孔喉结构对致密气微尺度渗流特征的影响

目前,关于孔喉结构对致密气微尺度渗流特征影响的研究较少,并且传统的数值模拟方法在研究微尺度流动时面临着诸多不适应。为此,从致密气藏实际的温度、压力及储层孔喉特征尺寸出发,研究致密气的流态及采用格子Boltzmann方法模拟致密气流动的合理性,在考虑微尺度效应、滑脱效应等因素影响的基础上,基于LBGK-D2Q9模型建立了致密气流动模型,并将该模型计算的结果同解析解及文献中数值解的计算结果进行对比,进而探讨了孔喉结构对致密气微尺度渗流特征的影响规律。研究结果表明:①当压力介于3～70 MPa、温度介于293.15～373.15 K时,克努森数(Kn)小于0.1,气体流态为滑脱流和弱连续流,采用LBGK-D2Q9模型模拟致密气流动是合理的;②流动通道的特征尺寸对Kn的影响远大于压力变化对其产生的影响,当孔喉比一定时,Kn沿喉道呈缓慢上升的趋势,且孔喉比越大,Kn上升的趋势越明显;③喉道的存在使得孔喉中压力的非线性分布特征显著,压力降主要处于喉道内,并且孔喉比越大喉道内的压降幅度越大;④压力的非线性分布使得气体的流动速度显著降低,从而降低了流动通道内气体的质量流量。结论认为,所建模型的模拟结果与解析解以及文献中DSMC、IP方法等数值解的计算结果吻合程度较高,证实了其可靠性;该研究成果揭示了致密气藏开发工程实践中"通缝扩喉"的重要性。

Effect of pore throat structure on micro-scale seepage characteristics of tight gas reservoirs

At present, the effects of pore throat structure on micro-scale seepage characteristics of tight gas reservoirs are less researched, and traditional numerical simulation methods are faced with a great number of challenges in the study of micro-scale flow. In this paper, the flow pattern of tight gas was studied based on the actual temperature and pressure of tight gas reservoir and the characteristic size of reservoir pore throat, and the rationality of tight gas flow was simulated by means of lattice Boltzmann method. Then, considering the influences of micro-scale effect, slippage effect and other factors, a tight gas flow model was established on the basis of LBGK-D2Q9 model, and its calculation results were compared with the analytical solutions and the numerical solutions listed in the literature. Finally, the influential laws of pore throat structure on the micro-scale seepage characteristics of tight gas were discussed. And the following research results were obtained. First, when the pressure is in the range of 3–70 MPa and the temperature is in the range of 293.15–373.15 K, the Knudsen number (Kn) is less than 0.1 and the gas flow is in the pattern of slippage flow and weak continuous flow. And in this case, it is reasonable to adopt the LBGK-D2Q9 model to simulate tight gas flow. Second, the effect of the characteristic size of the flow channel on the Kn is much greater than that of the pressure change. When the pore–throat ratio is constant, the Kn increases slowly along the throat. And its increasing trend gets more obvious with the increase of pore–throat ratio. Third, the presence of the throat makes the non-linear distribution characteristics of the pressure in the pore throat significant, and the pressure drop mainly lies in the throat. And the higher the pore–throat ratio is, the larger the pressure drop range in the throat is. Fourth, the non-linear distribution of pressure decreases the gas flow speed significantly, thus reducing the mass flow rate in the flow channel. In conclusion, the simulation result of the model established in this paper is highly coincident with the analytical solutions and the numerical solutions calculated by DSMC and IP methods in the literature, which verifies that this proposed model is reliable. The research results reveal the importance of "connecting fracture and expanding throat" in the practical development engineering of tight gas reservoirs.