Novel prediction methods for under-saturated coalbed methane wells: Effect of drainage schedules

A detailed study for production prediction methods under various drainage schedules for under-saturated coalbed methane wells is performed. In terms of the relationship between the value of average reservoir pressure and the critical desorption pressure, whole production life of under-saturated coalbed methane wells is divided into two periods, and the material balance equation in each period is derived respectively. Combining the two-period material balance equation and productivity equation under pseudo-steady state, a novel production prediction method is developed. Excellent agreements between predicted water/gas production rates from the proposed method and those from numerical simulator clarify the reliability successfully. Results demonstrate that (a) matrix shrinkage effect and effective permeable capability can significantly contribute to the production rise; (b) For the drainage schedule (FWFB), with the increase of the fixed water production rate, the drainage period will shorten; (c) For the drainage schedule (RDFB), sharp decrease of formation pressure will take place.