Novel method for optimizing the dewatering rate of a coal-bed methane well

Xiuqin Lu,Ning Wang,Zhiqi Wu,Xuefei Li,Yidong Cai,Mulian Huang,Chen Zhang,Zhengshuai Liu

doi:10.1177/0144598719898537

Abstract

The reasonable dewatering rate in the single-phase water flow plays an essential role in pressure propagation and coal-bed methane production. However, current fluid velocity sensitivity experiments cannot provide an optimum dewatering rate for field coal-bed methane production. This study proposes a new method to optimize the dewatering rate for coal-bed methane wells by assuming the investigation distance reaches the well boundary when the bottom hole pressure declines to the critical desorption pressure. The effect of the stress sensitivity and fluid velocity sensitivity on pressure propagation was first simulated with COMSOL Multiphysics software. The results showed that the expansion area considering the stress sensitivity is shorter than that neglecting the stress sensitivity when the bottom hole pressure reached to the critical desorption pressure at 200 days. The expansion area with high dewatering rate will be shorter about 35 m than that with low dewatering rate at 200 days. The relationship between the maximum investigation distance and required time was established to optimize the dewatering rate by combining the pressure profile considering the influence of stress sensitivity with material balance equation. The new model indicates that the initial permeability, porosity, and cleat compressibility have an important effect on investigation distance. The simulation of these parameters’ sensitivity suggests that the bigger the ratio of initial permeability and porosity, the longer the investigation distance is, and the smaller the cleat compressibility is, the longer the expansion area is. According to this model, we need to take more than 600 days at 0.58 m/d constant dewatering rate to reach the maximum investigation distance of 0.67 mD initial permeability. This work can be conducive to choose reasonable dewatering rate in single-phase water flow for coal-bed methane well production.

Highlights

The development of coal-bed methane (CBM) has supplied massive energy resource for the world and has successfully raised much attention in recent years (Cai et al, 2011, 2014, 2018; Moore, 2012)
Based on the characteristics of pressure propagation, a method was proposed to obtain the optimum dewatering rate for hydraulically fractured CBM well by assuming the investigation distance has reached the well boundary when the reservoir pressure decreases to the critical desorption pressure (CDP) (Xu et al, 2017), but this method neglects the variation of permeability and porosity brought by dewatering
The inner boundary condition is controlled by bottom-hole pressure (BHP) which varies with the dewatering and decreases to the CDP with a constant depressurization rate of 0.0182 MPa/d

Summary

Introduction

The development of coal-bed methane (CBM) has supplied massive energy resource for the world and has successfully raised much attention in recent years (Cai et al, 2011, 2014, 2018; Moore, 2012). Keywords Pressure propagation, investigation distance, permeability variation, porosity variation, depressurization rate Based on the characteristics of pressure propagation, a method was proposed to obtain the optimum dewatering rate for hydraulically fractured CBM well by assuming the investigation distance has reached the well boundary when the reservoir pressure decreases to the critical desorption pressure (CDP) (Xu et al, 2017), but this method neglects the variation of permeability and porosity brought by dewatering.

Results

Conclusion