Influence of geological factors on hydraulic fracturing effect of coalbed methane wells in Shizhuangnan Block,Qinshui Basin

Geological conditions are predominant factors for developing the coalbed methane (CBM),and directly affect the hydraulic fracturing process,which results in the varying performance of coalbed methane wells. In this study, a total of 34 CBM wells with similar construction parameters were selected to analyze the effect of geological factors on the hydraulic-fracturing performance. Geological factors including crustal stress,coal texture and the lithological signa- ture of roof and floor bed were analyzed and the fracturing effect were then evaluated. Results show that the magnitude of three-direction crustal stress controls the direction and the length of extended fractures. The type of coal texture de- termines whether effective fractures can be formed. The roof and floor of coal seam are mostly sandstones and mud- stones,and their relative thicknesses affect whether the fractures can penetrate the aquifer. Better fracturing perform- ance was found in the areas with smaller stress ratio,larger horizontal principal stress difference coefficient,higher pro- portion of the primary coal and larger thickness and proportion of the aquifer roof and floor. The production of CBM wells were recognized to be higher at such areas. The central part of the study area was characterized to develop verti- cal fractures,and to have a high proportion of the primary coal and high mudstone proportion in the top and floor, which makes it to be the primary area for hydraulic fracturing. The secondary targets for hydraulic fracturing are loca- ted in the south and southwest of the study area,where good crustal stress and roof-floor conditions could be found. In the north,northeast and southeast of the study area,however,the hydraulic fracturing should be avoided due to the large destruction of coal structure and poor sealing condition of roof and floor. It is suggested that the hydraulic fractu- ring scheme should be designed based on the geological conditions of coalbed methane wells.