Numerical Simulation Of Gas Water Two-phase Flow In Shale Gas Reservoirs

With the rapid development of national economy,the demand of energy is becoming more and more urgent.Meanwhile,the production of national major oil fields can no longer meet the needs of social development after secondary and tertiary oil recovery.Considering that China is rich in shale gas resources,the development and utilization of shale gas resources has become one of the best choices to ease the domestic energy tension and reduce the dependence on foreign energy imports.However,the main problems in the development of shale reservoirs are their low natural flow capacity.To achieve the economic development of shale reservoirs,the reservoirs usually need to be modified by horizontal well drilling and multi-hydraulic fracturing techniques.These approaches require the injection of large amounts of water into the reservoirs.Meanwhile,shale gas reservoirs also contain large amounts of native water.At present,the knowledge of fluid flow in shale reservoirs is often derived from single-gas phase flow,and there is a lack of in-depth understanding of its gas-water two phase flow mechanisms,which needs to be further explored.The purpose of this paper is to investigate the adsorption performance of shale gas in shale reservoirs through adsorption experiments as well as to study the flow properties of the modified reservoirs through numerical simulations.The main findings and innovations of this thesis are described as follows:1.A new extended pressure point method for dealing with Gibbs excess adsorption is proposed.The conventional treatment methods for Gibbs excess adsorption tend to assume a constant density of the adsorbed phase at different pressure conditions,this is consistently contrary to the findings of our molecular dynamics simulation.The new method is able to fit both the adsorption densities under different pressure conditions and the Langmuir parameters.The results of the experimental data show that our new method has a higher fitting accuracy compared with the conventional adsorption phase density treatment methods,and the fitted adsorption phase density variation pattern is also consistent with the molecular dynamics simulation conclusions.Meanwhile,the trend of adsorption volume calculated by this method is different from the conventional understanding.A reasonable explanation is given from the perspective of the competing relationship between the force of adsorbent surface on the gas molecules and the force field in the gas molecules.2.A cyclic iterative method applicable to the numerical simulation of multiphase flow in reservoirs is proposed.The method overcomes the saturation and capillary force discontinuities caused by the IMPES(Implicit Pressure and Explicit Saturation)method by solving the pressure and saturation terms cyclically,and satisfies the basic requirement of mass conservation in the numerical simulation results.The results show that the new method is fully mass conservative,better computational stable and more efficient.At the same time,the new method is suitable not only to homogeneous reservoirs but also to heterogeneous reservoirs,which greatly improves the applicability of the method.3.A mathematical model for gas-water two-phase flow in shale reservoirs is established and the corresponding numerical simulation software is written based on Visual Studio 2013.In this model,strong nonlinear factors such as Klinkenberg effect and adsorption effect of shale gas,rock deformation effect,are also considered.The results reveal that,the gas-water two-phase model can reproduce the results of the analytical solution in the literature and is closer to the actual flow conditions of the shale gas reservoirs,thus proving the correctness of the model in this thesis.4.The effects of reservoir parameters and engineering parameters on the production performance of shale gas reservoirs are investigated.The results show that the initial water saturation,rock deformation effect,adsorption effect,and Klinkenberg effect all have certain effects on the production of shale reservoirs.