Formation damage mitigation mechanism for coalbed methane wells via refracturing with fuzzy-ball fluid as temporary blocking agents

During drainage and production from coalbed methane (CBM) wells, the reservoir is prone to formation damage in the form of blockage of the seepage channels by coal fines or invasion of external water, which inevitably causes a significant decline in the productivity of the wells. To restore their productivity, temporary blocking materials (TBMs) are typically used to seal the original fractures for refracturing treatments. In this study, a special fuzzy-ball temporary blocking fluid (FTBF) was designed to effectively mitigate the formation damage through the refracturing operation. The performance of the FTBF was analysed via experiments, application to two case studies, and its formation damage mitigation mechanisms. The following results were obtained: (1) From the experimental analysis, it was observed that the FTBF increased the water resistance in the fracture and it was less than the gas resistance. (2) In Case one, there was no gas production before the refracturing; however after the refracturing, the daily gas production rose to 494 m3. In Case two, the daily gas production of the well before the refracturing was intermittent and less than 80 m3; however, after the refracturing, the gas production remained stable at 220 m3. (3) The analysis of damage mitigation mechanisms revealed that firstly, sodium dodecyl sulfate improved the viscoelasticity of FTBF and its chemical affinity to water, which enhanced the mitigation of the formation damage by water invasion; secondly, the viscoelastic characteristics of the vesicles enabled it to modify the mechanical properties of the rock and consequently improving the refracturing process; thirdly, sodium dodecyl benzene sulfonate provided excellent dispersion stability by adsorption of coal fines in the proppant pack to enhance the fracture conductivity. This study provided a reliable basis for understanding the mechanisms and application of FTBFs in mitigating the formation damage through refracturing CBM wells.