Study On Cave Collapse Mechanism And Crack Propagation Law Of Fractured-vuggy Carbonate Oil Reservoirs

With the rapid development of the economy and the rapid increase in energy demand,the exploration and development of oil and gas fields in our country are deepening gradually,the mining depth is bigger and bigger,and the types of oil and gas fields are becoming more complex.Therefore,the exploration and development of unconventional oil and gas reservoirs are becoming more important.Because of the large reserves and high production capacity,the fractured-vuggy carbonate reservoirs are of great importance to the oil strategy of our country.Unlike conventional reservoirs,carbonate reservoirs consist mainly of caves and fractures.The cave is the main reservoirs space of oil,and the fracture is both the reservoirs space and the seepage channel,which makes the fractured-vuggy reservoirs has a very strong heterogeneity and multi-scale characteristics,resulting in the development of such oil and gas reservoirs is very difficult.Especially in the development of fractured-vuggy carbonate reservoirs under 5000 meters,the collapse of cavities and the closure of fractures are more likely to occur,which seriously affects oil production.Therefore,it is important to study the mechanism of the collapse of carbonate caves and the law of fracture closure under the coupling of seepage field and stress field,which can provide the important scientific basis for the large-scale exploitation of oil resources and increasing oil production in China and has important theoretical significance and engineering application value.Therefore,this thesis took the Tahe fractured-vuggy carbonate reservoirs buried more than 5000 meters as the engineering background.Three-dimensional geomechanics model test and numerical simulation have been carried out to study the failure conditions,influencing factors and the law of crack propagation in the process of collapse.By means of the extended finite element method,the calculation model of fracture propagation under fluid-solid coupling was established.Based on ABAQUS platform,numerical simulation of fracture propagation under fluid-solid coupling was carried out.The main contents of this thesis include:(1)Triaxial compression test,uniaxial compression test and Brazil splitting test were carried out on the carbonate of fractured vuggy reservoirs in Tahe oilfield.The mechanical parameters of carbonate rocks buried depth in 5000m-7000m were obtained.The SEM analysis of fracture specimens of typical rock samples was carried out and compared with the macro mechanical test.The effects of microstructural changes and microcosmic composition on the macroscopic strength of the rocks were investigated,and the microscopic failure mechanism of the rocks under complex stress was revealed.(2)A three-dimensional loading model test system of ultra high pressure intelligent numerical control was developed.The model was subjected to three-dimensional gradient non-uniform loading by intelligent numerical control technology.The automatic test of displacement of any part of the model cavern was realized by photoelectric conversion technique.Through the high definition multi probe system,the destruction of the cave was observed dynamically.(3)Taking the fractured-vuggy reservoirs of Tahe oil field in Xinjiang province as the engineering background,the three-dimensional geomechanical model test of the collapse process of the karst reservoirs cave was carried out by using the similar material of iron sand cemented soil and the high-geostress three-dimensional loading model test system.The collapse failure modes,nonlinear deformation characteristics and stress variation laws of the ancient karst cave were revealed,and the regularity of the crack propagation around the caves was also revealed.And the model test was verified by numerical simulation.(4)Based on dichotomous depth reduction method,the prediction formula of cave collapse depth under the influence of multiple factors was established.The influence of cavern span,roof thickness and lateral pressure coefficient on cave collapse was analyzed.The calculation model of the fracture closure was established,and the influence of the crack angle,the crack length and the crack width on the closure of the fracture was revealed.(5)The fracture closure and propagation criteria were established by fracture mechanics method.(6)The extended finite element method was used to establish the calculation model of fracture propagation under fluid-solid coupling.Based on the ABAQUS platform,the fracture and closure calculation program of fractured-vuggy carbonate reservoirs was developed.(7)The numerical simulation of the crack propagation process in the fractured-vuggy reservoirs was carried out by using the numerical calculation model established in this thesis.The expansion law of different forms of fractures under seepage stress coupling was obtained.