Productivity Prediction and Production Parameter Optimization of Indirect Fracturing Coalbed Methane Wells

ABSTRACT The fractures formed in hydraulic fracturing in tectonic coal are relatively short. The indirect fracturing technology simultaneously fractures coal seam and its roof layer, which can significantly increase the fracture extension length. However, compared with conventional coalbed methane fracturing wells, the gas-water flow process in the reservoir becomes more complex. In this paper, a numerical simulation method for productivity prediction of indirect fracturing wells for coalbed methane is developed based on the isothermal adsorption equation, quasi-steady state diffusion equation, and seepage equation. The reliability of the seepage model is verified by the seepage simulation experiment of indirect fracturing wells. The pressure distribution, desorption area distribution, and gas production rate change during coal mining is analyzed. A method of optimizing the bottom hole flowing pressure by stages is developed, which optimizes the bottom hole flowing pressure in water drainage stage, pressure build-up stage, and pressure control stage, respectively. The average relative error between experimental and numerical results is 4.9%. In the production process, the desorption area can be divided into fast desorption area, slow desorption area, and non-desorption area according to the desorption rate. The whole rapid desorption region has a relatively high desorption rate. However, the desorption rate is relatively higher in the region near the roof layer in the slow desorption area. The roof layer provides a high permeability seepage channel for the fluid in the coal seam, and the fluid in the coal seam flows vertically driven by the pore pressure difference between the layers. Fluid flows through the pores of the roof layer and into the fractures. In the stable production stage, the gas production rate of roof layer can reach 40%. This study benefits productivity prediction and analysis of indirect fracturing coalbed methane wells. It can provide theoretical guidance for optimizing the strategy of coalbed methane extraction.