Dynamic Permeability Change During Coalbed Methane Production And Reasonable Drainage Speed Analysis

Coal reservoir permeability is one of the key parameters that affect coalbed methane yield and production efficiency, it changes continuously during the whole process of coal bed methane exploitation. A reasonable descending velocity of the producing fluid level is very important to reduce the damage of reservoir permeability, thus beneficial to the long-term stable production of CBM wells. In this paper, to find out the changing rules of the coal reservoir permeability and then put forward suggestions for reasonable drainage of CBM wells, CBM wells from FanZhuang-Zhengzhuang block in southern Qinshui Basin are adopted for a case study. The main results are as follows:(1)Considering the comprehensive influence of effective stress effect, matrix shrinkage effect and gas slippage effect, we derived a production data-based model for calculating the reservoir permeability change.(2)By using the actual production data of coalbed methane wells from southern Qinshui Basin, we tested the permeability model and accurately calculated the permeability change during CBM production process. According to the calculation result, we know that matrix shrinkage effect and effective stress effect are main effects which can make the permeability change greatly. However, slippage effect have only a weak influence and can be ignored.(3) According to the shape of permeability curve, CBM wells can be divided into three types, there are "decline type", "rebound type" and "rising type". What’s more, influence factors of the dynamic permeability are analyzed, results showed that cleat compressibility, specific surface area and pore diameter are the key geological factors that affects the permeability change. Pressure drop, daily gas production and final pressure are the key engineering factors that affect the permeability change.(4)In view of two different saturation state of CBM reservoir: supersaturated and undersaturated, the proper descending speed of fluid level in the initial stage is designed separately. For undersaturated reservoir, we established a fluid level drop speed calculation model. For supersaturated CBM reservoir, however, by using the method of statistical analysis, we obtained the reasonable drawdown speed ranges from 0 to 5 m/d, and the corresponding gas breakthrough time is 0 ~ 1.5 months(5)Defining the "rebound type" and "rising type" well as the potential well, based on this, predicting the potential zone of Coalbed methane reservoir and putting forward suggestions on CBM exploitation.