Technology Feasibility And Safety Study On Shale Gas Extraction With Supercritical CO₂

Shale gas is a special kind of unconventional natural gas,which mainly exists in adsorption or free status.Low pore pressure,low porosity,low permeability and high clay content are the key features of shale reservoir.Which make it more difficult and costly to exploration and development in shale gas reservoir.Practices have proved that multi-stage fracturing has been used in more than 90%shale gas reservoirs.Water sensitive is very critical for contact of clay minerals When the fracturing water into the shale reservoir.While which will pollute the reservoirs and cause the accidents.In addition,water resource would be seriously wasted by hydraulic fracturing.Combined with the characteristics of supercritical CO₂ fluid,compared of CO₂ and CH₄'s adsorption capacity with the shale reservoir,through analyzing of shale gas reservoir,we found that using supercritical CO₂ can reconstruct reservoir and exploit of shale gas effectively,also can enhance the shale gas recovery by the adsorption and desorption of shale gas.At the same time,it also can achieve the CO₂ effective geological storage in shale reservoirs.While we should have deep research on the feasibility and safety of using CO₂ to exploit shale gas,but not only in concept.In this paper,combined with the adsorption/desorption of CO₂/CH₄ in shale reservoir and the results of numerical simulation of the CO₂ exploitation shale gas,we study the CO₂ performance to determine the feasibility of using supercritical CO₂ extraction of shale gas.Combined with the CO₂ corrosion/protection of the shale wellbore and ground facilities,we have a security risk assessment and reveal the feasibility and safety of supercritical CO₂ extraction of shale gas.A suit of theoretical basis and technical support applies to China's shale gas reservoir development was provided.The paper carried out the following work:?1?It is concluded that the volume of CO₂ is very susceptible to compression or expansion along with the change of temperature and pressure.The High density of CO₂ has the resistance on the shale gas dissolved When environmental conditions beyond CO₂ critical condition.CO₂-shale gas multivariate system phase diagram shows it Will be prone to retrograde condensate phenomenon when the more to the left deviation,the wider to the two-phase region.?2?The experiments of CO₂ and CH₄ adsorption desorption showed that The CO₂ adsorption ability is far more than CH₄ adsorption ability in the shale,It can effectively utilize the difference of adsorption and desorption capacity to exploit the shale gas.CO₂ enhanced recovery of shale gas in the numerical simulation results show that using CO₂ to exploitat shale gas than the depletion development by 4.86%.?3?The risks of CO₂ exploiting shale gas projects may relate to four aspects,the risk factors identification,the risk factors analysis,the risk assessment and risk response.Using literature analysis method to establish the factors that should be first considered in the CO₂ mining process of shale gas,and hence the 19 major risk problems may occur in development of China's mining process for shale gas were proposed.Based on the fuzzy evaluation method to get the CO₂ exploitation of shale gas projects which membership degree is 2.8105,indicating the degree of risk for process is to the middle.While,the highest risk level is the investment risk,and technology risk,followed.Based on risk analysis and evaluation results,puts forward the measures in coping with CO₂ leaks and development process for corrosion control in technical levels.?4?The corrosion environment simulation data was determined refer to the conditions of typical gas reservoirs in home and abroad.From the analysis of corrosion factors,the capacitive reactance diameter at 90 ? are much smaller than which at 60 ?.However,the Corri?90?Corri?60?increased as the pH value reduced.The SEM analysis of corrosion product showed that the corrosion product is mainly contained three elements?Fe,C,O?,which indicating that corrosion is caused by CO₂,and the corrosion product films are mainly included by FeCO3 and Fe3O4.Besides,the electrochemical reaction is firstly occurred through the ionization of CO₂.?5?In order to inhibit corrosion caused by CO₂,PBTCA @ HP-CD inclusion compound was prepared and through FTIR and SEM,the polymer structure has been confirmed.Corrosion inhibition performance evaluation results of PBTCA @ HP-CD showed that the compound behaved better inhibition effects than single 2-phosphonic acid-1,2,4-tricarboxylic acid through both EIS and polarization curves.And,PBTCA @ HP-CD inclusion compound has achieved more than 90%of the inhibition efficiency.Based on the above researches,it is feasible to consider the mechanism of CO₂ mining shale gas.Although the risk factors gave the severities in the investment and corrosion,the corrosion can be solved by adding the inhibitor.