Acid fracturing technology of deep CBM wells and its field test in Daning-Jixian Block

There are abundant deep coalbed methane resources in China, but the coupling problem of coal reservoir reconstruction technology and deep geological quality conditions needs to be solved urgently. With Daning-Jixian Block as the geological background, the feasibility of volumetric fracturing is evaluated from the perspective of rock mechanical parameters of the No.8 coal seam in a deep layer of the block, and verified by the indoor triaxial acid fracturing model experiment, in order to explore the coal reservoir reconstruction technology under deep geological conditions. Based on the above basic research, the volume acid fracturing technology characterized by high displacement, low acid dosage, and the moderate sand ratio is proposed for the No.8 coal seam in this block with the composite technology featured by alternating acid injection, subsection sand addition, and variable rate injection. Field tests of complex hydrochloric acid and sulfamic acid volume fracturing are carried out based on this process principle. The results show that the accumulative daily production of 11 gas-producing wells reaches 20 469 m³, among which the maximum gas production of 10 vertical production wells reaches 5 791 m³/d. One horizontal production well has a maximum daily gas production of 11 000 m³. At the same time, there is a good correlation between the volume acid fracturing engineering factors (displacement, fluid intensity) and the monitored fracture area. It is suggested that the fracturing fluid flow rate should be further increased, and the optimal fracturing fluid flow rate 11−15 m³/min. The overall acid content should be reduced and the acid concentration should be further optimized. The low-density proppant is selected and the sand adding process is optimized to increase the sand adding scale. Clean fracturing fluids should preferably be added at a strength of 150−250 m³/m. At the same time, the quality of supporting equipment should be improved, such as upgrading casing steel grades and optimizing fracturing equipment. This study provides a technical reference for the exploration and development of deep coalbed methane in this block or under similar geological conditions from the perspective of volume acidizing fracturing engineering.