Study On The Physical Simulation Experimental With Supercritical CO₂ Fracturing Of Shale

Effective development of shale gas is an important strategic need for China to relieve energy imbalance between supply and demand and ensure the energy safety.In view of the shale gas reservoir with high clay content in the water shortage area of China,the existing horizontal well + multi-stage fracturing technology has the problems of large water consumption and easy expansion of clay.The research team proposed the ideas of using supercritical carbon dioxide fracturing to produce shale gas.In order to directly observe the fracturing and fracture morphology of shale supercritical carbon dioxide fractures and further understand the mechanism of supercritical carbon dioxide fracturing,it is very important to carry out the simulation test of shale true triaxial supercritical carbon dioxide fracturing.In this paper,the simulation experiment of true triaxial supercritical carbon dioxide fracturing was carried out in the outcrop shale of Yibin Longxaxi Formation in Sichuan Province.The high-energy CT and DR scans were observed and compared with hydraulic fracturing.The difference between initiation and expansion.The main results are as follows:(1)Investigation of shale's triaxial compression tests was carried out.The shale specimens were soaked in the friction reducing SC-CO2 for different time before tests.Evolution law of mechanical parameters with increasing soaking time was acquired.During the first 72 hours of soaking,peak strength and elastic modulus reduced rapidly.After the first 72 hours of soaking,peak strength and elastic modulus reduced slowly.Evolution law of failure patterns was summarized.Therefore,degradation law of the mechanical property of shale reservoir during the SC-CO2 fracturing period was revealed.(2)The true triaxial supercritical carbon dioxide fracturing test system consists of four parts: true triaxial stress loading system,pressurized system,carbon dioxide heating system and data acquisition system.According to the similarity principle,the indoor test parameters are obtained by referring to the site fracturing parameters.And the preparation of supercritical carbon dioxide fracturing specimen was completed by means of openhole completion.So a complete real triaxial supercritical carbon dioxide fracturing test system was constructed(3)Shale supercritical carbon dioxide fracturing is about 50.9% lower than that of hydraulic fracturing,and with the increase of temperature,the cracking pressure of shale supercritical CO2 is decreasing.(4)Hydraulic fracturing of shale is Lord of the single fracture,formation of crack type.And supercritical carbon dioxide fracturing can form more than the main fracture,compared with the hydraulic fracturing,produced a lot of pressure after open bedding seam cracks;Supercritical carbon dioxide fracturing formation of the cracks is more complex,more rough fracture surface,this shows that the supercritical carbon dioxide are more likely to stimulate the larger reservoir space,achieve similar volume of fracturing effect.The results show that the effect of horizontal ground stress on the fracturing morphology of shale supercritical carbon dioxide fractures is consistent with the conclusion of shale hydraulic fracturing.