Study On The CO₂ Enhanced Shale Gas Recovery Technology In Ordos Basin,China

In the face of the severe energy shortage and the immense pressure to address global climate change,the fossil fuels mainly made up of oil,coal and natural gas can't meet the needs of social and economic development any more.The clean and efficient shale gas resource becomes the new development hotspots in changing the energy structure and getting rid of the high dependency of the conventional fossil fuels.But due to the low permeability,90% of shale reservoir should be stimulated to realize commercial exploitation,and the horizontal well with hydro-fracture is the most widely used stimulation technology in recent years.The defections of high water consumption,the damage to the reservoir,the high investment and potential severe environment problems of hydro-fracture reduce the economic and technical recoverable reserves of shale gas.The proposal of CO₂ enhanced shale gas recovery technology?CO₂-ESGR?can be good to make up for those defects.The CO₂ enhanced shale gas recovery technology is an new type shale gas development technology which replace water with supercritical or liquid CO₂ as fracturing fluid and taking advantage of high adsorption capacity of CO₂ over CH₄ to promotes the desorption of CH₄ and enhance the shale gas production and realize the CO₂ geological sequestration to reduce greenhouse gas emission at the same time.Although the United States has carried out related indoor experiments and numerical simulation works to prove the feasibility and potential of this technology,this technology is still in its infancy,and most of the researches are for marine shale.China possesses abundant shale gas resources both in marine and continental facies,and the Ordos basin is a typical massive continental oil and gas bearing basin,which developmultiple sets of shale in the upper Paleozoic and Mesozoic,and the Ordovician pingliang shale,the Permo-carboniferous benxi,Taiyuan and shanxi shale and the upper Triassic yanchang shale possesses the most potential.In this study,the upper Triassic yanchang shale in Fuxian area from the Ordos basin is chosen as the study area to explore the CO₂ sequestration in shale reservoir and the combined shale gas recovery technology with numerical simulation by employing the CMG-GEM and TOUGH+ simulators.The study mainly focus on the CO₂ storage mechanisms in shale reservoir,the continental shale gas production characteristics and influential factors of CO₂-ESGR and the optimization model establishment both in maximizing CO₂ storage and shale gas production.To explore the long-term trapping mechanisms of CO₂ in shale gas reservoirs by considering the interaction between gas adsorption and CO₂-water-rock reactions.A 2-D simplified reactive transport model is established for the Yanchang shale in the Ordos Basin of China using the GEM code.In the short to middle term,CO₂ is mainly trapped in the supercritical phase and adsorbed phase,but in the long term mineral trapping gradually plays a dominant role among other storage mechanism which continually increases the CO₂ sequestration in the dissolved phase and the mineral phase,thus reducing the supercritical phase and adsorbed phase of CO₂.The displacement of CH₄ by CO₂ occurs in two stages.First is the pre-displacement stage during which adsorbedconcentrations of both CH₄ and CO₂ increase due to pressure build-up,however due to the low concentration of CO₂,the displacement does not happen at this stage.Second is displacement stage when CO₂ concentration is high enough to displace CH₄ from the host rock and the higher pressure benefits the displacement of CH₄ by CO₂.The interaction between gas adsorption and CO₂-water-rock reactions is also explored.By buffering the reservoir pressure during CO₂ injection into the subsurface,the gas adsorption effect can impact the solubility trapping process so as to interfere with the CO₂-water-rock reactions.Furthermore,this interference is intensified with reduction in the CO₂ concentration in the reservoir over time.Also,the porosity and the permeability change induced by the CO₂-water-rock reactions over time change the pressure in the reservoir which affects the gas adsorption on the host rock.Limited sensitivity simulations have also been performed.Variations in the CO₂ adsorption isotherm,abundance of smectite and chlorite,reaction rate and reactive surface area of smectite significantly affect the CO₂ storage in different mechanisms.High CO₂ adsorption capacity to shale is beneficial for the CO₂ injection into shale reservoir;the amount of adsorbed CO₂ is proportional to CO₂ adsorption isotherm,while the other phase CO₂?gas/supercritical phase,dissolved phase and mineral phase?is negatively related to the CO₂ adsorption isotherm.Simulation results also show that smectite and chlorite are the key minerals that influence the CO₂-water-rock reactions in Yanchang shale.To study the impact of residual trapping on the CO₂ storage in shale reservoirs,a model fully coupled all the CO₂ storage mechanisms including the hydrodynamic trapping,adsorption trapping,residual trapping,solubility trapping and mineral trapping is constructed.To isolate the effect of residual trapping on gas adsorption and long-term CO₂-water-shale reactions,a transport model and reactive-transport model were established separately.The results suggest that hysteresis effect can enhance the injectivity of shale reservoir as well as the storage amount of CO₂,which directly leads to an increase in content of free phase CO₂.However,the adsorbed and dissolved phases CO₂ are inhibited by hysteresis effect.On one hand,it is because part of free phase CO₂ transfers into residual gas,and on the other hand,the hysteresis effect changes the formation pressure distribution.The influence of hysteresis effect on formation pressure varies at different locations.In the area saturated with CO₂,the capillary pressure caused by hysteresis effect can be negligible and formation pressure is uplift due to an increase in CO₂ injectivity;while in the area with low concentration of CO₂,the negative capillary pressure leads to a decrease in formation pressure.The change of formation pressure will cause the corresponding gas adsorption and dissolution change so as to the CO₂-water-rock reactions.Shale reservoir can be divided into acidification zone and alkalization zone along the X axis according to the pH distribution,the influence of hysteresis effect on pH value mainly reflected in the alkalization zone,showing to slow down the alkalization of the formation water.At the pre-displacement stage,the hysteresis effect increases the adsorption capacity of CH₄ by improving the formation pressure.However,at the displacement stage,the influence of hysteresis effect also shows partition on the competitive adsorption between CO₂ and CH₄,which plays a promotion role in the area near the well and an inhibition role in the area far from the well.The hysteresis effect is beneficial for improving the injectivity of reservoir,but its intensity variation exerts no effect on injectivity.The content of residual phase CO₂ is intensified with hysteresis effect,while the other phases shows the opposite change tendency.The formations pressure increase with hysteresis effect at the zone near the injection well but shows in negative relation with hysteresis effect intensity at the zone far away from the injection well.By addressing the increasingly serious global energy and environmental crisis through promoting the production of shale gas from organic-rich shale and reducing CO₂ emission,CO₂-ESGR technology wins more and more attention.To study the potential and characteristics of employing CO₂-ESGR on yanchang shale,a 3-D numerical model according to typical shale reservoir parameters from the Fuxian area was established with the TOUGH+ simulator.With a constant CO₂ injection rate of 0.03 kg/s for 50 years,the shale gas production in the producer can be enhanced 6.74%,while a total of 9.467×107 kg CO₂ can be stable stored.Production pressure increase induced by continually CO₂ injection and the competitive adsorption between CO₂ and CH₄ are the two key mechanisms contributing to the enhancing of shale gas production.By analyzing the impact of CO₂ injection rate and well distance between injector and producer,the conclusions that shale gas recovery shows non-monotonous variation with CO₂ injection rate and decrease with the increase of well distance before CO₂-breakthrough can be drowned.In order to find the impact of temperature and salinity on CO₂-ESGR,a 3D geological model coupled thermal and salting-out effecs are constructed.It is conclude that the shale gas production can lead to the corresponding decreasing of reservoir pressure and temperature which reduce 3.73 MPa and 0.4? after 10 years continually production,separately.The injection of CO₂ can induce the pressure accumulation around the injector,while the temperature evolution affects by two factors: reservoir pressure and injecting fluid temperature.Due to the higher transmission rate of pressure,reservoir pressure is more important at the beginning of CO₂ injection.Except the dissolution CO₂,free phase and adsorption CO₂ increase with the formation water salinity.Reservoir pressure uplifts with the increase of salinity,while temperature decreases with salinity.Alhtough salinity exerts tiny effect on shale gas recovery,it is still play positive role on it,however it also increase the CO₂ leakage risk by increasing the amount of free phase CO₂.Senstivity analsysi reveals that CO₂ injection rate is the most significant impact factor for reservoir pressure,temperature and shale gas recovery ratio.Finally,to provide a greater economic and environmental incentive for the implication of CO₂-ESGR,an optimization framework was proposed to optimize shale gas production as well CO₂ storage at different well distance.The constant rate injection?CRI?mode and the constant pressure injection?CPI?mode were also compared.When CO₂-breakthorugh was the only constraint for the CRI mode,the further the well distance,the better,and the optimal injection rate at well distance of 390 m is 0.0609 kg/s?? model?with a total CH₄ production of 4.252×106kg and CO₂ storage of 96.08×106kg.When adding the constraint of injection pressure,the optimal scenario should be restricting the CO₂ injection rate as 0.031796 kg/s and set the well distance as 360 m.While,with the same maximal injection pressure limitation,CPI mode shows more superiority than CRI mode both in CO₂ storage and shale gas production.