Research On Water-rock Reaction During Carbon Dioxide Flooding In Tight Sandstone

With the difficulty of conventional oil and gas exploitation,tight oil Development has become a new method to solve the growing demand for oil and gas.The characteristics of low porosity and low permeability of tight oil reservoirs resulting in low oil production and poor efficiency.It is of great significance to improve the recovery of tight oil.The practice of tight oil development in many oilfields domestic and abroad shows that gas injection is an effective technology for increasing tight oil production.Supercritical CO₂ can be mixed with oil and gas to make its viscosity and volume expand.This unique property makes CO₂ injection has great potential for increasing production.CO₂ dissolves in water to form carbonic acid,and carbonic acid can undergo a series of chemical reactions with various minerals in the reservoir,resulting in the dissolution of minerals,or the generation of new minerals.Dissolving of the minerals can increase the porosity and permeability of the reservoir and improve the properties of the reservoir.The newly formed mineral particles can block the pores and throats.This article relies on the“Research on energy supplementation of tight oilfield based on digital core”,through a series of microscopic pore throat structure characterization studies on tight sandstone reservoirs,carries out indoor CO₂ flooding.Experiments and acidification experiments,using MAPS image analysis and QEMSCAN mineral analysis techniques,study the pore structure and mineral change characteristics at the same position before and after acidification of tight sandstone.The experimental results show that the water injection has a certain degree of influence on the microscopic pore structure of the tight reservoir in the study area,which is reflected in the blockage of some microscopic pores and micro-fractures.The average plane porosity in the six sights prior to water injection is 14.12%,after water injection,the plane porosity dropped slightly,and the decrease ranged from 0.11%to 1.62%,with an average decrease of 0.84%.The comparison of SEM images of tight reservoirs before and after water injection indicates that pores and throats are filled with clay minerals.Water injection causes hydration of clay minerals,resulting in densely distributed areas of clay minerals.Water-sensitive clay minerals absorb water and expand to block pores and throats.Based on MAPS and SEM techniques,the influence of acid etching on the pore structure was analyzed using the in-situ comparison method of rock samples.The ultra-micro pores between 50 nm and 400 nm in the B4 sample were larger after the dissolution and the pore size was between 400 nm and 1000 nm.The pore size of E1 samples between 50nm and 1000nm increased uniformly after dissolution.The plane porosity of B4 samples increased from 6.53%to 25.91%;the plane porosity of E1 samples increased from 7.7%to 10.47%.There is little change in quartz minerals before and after acid etching;carbonate minerals are completely dissolved;a certain amount of dissolution of albite with incomplete crystalline;K-feldspar with a complete crystalline remains stable;Clay minerals absorb water then swelling and cause a decreases of porosity and permeability;pyrite is less affected by acid etching.