Geological conditions of coalbed methane accumulation in the Hancheng area, southeastern Ordos Basin, China: Implications for coalbed methane high-yield potential

Chen Guo,Lingling Lu,Xueyang Sun,Gelian Dai,Yue Chen,Dongmin Ma,Jian Shen,Yucheng Xia

doi:10.1177/0144598719838117

Abstract

The Hancheng area is a hot spot for coalbed methane exploration and exploitation in China. Structure is a key factor affecting coalbed methane accumulation and production in the Hancheng area. For a better understanding of the coalbed methane accumulation conditions and high-yield potential, this study investigates the structural patterns and evolution, the hydrogeological conditions, and the geothermal field in the coal-bearing strata in the Hancheng area. Then, the spatial distribution of the coalbed methane content and the tectonic deformation of the coal seam are evaluated. Finally, the critical depth for coalbed methane enrichment and a high-yield potential are revealed, and the favorable areas for coalbed methane development are predicted. The following conclusions are obtained: (1) Under the Yanshanian SE–NW trending maximum principle stress, the Hancheng overturned anticline was formed and subsequently subjected to uplift and erosion along its axis, which led to the NW limb of the anticline forming the current uniclinal structure of the Hancheng area; (2) Four degrees of tectonic deformation in the coal seam are identified based on structural curvature analysis. The moderately deformed area shallower than 800 m would benefit coalbed methane production with higher permeability. Most of the locations of coal and gas outburst events that occur during coal mining were distributed along the highly and very highly deformed areas; (3) The gas content gradually increases along the NW-trending inclination of the coal seam. 400 m and 800 m are discriminated as the critical depth levels for controlling coalbed methane accumulation and a high yield. Secondary biogenic methane was generated in the shallow formations; and (4) The Hancheng area is divided into four ranks for determining coalbed methane development potential. From high to low, they are ranked A, B-1, B-2, and C. Most of the high-yield wells are located in the areas ranked A and B-1.

Highlights

The exploration and development of coalbed methane (CBM) have rapidly increased in China during the past several decades, and China has gone into early-stage large-scale CBM development (Qin et al, 2018)
We revealed the critical depth for CBM accumulation and identified the probable CBM high-yield areas
The geothermal temperature gradually increases from the SE to NW, consistent with the trend of burial depth of the coal seam (Figure 7)

Summary

Introduction

The exploration and development of coalbed methane (CBM) have rapidly increased in China during the past several decades, and China has gone into early-stage large-scale CBM development (Qin et al, 2018). Significant linear form, indicating a stronger influence of burial depth and a triaxial compressive state of the coal reservoir where vertical stress plays a leading role This state determines the low permeability and poor development potential of CBM. For deep coal seams in the Hancheng area (>800 m), coal reservoirs are characterized by extremely low permeability, high in situ stress, and poor fracturing feasibility, meaning that these CBM resources are difficult to develop. The vertical variations of permeability, fluid pressure, gas content and composition are in good agreement with the critical depth we discriminated in this study, the essence of which is the coupling effect of coal reservoir properties and geological environments (e.g., in situ stress, tectonic deformation and geothermal temperature). The partition method is: 1. Rank C with a lower CBM potential: Only one of the conditions listed in Table 3 (deformation, gas content, thickness, and structural unit) needs to be satisfied for determining rank C

Rank A with a higher CBM potential

Findings

Conclusions