Study On Collapse Condition Of Karst Cave In Fractured-vuggy Carbonate Reservoir In Tahe Oilfield

Karst cave collapse usually occurs during the process of drilling and reservoir development.Karst cave collapse increases drilling risk and seriously affects oil production.In order to reveal the collapse mechanism of karst caves in fractured-vuggy reservoirs in Tahe oil field,four aspects of research were conducted,including reservoir characteristics analysis,mechanics experiments,numerical simulation studies and prediction of cave collapse conditions.Based on core,seismic and well logging data in the study area,the numerical simulation models of karst cave in Tahe oil field were established,and the influence factors related to the collapse conditions of karst cave were analyzed.The main achievements of the paper are as follows:(1)By analyzing the characteristics of karst caves,the karst caves in Tahe oil field are highly heterogeneous and unevenly distributed in both horizontal and vertical directions due to the influence of geological structure and karstification.Using outcrop,core,well logging,formation microscanner image(FMI)and seismic data and other means can effectively identify karst caves,which is characterized by blowdown loss in drilling,decrease in drilling time,enlargement of well diameter,low resistivity,increase in natural gamma and decrease in density.(2)In rock mechanics experiments,three types of rock mechanics tests(uniaxial compression,triaxial compression,and Brazilian splitting)were carried out to obtain the basic rock mechanics parameters.Compressive strength is 74.2 MPa,elastic modulus is 36.3 GPa,Poisson's ratio is 0.25,cohesive force is 12 MPa,and internal friction angle is 36°.Based on the theoretical analysis of the stability of karst caves,it is found that the main influencing factors are the span,height,shape and rock properties of karst caves.(3)In numerical simulation studies,the collapse conditions of a single karst cave and a cave system were studied.Numerical simulation results indicate that: for a single karst cave,size and geometrical feature of karst caves are main factors influencing collapse condition.The larger the burial depth,the larger the cave span,and the smaller the roof thickness,the more likely the cave is to collapse.A cylindrical cave is the most likely to collapse,a cubic cave and a spherical cave are harder to collapse.For a cave system,combination pattern and distance between caves are two main factors affecting collapse condition.A horizontal cave system is more likely to collapse than a vertical cave system.The closer the distance between the caves,the easier the cave system will collapse.When distance is greater than cave span,it will be difficult for the cave system to collapse.(4)By analyzing the numerical simulation results,calculation formulas and a prediction chart for cave collapse are obtained.The drilling data shows that the chart is accurate and reliable.By using the formulas and chart presented in this paper,the collapse potential of karst caves can be judged in the field quickly.The formulas and chart can improve the success rate of drilling.