A Method for Dynamic Reserve Calculation and Desorption Field Characterization of Coalbed Methane Wells

Abstract Dynamic reserves are an important dynamic parameter for evaluating well pattern adaptability and predicting development index. Due to the characteristics of low permeability, easy powder production and adsorption/desorption mechanism of coalbed methane wells, there are few static pressure data and long time for gas wells to reach pseudo-steady state. As a result, the commonly used dynamic reserve methods such as pressure drop method, elastic two-phase method and modern production decline analysis are not suitable for coalbed methane wells, especially for gas wells in the early stage of production. Based on the adsorption/desorption mechanism of coalbed methane, this paper proposes a new method of comprehensive analytical solution and numerical simulation, which can be applied to the dynamic reserve calculation of mid-early coalbed methane wells, and can generate real-time formation pressure field data for the characterization of underground desorption field. The simulation methods mainly include three aspects: (1) The fracturing fracture length and reservoir permeability are evaluated by using the early single-phase drainage and depressurization stage. Its advantage is that compared with the gas-water two-phase flow stage, it can effectively avoid the error caused by the uncertainty of gas-water two-phase seepage. (2) By carrying out the water production decline analysis, the final water production of gas wells is predicted, and the volumetric method is used to reverse calculate the swept area of gas well drainage and depressurization. (3) Using the above formation parameters, a single well numerical model is established to carry out the historical simulation and prediction of production and pressure, and obtain the final desorption radius, dynamic reserves, recoverable reserves and other parameters of the gas well. Based on the pre-production data of block AA and the data of the whole production cycle, the dynamic reserves predicted by the pre-production data are consistent with the dynamic reserves predicted by the whole production cycle, and the predicted recoverable reserves are basically consistent with the predicted results of EUR, indicating that the prediction results of this method are reliable. This method overcomes the shortcomings of traditional methods in the interpretation of coalbed methane wells, and can be applied to the assessment of gas wells in the middle and early stage.