考虑页岩储层微观渗流的压裂产能数值模拟

考虑页岩微观渗流特征下的产能评价方法有利于提高压后动态分析的准确性和可靠性。压裂改造后页岩储层中,页岩气将在纳米孔隙中通过解吸附、扩散和滑脱流进入天然裂缝,再由天然裂缝流向人工裂缝,常规的产能评价数学模型已无法进行刻画和描述。为此,在考虑页岩气生产过程中基岩纳米孔隙中Knudsen扩散、滑脱流、吸附解吸微观流动特征,天然裂缝应力敏感以及人工裂缝非达西流效应基础上,基于双重介质模型,人工裂缝考虑为离散裂缝,建立了页岩储层基质—天然裂缝—人工裂缝的渗流数学模型,并给出了数值解法。模拟分析了页岩水平井压裂裂缝与储层参数对生产动态的影响。研究表明:吸附解吸效应、Knudsen扩散与滑脱流、天然裂缝渗透率、应力敏感系数、裂缝导流能力、裂缝半长与压裂段数对页岩气井生产具有重要影响。该研究为完善页岩气渗流理论,建立适合页岩气的动态评价模型,准确评价页岩气产能具有重要意义。

Numerical simulation of productivity after fracturing with consideration to micro-seepage in shale reservoirs

Effective productivity assessment methods with consideration to micro-seepage in shale reservoirs may effectively enhance the accuracy and reliability of post-fracturing performance analysis. In fractured shale reservoirs, shale gas may enter natural fractures through desorption, dispersion and slipping in pores of nanometer scales before travelling from natural fractures to artificial fractures. Those conventional numerical models for productivity assessment may not effectively describe such processes. With consideration to Knudsen dispersion, slippage flow, absorption and desorption micro-flow features of shale gas in nanometer pores in matrix, together with stress sensitivity of natural fractures and non-Darcy flow effects of artificial fractures, a numerical model for seepage in matrix-natural fracture-artificial fracture system in shale was established on the basis of dual-media model and with artificial fractures as discrete fractures. In addition, numerical solutions for the model were proposed. The model was used to simulate the impacts of artificial fractures in horizontal wells in shale and reservoir parameters on production performance. Research results show that absorption-desorption effects, Knudsen dispersion and slippage flow, permeability of natural fractures, stress sensitivity coefficients, conductivity of fractures, half length of fractures and the number of fracturing intervals may have impacts over the performances of shale gas wells. These relevant research results may play important roles in improving the seepage theories of shale gas, building a performance assessment model for shale gas, and predicting the shale gas productivity with high accuracy.