Reservoir Damage in Coalbed Methane Commingled Drainage Wells and Its Fatal Impact on Well Recovery

Fluid backflow behavior and reservoir damage during methane commingled drainage from multiple coal seams were investigated via physical experiments and reservoir modeling. First, the initial water saturation of coal reservoir was determined by an experimental method, and then based on low-field nuclear magnetic resonance technology, coal imbibition and centrifugation tests were conducted to obtain gas permeability under different water saturation. Finally, a reservoir model was used to calculate the shut-in pressure and interlayer interference performance of the commingled drainage well. The following conclusions were derived from a case study of GP-2 well in the western Guizhou Basin. At the initial stage of commingled drainage, the wellbore serves as a medium for coordinating the energy of each production layer, allowing the fluids from the high-pressure production layer to flow back into low-pressure layers, thus creating a new stable system. Using inflow performance relationship curves, the initial shut-in pressure of GP-2 was determined to be 6753 kPa. Simulations demonstrate that pressure anomalies are a prerequisite for the presence of reverse fluids, the water imbibition and gas lock phenomenon would result in the loss of the irrecoverable permeability to reservoirs. The ideal average gas production rate of the commingled well is 3846 m(3) d(-1) for 3000 days of drainage, when ignoring reservoir damage only 6% interlayer interference was detected in the commingled drainage well, which may be disregarded in operation. In comparison, the ultimate interference coefficient of the commingled drainage well might be as low as -50% when water imbibition is factored in, and as much as 90% of CBM resource could not be produced with both water imbibition and gas lock in the reservoir. This research bridges the gap between reservoir dynamics and interference analysis and provides a basis for the favorable layer selection and drainage system establishment in commingled drainage wells.