Research On Production Mechanism And Numerical Simulation Of Shale Gas

Shale gas is a promising unconventional natural gas resource.The production of gas from organic rich shale has increased dramatically in North America in recent years,and future worldwide growth in the use of this resource of energy is also projected to increase.However,the formation of shale gas is very complex.Shale is both the source rock and the reservoir,gas production differs significantly between shale and conventional gas reservoirs.How to efficiently exploit shale gas,which plays a key role in the global energy security,is also of great significance to improve the energy structure.This thesis is supported by the National Natural Science Foundation of China（Grant No.51374527）and the New Century Excellent Talent Foundation from MOE of China（Grant No.NCET-09-0844）,using TOUGH + REALGASBRINE（developed by Lawrence Berkeley National Laboratory）to simulate.In order to verify the reliability of the numerical simulation,the results of simulation compared with experimental results,which can achieve a good agreement under same conditions.Then simulate multi-component adsorption,CH₄-CO₂ displacement,supercritical CO₂ displace CH₄,and an engineering case.Through the above experiments and simulation analysis,the research results as follows:（1）Firstly,this paper simulated the single component adsorption of shale,and compared with the experimental result under same conditions.The conclusion as follows: the adsorption of shale was effected by temperature and pressure,the sorbed capacity and pressure were positively correlated,while negatively correlated with temperature.The cumulative production of shale gas gradually increased with time and reached its maximum at last.（2）On the basis of simulation and experiment of single component adsorption,this paper simulated the multi-component adsorption.Then found that the sorbed capacities of different gases are not the same even under the same conditions.Because the different of VDW（Van der Waals’ force）due to the different of sorbed capacities,the bigger force due to the stronger sorbed capacity.At the same time,the adsorption ability of different gases occurs competitive adsorption among mixed gases.The greater adsorption ability of gas has more proportion in the mixed gases,which will have more sorbed capacity.（3）On the basis of flow experiment of shale with a split fracture,the paper also simulated shale flow with a split fracture.Founding that the permeability with a split fracture is bigger than without a split fracture,the permeability increased with the increase of pore pressure,the sensitivity of shale to pore pressure with a split fracture is higher.At the same time,the permeability of shale with a macro-fracture will decrease because the temperature due to the thermal stress.Through simulation,it can be found that the sorbed capacity of shale with a macro-fracture is lower than without a macro-fracture,but it’s beneficial to increase the production of shale gas.（4）In the multi-component adsorption simulation,it can found that different gases will compete with each other.By using this principle,CO₂ with strong adsorption capacity can displace CH₄ with weak adsorption capacity to increase the production of shale gas.Founding that the more production with higher temperature,because the adsorbed gas is easy to be desorbed due to the higher temperature.At the same time,the greater pressure will increase the production of shale gas at the same temperature.（5）Because shale buried deep underground,both of temperature and pressure are high,so actually CO₂ is under the supercritical state when using CO₂ to exploit CH₄.In order to more real reflect the exploit condition and achieve efficient production of shale gas,the paper carried out research about CH₄ displaced by supercritical CO₂.The study found that the rule of CH₄ displaced by supercritical CO₂ is consistent with the rule of CH₄–CO₂ displacement.At the same time,it found that due to the some physical properties of supercritical CO₂ have changed,which leads to more favorable exploitation of shale gas and increase the production.（6）On the basis of the above theoretical and simulation studies,this paper then studied the real engineering case.It found that the production of shale gas increased by injecting CO₂,which accords with the previous research.At the same time,this paper studied the transient flow of gas,it found that the flow velocity of gas is greater when it nears the well.Because the pressure around the well is small due to the gas flow rapidly.It also found that gas saturation with macro-fracture is relatively low and the flow velocity is great.（7）This paper also studied the influence of Klinkenberg effect on gas production.Klinkenberg effect should be taken into account when the gas flow velocity is very slow in low permeability porous media.It found that whether Klinkenberg effect is considered have no influence on the gas production in the initial stage.But the cumulative gas production when considered Klinkenberg effect is obviously higher withthe time increase.So the Klinkenberg effect should not be ignored during the long time exploitation of shale gas.