Study On The Effect Of Water Flooding - Gas - Rock Reaction On The Physical Properties Of Loose Sandstone Reservoirs

CO2flooding can not only reduce the emission of greenhouse gas, but also can effectively enhance oil recovery. CO2has the property of making crude oil expand, making oil viscosity and interfacial tension decrease, and improving oil fluidity. As a kind of compressible fluid, CO2is easy to transport and store, and can be injected in liquid phase at low temperature. CO2flooding is suitable to be promoted and applied in offshore oil fields. The results of CO2flooding tests in North Sea Oilfield and Tulan Oilfield are good. The results of experiment and simulation show that immiscible displacement can increase oil recovery by5-8percent. The domestic conventional viscous oil reservoirs such as Bohai Gulf Oilfield have proved the feasibility of CO2flooding. The depth of such reservoir is shallow, and the rock is unconsolidated. After injection, CO2can react with formation water and formation rock, which causes the property change of fluid and rock, and affects the reservoir properties and seepage ability. Current studies on the reactions of CO2-formation water-unconsolidated sandstone are less. In this paper, based on the relevant theoretical and experimental results, the changes of petrophysical properties of unconsolidated sandstone caused by CO2injection are studied.In this paper, a certain unconsolidated sandstone reservoir is selected as the object of research. First, CO2-formation water-formation rock reaction experiments are carried out. Based on the experiment results, simulation models are built and fit to verify the experiments, and the influential factors are also predicted by the simulation. To reduce the effect of stress sensitivity and the effet of CO2on different permeabilities,16short steel sand tubes are produced. The unconsolidated and rock samples from the field are washed with water and filled into the sand tubes.9groups of paralleled rock samples with different permeabilities are made.18high temperature and pressure reaction tests with different injcted CO2saturation are carried out in9groups, and each test lasts72hours.23groups of simulations are carried out with5influential factors. Substance compositions and influential mechanisms are analyzed, and the following conclusions are drawn:(1)Experiment results show that, the porosity and permeability of unconsolidated sandstone sample decrease a little after its contacting and reacting with water and CO2under formation condition. The overall effect is not obvious, so it can be neglected in field production.(2)The greater the initial porosity and porosity are, the smaller the decreases of porosity and permeability are. CO2saturation in the injected water has little effect on the reservoir properties.(3)The simulation results show that, except the temperature and concentration change of CaCl2, other factors have little effect on reservoir properties during the simulation time, and the effect could be even neglected. The primary reason of reservoir property change is the mass change of carbonate precipitation. Because the carbonate content of unconsolidated sandstone is relatively small, the overall effect is not obvious.The results of this paper are from the research project of the oilfield. The research project has passed the inspection, and has been applied to the production of the oilfield, which has supported the engineering design of CO2injection of the oilfield.