| Unconventional reservoir resources play an important role in supplementing energy consumption and maintaining the balance between supply and demand in the oil and gas market.Shale reservoir,tight sandstone reservoir and carbonate reservoir have the characteristics of low porosity,low pore throat diameter and low permeability.In carbonate reservoir,there are large scale vugs,and large-scale volume fracturing is widely used to develop them.They are all developed by hydraulic fracturing.Therefore,there may be a variety of porous media of different scales in the reservoir:micro and nano pores,natural fractures,hydraulic fractures,vugs.There are different flow mechanisms in porous media of different scales,so how to make correct simulation calculation is always a difficult problem in unconventional oil and gas development.In this paper,the flow mechanism in the multi-scale porous media of the above reservoir was studied,and the flow models at micro and nano pores,hydraulic fractures and vugs were proposed.Numerical simulation was carried out to reveal the relevant laws and characteristics.The main research achievements and innovations of this paper are as follows:1.The model of gas-water flow in nanoscale pores is improved.The analysis of molecular dynamics simulation results showed that the gas-water velocity distribution profile was different from the traditional gas-water two-phase model due to the coexistence of gaswater molecules at the gas-water interface.According to this discovery,the region is defined as miscible region,and then the gas-water velocity model in the pore is modified.Finally,the apparent permeability model is derived.The influence of miscible zone on pressure and yield was studied by using permeability correction coefficient in numerical simulation.2.An oil-water two-phase model considering permeability and capillary stress sensitivity is proposed.In the fracturing process,with the increase of pressure and pore size,permeability and capillary pressure change significantly.A new oil-water two-phase flow model was established by coupling stress-sensitive permeability model and capillary pressure model.The numerical simulation results show that the stress sensitivity of capillary pressure significantly changes the saturation distribution around the fracture.The results of this study are verified by previous experiments,namely,the measured distribution profile of special water saturation near fractures is consistent with the numerical simulation results of this model.This proves that the stress sensitive effect of capillary pressure is the physical mechanism that causes this particular profile.In addition,the relationship between the variation of water saturation near the fracture and the stress sensitivity effect and capillary pressure under different shut-in time is also studied numerically.3.A discontinuum single-phase oil model is proposed to study the effect of vugs on transient pressure response.In this paper,the large-scale vug is regarded as an aquifer and the pressure of vugs and formation are decoupled.This processing method can not only more accurately describe the effect of various parameters of the vug on the transient pressure response curve,and correctly reflect the information of vugs on the curve,but also greatly speed up the calculation speed.This model can effectively reveal the effects of cave size,distance from cave to well,number of cave and distribution mode on pressure transient response. |
