff-discharge" hydrodynamic system [5,6]. The hydrodynamic field may have positive or negative effects on the CBM enrichment. On the one hand, hydraulic dissipation will destroy the gas reservoir in the runoff area. One the other hand, hydraulic sealing has a  8 Geofluids protective effect on the CBM reservoir in the stagnant area [10,13]. Geostress is a criterion which indicates the amount of driving energy in coal reservoirs, which not only plays a controlling role in the process of CBM generation but also affects the development of CBM. Geostress has an important influence on the reservoir physical property such as coal structure, permeability, and pore-fracture morphology [29][30][31]. Generally, areas with relatively high geostress also have relatively high gas content of coal reservoir. Various tectonic stress and their internal stress distributions would result in changes in the property of coal reservoir, capping layers, and groundwater runoff conditions during the formation of different types of geological structure, which would further affect the content of CBM [19]. Figure 11 shows the coupling effect of hydrodynamic field and geostress field. As shown in this figure, the high gas content area is mainly distributed in the coupled area of weak hydrodynamic field and high horizontal stress difference field,  9 Geofluids while the gas content of coal reservoir is low where the hydrodynamic is strong or the horizontal stress difference is small. The burial depth of coal seam is shallow in the area with strong groundwater hydrodynamic, which has less vertical geostress as well as less reservoir pressure, leads to a lower adsorption capacity of the coal reservoir for CBM and a lower gas content, thus, it is not conducive to CBM enrichment, while in the stagnant area with inactive groundwater, the horizontal stress difference of the coal res-ervoir is large, and the hydrodynamic activity is small; in addition, the higher geostress promotes the increase of the CBM adsorption capacity of coal reservoirs, which is favorable for CBM enrichment ( Figure 11). Therefore, areas with low TDS value indicate that a relatively open hydrodynamic environment and low horizontal stress difference are not conducive to CBM preservation and have low coalbed gas content. On the contrary, inactive hydrodynamic environment is favorable for CBM preservation. FuCheng H e C h u a n