Dewatering rate optimization for coal-bed methane well based on the characteristics of pressure propagation

- A principle of dewatering rate in early stage for CBM well is proposed.
- The method is based on the characteristic of pressure propagation.
- A mathematic model was established to optimize the dewatering rate.
- The proposed model was applied in Han-Cheng CBM field and got reliable results.

In coal-bed methane (CBM) reservoirs, gas is mainly stored in matrix as adsorbed gas and the cleats are commonly saturated with water. To produce the adsorbed gas, the cleat pressure should be reduced to critical desorption pressure by dewatering first. But how to dewater more efficiently is a difficult issue. Apparently low dewatering rate is uneconomical due to excessive development times required. But from the field experiences, high dewatering rate will bring more problems such as coal break, stress sensitivity and two-phase flow in earliest stage, which will reduce the productivity, or sometimes make the well abandoned. Literatures have studied much about the effect of coal break and stress sensitivity. However, there is no discussion about the relationship between two-phase flow and dewatering rate. High dewatering rate is companied with high pressure drawdown which will cause gas desorption near wellbore area and simultaneously cause two-phase flow. Two-phase flow has significant negative effect on pressure propagation, which controls investigation distance or drainage area in the early stage, and therefore affects the overall productivity. Since the purpose of dewatering is to maximize the drainage area and maximize the depressurization for whole reservoir, the dewatering rate should be controlled to ensure well-bottom hole flowing pressure (BHFP) is larger than critical desorption pressure in order to avoid the occurrence of two-phase flow in the dewatering stage. Based on this principle, a procedure for optimal dewatering rate is presented and a mathematic model is established. It is shown that initial pressure, critical desorption pressure, permeability, porosity, and fracture half-length play significant roles in dewatering rate determination. This method is easy, and has applied in CBM development region of Han-Cheng, China.