Impacts of vertical variation of coal seam structure on hydraulic fracturing and resultant gas and water production: A case study on the Shizhuangnan Block, Southern Qinshui Basin, China

Abstract

Hydraulic fracturing plays a vital role in the development of coalbed methane (CBM). Coal seam structures can, however, affect the fracturing operations, further affecting gas and water production from the coal seam. Within the research area—the Shizhuangnan Block, Southern Qinshui Basin, China—three types of coal body textures are relevant, including original, cataclastic, and granulated structure. This research describes the impacts of different coal seam types on fracturing operations and gas and water production. The results show that different types of coal seams have different impacts on hydraulic fracturing and the resultant production of gas and water. The coal seams are classified according to the vertical variation characteristics of the coal seam structures. The first type of coal seam (Type I) only develops the original and cataclastic coal structure. The effects of fracturing reconstruction for Type I are good. A high degree of coal breakage and fracture formation is observed. Gas production from CBM wells in Type I coalbeds is usually high because the water in the coal reservoir can be discharged smoothly at an early stage. The second type of coal seam (Type II) develops an additional layer of granulated coal compared with Type I. During the fracturing process, breakage of the coal seam is obvious, and the granulated coal can easily produce fine-grained coal that can block the pores and fractures, which causes an increase and fluctuation of oil pressure and can affect the effectiveness of fracturing. The gas production curve from CBM wells in Type II coal seams is mostly bimodal and water production depends on whether the pulverized coal blocks the pores and fractures. The third type of coal seam (Type III) develops two additional layers of granulated coal compared with Type I, with a greater proportion of granulated coal present. A low degree of coal breakage and fracture formation is observed, and fractures are easily blocked by pulverized coal. The effects of fracturing reconstruction for Type III are bad. Forming an effective seepage channel in this type of coal seam and extending the fracture to the far end is difficult. The gas production from CBM wells in Type III coal seams is usually low, and water production is generally low during the whole drainage period.