Experimental Study On Adaptability Of Two-stage Channeling Sealing Technology For CO₂ Flooding In Tight Sandstone Reservoirs

Reservoir characteristics of tight sandstone reservoirs in China are mainly characterized by ultra-low porosity,matrix permeability and low pressure coefficient.It is difficult to inject water.Gases with good injectability,such as CO₂,are mostly used to maintain formation energy.However,due to the existence of fracture networks composed of natural and artificial fractures of different scales,low fluidity of CO₂ escapes seriously,and it is difficult to start crude oil in the matrix.Sealing channeling with different scales of fracture net can improve formation energy supplement and gas drive effect.Firstly,according to the principle that matrix permeability and fracture scale can simulate accurately and repeatedly,we have developed the manufacturing method of different sizes of tight sandstone slit network models,and study the adaptation limits of the optimized starch gel system and ethylenediamine system to seal the seams.After optimization of the experimental parameters by injection parameters,the high strength gel and ethylenediamine two level sealing system are optimized.The experimental results showed that the high strength starch gel with 4%concentration was low in viscosity before gelation,which was conducive to the smooth injection of the system.The gel strength was high,and it could be used for strong sealing of cracks,changing the 3 kinds of crack opening,3 kinds of matrix permeability and 3 kinds of starch gel concentration.It could be concluded that under the condition of 0.42mm crack opening and 0.4×10^-3?m² matrix permeability,the starch gel with 4%concentration could achieve more than 99%plugging.The breakthrough pressure was up to 24.9 MPa,which effectively started the remaining oil in the matrix,improved the oil recovery by 32%,and had good plugging adaptability.Under the conditions of 0.08mm fracture opening and 0.4×10^-3?m²matrix permeability,ethylenediamine could achieve more than 99%plugging efficiency,the plugging effect was the best,and the gas sweep range could be expanded to a greater extent,and the recovery rate could be increased by 23.1%.Plug assembly,different injection volume and injection mode experiments showed that the slug combination of starch gel+ethylenediamine injection with 2 times crack volume could be obtained.When plugging was applied,the sealing effect of fracture network is the best and the recovery factor was 53%.Therefore,the adaptability of CO₂ flooding two level channeling technology to tight sandstone seams with different openings could be obtained.