Study On Pressure Transmission And Development Dynamic Law Of Tight Oil Reservoir During Huff And Puff Process

In recent years,the output of conventional oil and gas resources is difficult to meet the increasing energy demand,more and more attention is paid to tight oil and gas production.However,the latter usually shows problems such as too fast production decline and insufficient natural formation energy to maintain production capacity in the development process due to its complex reservoir pore structures.Therefore,it is necessary to supplement the formation energy in the development of tight oil and gas resources.Based on the physical simulation experiment,in this work,the pressure transmission law in the process of gas injection and energy supplement in tight reservoirs and the replacement mechanism of different types of energy supplement media and crude oil were studied,achieving the accurate characterization of the development dynamics of tight reservoir.To clarify the influence of pore structure on reservoir seepage ability,a method for dividing different types of pore throats based on permeability contribution rate and movable fluid saturation is proposed by combining the results of mercury injection test and nuclear magnetic resonance test.This method divides the microscopic pore throats of tight reservoirs into three categories:the invalid pore throat（<0.08μm）;the low contribution pore throat（0.08-0.6μm）;and the mainstream pore-throat（>0.6μm）,which provides a theoretical basis for the occurrence of residual oil in different levels of pore throats in subsequent studies.From the experimental results of long core pressure transmission under different oil saturations,it can be seen that the pressure transmission coefficient in the process of gas injection and energy supplement in tight reservoirs has three different characteristics.First of all,there is a slight decrease of the pressure conductivity coefficient with an increase in the oil saturation when the oil saturation is higher than 79%,which is named as the gradient stage of the low pressure conductivity(2.1×10^-6m²/s→0.9×10^-6m²/s).Then,with the further decrease of the oil saturation,the pressure transmission coefficient will jump from the order of magnitude of 10^-6m²/s to the order of magnitude of 10^-4m²/s when the oil saturation is between 79%and 68%,which is called the sudden change stage of pressure transmission coefficient.Finally,when the oil saturation is lower than 68%,the pressure conductivity coefficient slowly increases with the decrease of oil saturation,which is considered as a transitional stage of the high pressure conductivity coefficient(1.3×10^-4m²/s→4.4×10^-4m²/s).Furthermore,it is found that reservoir physical properties,reservoir pressure and working pressure difference have a significant positive effect on pressure transmission coefficient,while fluid viscosity has a significant negative effect on pressure transmission coefficient.Based on the above experimental results,not only a variable coefficient two-phase pressure transmission model is innovatively established,but also a prediction method of pressure distribution and pressure sweep range in the process of energy replenishment in tight reservoirs is proposed,which provides theoretical support for optimizing the fracture spacing in horizontal wells.Based on the imbibition and huff and puff experiments of the one-dimensional columnar core and the two-dimensional flat core,the main replacement mechanism of different types of energy supplement media and crude oil is clarified.As the main representative of liquid energy supplement medium,the mechanism of formation water replacement with crude oil is imbibition exchange.As a gaseous energy supplement medium,the replacement mechanism of oxygen-reducing air and crude oil is mainly differential pressure flow,followed by molecular diffusion.Compared with the liquid medium,the gaseous medium has a larger macroscopic wave coverage range and a lower limit for micro pore throat utilization,making it more suitable for energy replenishment in tight reservoirs.Combined with the large-scale two-dimensional plate model and the supporting large-scale high-temperature and high-pressure physical simulation system,the whole process simulation of the development of oxygen-reducing air huff and puff in tight reservoirs was carried out in the study of the dynamic law of the development of oxygen-reducing air huff and puff.By analyzing the variation of oil production rate,bottom hole pressure and gas-oil ratio in the development process,it is found that the whole oil production process can be divided into four stages:single-phase gas flowback;free gas expansion liquid production;dissolved gas flooding;and oil production failure,among which the free gas expansion liquid production is the main oil production stage in the development process of oxygen-reducing air huff and puff.Through the analysis of the influencing factors of the development effect,it can be concluded that the first four rounds of throughput have better development effect,while the larger the gas injection volume,the higher the production pressure difference,and the longer the soaking time,the better the development effect.When the formation deficit coefficient is 70%-80%,it is the best time to replenish the oxygen-reducing air.Based on the theory of material balance and molecular diffusion,a production prediction model for tight reservoir depletion and oxygen-reducing air huff and puff development is established.By comparing with the experimental results,it is confirmed that the model has high accuracy with the maximum relative error of 5.8%and the average error of3.9%.