Numerical Study on Interlayer Interference in Different Types of Coal-Bearing Strata

Abstract

China is rich in coal-bearing gas resources, but the strata of coalbed methane, shale gas and tight sandstone gas are often stacked together with several layers with one type of gas, two types of gas or even three types of gas. The reservoir conditions are complex with great different physical properties and low permeability, which make exploitation difficult. The multi-layer co-production is often adopted to improve the productivity of a single well. However, different types of gas reservoirs demonstrate different gas production characteristics and different interlayer interference, which brings difficulty in the optimization of gas production in coal-bearing strata. In order to strengthen the analysis of production characteristics and interlayer interference in coal-bearing strata, three two-layer co-production models for coalbed methane, shale gas and tight sandstone gas respectively were established using reservoir numerical simulation method based on the reservoir physical characteristics of coalbed methane, shale gas and tight sandstone gas. A virtual wellbore setting was proposed, and the interference factor was defined to be the ratio of co-production and separate production, which can simulate and characterize the phenomena of interlayer interference of production more reasonably. Then the gas production and interlayer interference in different types of tight gas reservoirs were predicted, analyzed and compared comprehensively. The results show that due to the different occurrence and production mechanisms of coalbed methane, shale gas and tight sandstone gas, in the same coal-bearing strata, the tight sandstone gas has the highest production rate at the beginning of production, but it is easily affected by the interlayer interference induced by the pressure difference between two layers, while the release of the productivity of coalbed methane and shale gas is usually lagged behind. The desorption pressure and water saturation are the sensitive factors for interlayer interference. In addition, the influence of water loading at the bottom of the wellbore on gas production cannot be ignored. The results have a guiding significance to the multi-layer co-production in coal-bearing strata.KeywordsCoal-bearing strataMulti-layer co-productionNumerical simulationProductivity predictionInter-layer interferenceSensitivity analysis