A Comprehensive Semi-Analytical Model for Predicting Gas–Water Two-Phase Productivity of Fractured Horizontal Wells in Deep Coalbed Methane Reservoirs

Abstract

The development of deep coalbed methane is becoming increasingly important in the energy sector. Faced with deep coal seam conditions, there is a lack of a comprehensive productivity evaluation model that considers the triple media of matrix, cleat fractures, and hydraulic fractures, as well as matrix shrinkage and stress sensitivity. This paper analyzes the geological characteristics and flow mechanisms of coalbed methane (CBM), including two-dimensional flow in cleat fractures, considering stress sensitivity and matrix shrinkage, and one-dimensional flow in hydraulic fractures, also considering stress sensitivity. A comprehensive CBM productivity evaluation model is established that incorporates the complexities of dynamic fracture permeability and matrix shrinkage. The model is based on the interactions among hydraulic fractures, cleat fractures, and matrix flow. The triple-medium-flow model is solved by coupling analytical solutions for matrix flow and cleat fracture flow with numerical solutions for hydraulic fracture flow. Through the analysis of typical wells, the influence and controlling factors of geological parameters and development factors on deep CBM productivity are elucidated. The results demonstrate that the proposed CBM productivity evaluation model effectively explains the seepage characteristics of triple media under complex conditions. The daily gas production at the current time can be determined on the basis of the average formation pressure and average gas saturation at a certain moment. For example, when the average pressure of deep coal seams is 26 MPa, the average gas saturation is 0.6, and the stress sensitivity index of cleat fractures increases from 0.1 to 0.5, the gas production rate decreases by 61%. When the stress sensitivity coefficient of hydraulic fractures increases from 0.02 to 0.08, the peak gas production rate of the inflow performance relationship (IPR) curve decreases by 49%. The model can be used to predict the total capacity of a certain production stage and the capacity of each fracturing cluster and provide guidance for the next step of production. The model proposed in this paper considers the complex fracture network morphology under the triple-medium condition of hydraulic fracture-cleat fractures-matrix, and the focus is on distinguishing the different effects of deep coal seam cleat fractures and hydraulic fractures on production, which enhances the understanding of the stress-sensitive mechanism under deep conditions and provides a basis for the study of coalbed methane productivity changes, controlling factors, and efficient coalbed methane exploitation at the production site.