Productivity prediction model and its application of CBM test well under the condition of nonlinear flowing bottom hole pressures

Abstract：In order to explore the permeability mechanism and production law of coalbed methane in low permeability coal seam (soft coal, soft top and soft bottom) in Jiaozuo“three soft”mining area under hydraulic fracturing condition, the reservior tracture permeable model was established based on the reservoir fracture propagation model and the relationship between fracture porosity and permeability after fracturing.At the same time, the influence of nonlinear dynamic change of bottom hole flow pressure on coalbed methane production was mainly considered, and the productivity prediction model was established.The application and analysis of single test well were carried out.Then, theoretical gas production during test period was calculated according to the geometric parameters of fracture crack and flowing bottom hole pressures.The data were collected during the test period, the theoretical output during the test period and the simulated output during the test period were compared and analyzed through the historical fitting method, and the correctness of the productivity prediction model was verified.Finally, combining with the relevant parameters, the productivity prediction model was used to predict GW-002 test well for 2 450 d, and to calculate the recovery ratio during the economic evaluation period.The results showed that the average daily gas production, cumulative gas production and recovery ratio could reach 587 m³, 1.05×10⁶m³and 32.86% respectively, which met the technical requirements of CBM mining.The results could guide fracturing extraction practices and productivity prediction in the “three soft” mining area.

Keyword：low permeability coal seam;hydraulic fracturing;permeable model;flowing bottom hole pressure;recovery ratio;productivity prediction;