Expansion Mechanism Of Desorption Area And Production Prediction Method For Coalbed Methane Reservoirs

Accurately characterizing the gas-water distribution characteristics in the pores of the matrix system of coalbed gas reservoirs is a necessary prerequisite for correctly assessing the gas transport ability from the matrix system to the microfracture system.During the long geological evolution of coalbed methane,the thermal oxidation degree of organic components of coal rocks,sedimentary environment,and geological burial history will affect the development characteristics of coal rock matrix pores and gas-water distribution.Therefore,it is meaningful to study the pore development characteristics of coal strata of different coal ranks.Conventional coalbed methane reservoir development theory still uses the Fick diffusion theory as the driving force to describe the mass transfer process of gas from the matrix system to the micro-fracture system.However,the main component of the coalbed methane is methane,and there is no concentration difference in the interior.At the same time,Fick diffusion fails to capture the gas-water distribution in the actual coal rock,so it is necessary to re-examine the mechanism suitable for describing the gas transmission from the matrix system to the micro-fracture system.Interwell interference is an effective means to improve the efficiency of coalbed methane development,and an accurate grasp of the pressure propagation law of coalbed methane wells is a necessary prerequisite.It helps to determine the effective desorption area of coalbeds and the calculation of dynamic reserves and provides technical guidance for real-time adjustment of development plans.Correctly understanding the pore development characteristics of the micro-matrix of the coal rock,the pressure transmission process in the coal cleats,and the gas transmission mechanism will help grasp the characteristics of the CBM well exploitation,formulate a reasonable production system,and improve the CBM development efficiency.First,the micro-pore development of coal rocks during the long geological evolution process is studied,and the occurrence state of formation water in matrix pores is mainly investigated to complete the characterization of the micro-gas-water distribution of the original coal reservoir.Different from the premise of previous studies of solid-gas systems,based on the micro-gas-water distribution of coal reservoirs,this paper considers that the desorption and transport process of gas is affected by the combined effects of solid-gas and solid-liquid systems.Transport models for the solid-air interface and the solid-liquid interface were established for gas-pores and plant tissue pores respectively.The limitations of the conventional Fick diffusion theory to describe the mass transfer from the matrix system to the cleat system were evaluated.The transmission process was considered as a pressure-driven seepage process.The transmission capacity of the Fick diffusion model driven by the concentration difference and the channeling model driven by the pressure difference is compared.It is found that the desorption gas always enters the micro-cracks in a short time regardless of the channeling or the Fick diffusion,revealing that the main controlling factor of the gas seepage ability from the matrix to the micro-fracture system is the ease of gas desorption,not the apparent permeability of the matrix system.Then,the possible flow patterns in the micro-fractures of coal and rock during the production process were evaluated and the respective pressure transmission speeds were quantified.Single-phase water has the fastest pressure transmission speed,followed by single-phase gas,and the slowest pressure transmission speed of gas-water two-phase flow.Based on the relationship between coalbed gas well pressure transmission and well production performance,a prediction model for desorption-area suitable for unfractured and fractured coalbed gas straight wells was established.The stronger the desorption capacity of the coal seam,the more obvious the effect of "supply pressurization",which will delay the pressure transmission speed and limit the expansion of the desorption zone.Combining the relationship between the expansion of the desorption zone and the gas production capacity,a production prediction method that couples the expansion of the desorption area with the gas-water two-phase capacity equation is established.An optimization method for the drainage and extraction system with the objective function of maximizing the desorption area is proposed,and suggestions for the optimization of the drainage and extraction system that can guide on-site actual production and improve development efficiency are formed.Finally,considering that it is difficult to accurately calibrate the physical properties of coal seams through laboratory experiments or field tests,this paper proposes a series of coal seam gas production data analysis methods that combine gas-reservoir engineering methods and pressure propagation processes.Based on the characteristics of pressure propagation process and fluid-phase change in coal seams,production data analysis methods suitable for coal mine gas well drainage phase and gas-water two-phase flow phase were established.A gas-water two-phase permeability curve prediction method suitable for coalbed gas wells was established,which can provide theoretical methods for determining coalbed physical parameters and accurate prediction of coalbed gas well production.