A case study on the effective stimulation techniques practiced in the superposed gas reservoirs of coal-bearing series with multiple thin coal seams in Guizhou, China

Abstract

The superposed reservoirs of coal-bearing series represent a new type of gas reservoir associated with coal seams, and those with multiple thin coal seams are an important subtype popular in Guizhou, an emerging area for coal-bearing series gas exploitation in China. So it is significance to accommodate hydraulic fracturing, a common stimulation technique for enhanced gas recovery from low permeability reservoirs, to the reservoirs there. Based on a number of engineering and test data from Songhe pilot project for coal seam gas recovery in Guizhou, this paper presents some effective stimulation techniques adapting to the superposed reservoirs of coal-bearing series with multiple thin coal seams, including drilling and well completion selection, gas-bearing intervals targeting, small-layer perforation, staged fracturing, ball sealer diversion and graded proppant injection. The study also focuses on the mechanism on adaptable hydraulic fracturing of the superposed reservoirs associated with multiple thin coal seams, by combining theoretical analyses and actual fracturing curve diagnoses. The results indicate that the fractures opening exhibit a certain temporal order depend on mechanical and other physical properties differences among individual perforated intervals, and the fracture opening sequence can be controlled by adjusting pumping scheme and dropping ball sealers; the fracture height extended into the adjacent rocks is a critical geometric parameter affecting scale of fracture network, and negatively correlated with tensile strength difference between coal seams and adjacent rocks; under a condition of the current maximum pump rate, the fracturing treatment in both multiple thin coal seams and interlayered sandstones have been implemented, and a fracture network has been generated with a uniform fracture geometry and strong flow conductivity by the combined application of the effective stimulation techniques.