Experimental Study On The Nonlinear Flow Model In Tight Reservoirs And The Corresponding Applications In The Numerical Reservoir Simulation Software

The geological reserves of oil and gas in tight reservoirs have an significant share of the total oil and gas resources.As the oilfield development technique has been improved greatly,the exploitation of oil from low permeability and even ultra-low permeability reservoirs has industrial values.Special reservoir numerical simulation techniques are necessary for realizing the efficient development of tight reservoirs.At present,the nonlinear flow effects in tight reservoirs have not been fully considered in commercial reservoir numerical simulation softwares.For tight reservoirs,the pore size is usually of the order of microscale or nanoscale,and the flow is often non-Dacry's,which can not be described by the traditional linear Darcy's law.In order to perform the numerical simulation for tight reservoirs accurately,it is necessary to establish an appropriate nonlinear governing equation,and this is one of the core problems in tight reservoir research fields.It is also crucial to apply the nonlinear governing equation in the tight reservoir numerical simulation softwares and to understand and master its influence on water or CO2 flooding processes.It is of important scientific research significances and huge engineering application values for improving the recovery of tight reservoir and even for realizing the "double carbon" goals in China.This thesis focuses on establishing the nonlinear law for the flow in tight reservoirs and revealing its impact on water or CO2 flooding processesof seepage in tight reservoir and the influence of nonlinear seepage on water drive and CO2 drive in tight reservoir.Through theoretical analysis,experimental measurements and numerical simulations,the nonlinear flow phenomenon is investigated.Based on the numerical tests performed on the self-developed software,the influence characteristic of the nonlinear flow in tight reservoirs is discussed.The specific research contents and relevant conclusions are as follows.1.A set of nonlinear flow models,named as "the quasi starting pressure gradient model" is proposed for tight reservoirs.This set of proposed models has three characteristics:strict monotonicity,slow start and asymptotic linearity.In this set of models,it is recommended to use the "the quasi starting pressure gradient model with a single parameter in exponential function form".since only one parameter which is just the quasi starting pressure gradient is contained in it.The recommended nonlinear model is concise and easy to apply in numerical simulations.2.The experiments of fluid flows in low-permeability media have been performed for silicon oil in nanochannels and for water or oil in the core of the tight reservoirs.By fitting the measured velocity-pressure difference curve with the proposed nonlinear flow model,the value of the pseudo starting pressure gradient is obtained.From the experimental results,it is shown that the pseudo starting pressure gradient is related to the core permeability.Their quantitative relation can be approximated by a power low function.According to the results of core experiments,an empirical formula is summarized for this relation.For water phase,the power exponent between the pseudo starting pressure gradient and the the permeability is about-1.35.For oil phase,the power exponent about-0.46.3.The framework design and main module functions of the self-developed numerical simulation software for tight reservoirs are introduced.The self-developed software realizes the modular design of the core codes,and achieves some progress on the description of the nonlinear flow and the fracture treatment compared to the existing commercial numerical reservoir softwares.On the aspects of the computing power and efficiency,the computable grid scale of the self-developed software on a personal computer can exceed 5 million,which is larger than the serial ECLIPSE software.With the help of the advanced embedded discrete fracture model,the self-developed software shows its advantages on the numerical simulation for hydraulic fractures.4.The numerical simulations of water or CO2 flooding in tight reservoir are performed by using the self-developed software,and the influences of the nonlinear flow model on the numerical simulation results are investigated.The numerical simulation results show that the nonlinear flow model has a great influence on the water and CO2 flooding process in tight reservoirs due to its low or extra-low permeability.Under the condition of constant pressure wells,the water or CO2 flooding processes predicted by the nonlinear flow model are significantly slower than the results predicted by the Darcy's law.Correspondingly,the injection and oil production rates are significantly less than those obtained from the simulation results based on the Darcy's law.The greater the quasi starting pressure gradient,the more effects of the nonlinear flow model.In addition,because the formation resistance corresponding to the nonlinear flow is larger than the Darcy's flow,the effects of resistance reduction and production increase of hydraulic fractures are more obvious for the nonlinear flow model.