Fractability Evaluation Of Shale Gas Reservoirs Based On Laboratory Tests

Shale gas as a clean and efficient energy burst into people’s attention.However,most of the shale gas resources in China are buried in deep strata,which is different from the shallow shale gas resources in the United States.The geological conditions of deep shale gas reservoirs in China are complicated,and fold faults are developed.Therefore,the theories and techniques of shallow shale gas exploitation in the United States are not suitable for deep shale gas reservoirs in China.A deep shale gas resource exploitation theory with Chinese characteristics needs to be developed independently.Therefore,centering on the subject of indoor fracturing evaluation of deep shale gas reservoir,this paper takes shale of Silurian Longmaxi Formation in Changning County,Sichuan Province as the research object.Through real-time high-temperature triaxial test,hydraulic fracturing test and uniaxial strain test,and based on energy balance,theoretical analysis and normalization principle,the mechanical properties of shale in different formation conditions was studied,brittle characteristics,the fracture toughness and fracture can be change rule,the stress strain curve of the shale under different test conditions and pump pressure curve were analyzed,the confining pressure and temperature on the rock failure mode(including high temperature triaxial compression test and hydraulic fracturing test)of the control function were explored,the influence mechanism of temperature on the mechanical properties of shale was revealed.Meanwhile,the temperature damage threshold of shale is obtained,and based on the new temperature damage,a damage constitutive model was established which comprehensively considers the characteristics of pre-peak and post-peak stress-strain curves of rock.Besides,based on fracture mechanics theory,a new fracture toughness measurement method was designed.The influence mechanism of formation conditions on shale fracture toughness is discussed.Finally,A comprehensive evaluation method for fracturing ability was developed,including brittleness index,fracture toughness and fracture initiation pressure without prefabricated crack,and the function relationship with different burial depth and different overconsolidation ratio was established.The main contents are as follows:(1)In the high temperature triaxial compression test,when the temperature is lower than140℃,the stress-strain curve shows a class Ⅰ,with obvious step shape after the peak.At low confining pressure,it shows tensile failure,and under high confining pressure,it shows shear failure.When the temperature is higher than 140℃,the stress-strain curve is class Ⅱ.There are obvious compaction stage and yield stage before the peak no step after the peak.At low confining pressure,the stress failure also appears as tension failure,but at high confining pressure,the failure model appears as scattered failure.(2)Based on energy conservation theory and high temperature triaxial compression test,a new temperature damage was defined by considering the fracture closure stage,elastic compression stage,plastic compression and failure condition of rock.Meanwhile,a constitutive model of rock damage was established considering the characteristics of prepeak and post-peak stress-strain curves of rock,which could better characterize the deformation and failure characteristics of rock under high temperatures and confining pressures.(3)In hydraulic fracturing test,the fracturing pressure of shale samples increases with the increase of confining pressures,but decreases with the increase of temperatures.The first fracture stress is the highest,followed by the second,and the third fracture pressure is close to the confining pressure.(4)In hydraulic fracturing test,rock fracture surface is uneven and rough under low temperature and pressure.Under the condition of high temperature and pressure,the rock fracture surface is relatively smooth without obvious concave and convex shape.From the fracture face of shale samples,the fractures of shale under low temperature and pressure show serrated fractures,while the fractures of shale samples under high temperature and pressure show straight fractures.(5)In hydraulic fracturing test,under the low temperature and pressure,internal pore pressure is not enough to open stratification plane,and the crack expands along the direction of the hydro-crack.However,under high temperature and pressure,pore pressure can direction along the prefabricated crack extension and is enough to open weak bedding.In other conditions,the crack propagation direction will probably be turned,extend in the direction of the bedding.(6)Based on the high temperature triaxial compression test,hydraulic fracturing test and uniaxial strain test,the brittleness index and fracture toughness under different test conditions were combined,as well as the fracture stress without prefabricated crack,the fracability index of shale reservoir was defined,and the indoor compressibility evaluation method based on burial depth and overconsolidation ratio OCR was established respectively.