Research Into The Mechanism Of Enhanced Oil Recovery By Air And CO₂ Huff-n-puff In Tight Reservoir

Tight oil is widely distributed and has huge reserves around the world,but it is a typical unconventional oil and gas resource because it is difficult to develop and the production decreases very fast.Hence,it is difficult to obtain more commercial value by conventional techniques.At present,such as Bakken tight reservoirs in North America mainly used the techniques of CO2 huff-n-puff,which effectively improved the tight oil recovery.Because of Chinese CO2 gas source is insufficient and abundant air resources,it is significant to research the mechanism of enhanced oil recovery by air and CO2 huff-n-puff in tight reservoirs.In the first part,it provides lots of investigation about gas huff-n-puff in tight reservoirs around the world in this paper.The feasibility of the development is analyzed with air and CO2 puff-n-puff in tight reservoirs based on the successful cases of gas puff-n-puff in tight reservoirs.At the same time,the potential of air and CO2 huff-n-puff in M tight reservoir is estimated by analyzed and compared the reservoir structural characteristics,fluid properties,formation pressure and temperature,and production characteristics.Based on the field data,the tight core physical simulation of air and CO2 huff-n-puff was carried out to research into the effects of different air and CO2 ratio,different huff-n-puff cycle and different soaking time on the oil recovery.The experimental results show that with the increase of CO2 content,the huff-n-puff recovery is higher,which the CO2 huff-n-puff enhanced recovery is 21.10%and the air huff-n-puff enhanced recovery is 7.84%.With increase of the huff-n-puff cycles,the single cycle huff-n-puff enhanced oil recovery is lower,and the enhanced oil recovery is mainly concentrated in the first cycle,in addition,the fourth cycle even no oil is produced.With the increase of soak time,the air and CO2 huff-n-puff recovery factor also will increase,but the extent of increase is very small,however,the gas is achieved balance with dissolved and diffusion in the tight core when the soak time is more than 6 hours.The results for tight reservoir stimulation of air and CO2 huff-n-puff provides an experimental basis.Air and CO2 mixture oxidation experiment was studied by TG/DSC thermal analysis.With the increase of temperature,the oxidation reaction of crude oil and mixture gas main include the light component of crude oil volatilization,heavy component splitting decomposition,heavy component oxidation reaction at low temperature,fuel deposition and high temperature oxidation.The Coats model is used to calculate the mixture gas oxidation kinetic parameters of crude oil,which oxidation activation energy is between the 24.67 KJ/mol and 26.39 KJ/mol at low temperature.And high temperature oxidation activation energy change is larger.TG curve of crude oil with air and CO2(1:1)was matched by trial and error method.It is determined the oxidation equation of crude oil,which provided the reaction condition for reservoir numerical simulation.Combined with physical simulation of tight core and crude oil oxidation thermal analysis experiment,core numerical model is set by CMG-STARS of reservoir numerical simulation software.The sensitivity factor has been studied by numerical simulation,which results show the mechanism of enhanced oil recovery by air and CO2 huff-n-puff.In the huff-n-puff process,air main show the supercharging and CO2 main show the dissolving viscosity decresed.Based on the core numerical model,the crude oil oxidation reaction equation is applied to the reservoir numerical model.And the reservoir model was history matched under the condition of air and CO2 ratio of 1:1.The sensitivity parameters include gas injection rate,injection time,soaking time and huff-n-puff cycles are analyzed,which determined the best stimulation construction parameters are:gas injection rate of 10000 m3/d,gas injection for 10 days,soak time for 10 days,and four huff-n-puff cycles.Due to the low reservoir temperature,high content of light components in crude oil and short gas huff-n-puff time,the low-temperature oxidation reaction of crude oil could not be detected.The simulation results provided technical support for the successful implementation of in-situ air and CO2 huff-n-puff.Based on the theory of enhanced oil recovery by air and CO2 huff-n-puff in tight reservoir,which provides the theoretical support for air and CO2 huff-n-puff development in tight reservoir.The research could help to promote the new theory and new method of development the tight reservoir by air and CO2 huff-n-puff,which provide the technical reserves for the tight reservoir efficient development in our country.