Study On The Phase Behavior Of CO₂ Natural Gas In Tight Gas Reservoirs

A tight gas reservoir is a gas reservoir with permeability less than 0.1 mD.Tight gas reservoirs are characterized by strong heterogeneity,high water saturation,complex pore structure and low recovery.At present,the recovery rate is only about 20%.In order to improve the recovery of tight gas reservoirs,it is urgent to develop new methods to improve the recovery.Injecting CO2 into tight gas reservoirs can not only reduce the greenhouse effect but also improve the recovery of tight gas reservoirs.Therefore,CO2 injection is considered as a potential technology to improve the recovery of tight gas reservoirs.Because the high CO2 gas reservoir is very special and difficult to develop,which is mainly reflected in the phase characteristics of high CO2 gas fluids,it is of great practical significance to study the phase characteristics of tight gas reservoirs after CO2 injection for enhancing recovery of tight gas reservoirs.At present,CO2 injection into reservoirs to enhance oil recovery has become a relatively conventional production technology,but in the field of CO2 injection to improve gas reservoir recovery is still a very cutting-edge research topic.At home and abroad,only a few experiments and mechanism simulation studies have been carried out on CO2 injection to enhance the recovery of tight and low permeability gas reservoirs.As a whole,it is still in the exploratory stage,and there are still technical problems to be solved urgently.On the basis of formation fluid matching,the phase characteristics of CO2 injection in tight gas reservoirs are tested;a new method for testing the phase characteristics of natural gas in porous media is designed,and the phase characteristics of tight gas reservoirs in porous media are obtained by analysis;the adsorption characteristics of CO2 and natural gas in tight sandstone are tested by fully automatic adsorption gas content testing device;based on the experimental results,the adsorption characteristics of CO2 and natural gas in tight sandstone are tested.Based on SRK equation of state,a prediction model for predicting the deviation factor of CO2-natural gas mixtures in PVT cylinder and porous media was established by modifying the gravitational term of the equation of state and establishing a new interaction correlation.The main conclusions and achievements are as follows:(1)Under different CO2 content,the relative volume,volume coefficient and compression coefficient of natural gas have little difference.With the increase of CO2 content,the change of relative volume,volume coefficient and compression coefficient is not obvious,while the deviation factor of natural gas decreases significantly,while the density and viscosity increase.After saturated water containing CO2,deviation factor and volume coefficient of natural gas increase at low pressure and decrease at high pressure,while density and viscosity decrease slightly at low pressure and increase slightly at high pressure.(2)With the increase of pressure,the relative volume of natural gas in porous media and PVT cylinder decreases,and the decrease under high pressure is much smaller than that under low pressure.The influence of CO2 content and core permeability on P-V curve is very small.Moreover,the existence of porous media changes the critical properties of natural gas components to a certain extent,and makes the deviation factor increase slightly.(3)Under the same conditions,the order of adsorption capacity of CO2,CH4 and mixture(CO2:CH4=1:1)in sandstone is:CO2>mixture>CH4,which indicates that CO2 has the ability to displace natural gas in sandstone;the smaller the permeability of sandstone core,the larger the adsorption capacity;the larger the bound water content in core,the smaller the adsorption capacity.(4)By modifying the parameter a(T)of the attraction term in SRK equation of state and establishing a new interaction correlation,a prediction model of deviation factor is established which can effectively simulate CO2 gas injection into PVT cylinder and porous media.The simulation results are in good agreement with the experimental values,and the absolute average error is less than 1%.