| Oil and gas resources contained in deep-buried carbonate rocks will be China’s the main battlefield of second pioneering work with regard to oil and gas exploration. As the main distribution area of carbonate reservoirs, Tarim Basin has the typical characteristics of the collective development of fractured reservoirs. Previous prediction methods of cave, such as eophysical methods, drilling empty distance, are not perfect. It cannot effectively predict the cave location, shape and size, so that the cave drilling has features of random and sudden. So, cave stability prediction has led to a certain blindness.The effect of wellbore pressure and water injection process fluid properties change is likely to change the original stress condition of the karst cave, which will make cave instability, or even collapse. So the research of stability of karst caves helps to guide drilling deep carbonate/production process design and construction.The research object of this paper is the12area in tahe oil field ordovician carbonate karst cave reservoir. And combined with drilling data, the paper studied karst cave’s distribution and devlopment in the work area. Through the mechanical models of the factors influencing stability of karst cave and karst cave analysis, in combination with the engineering assessment method of cavern stability, the work area cave evaluation standard is formed. Then, by using the model of single karst rcaves and complex multilayer, the stability analysis is finished.The distribution of cave have been studied. The ordovician carbonate reservoir in the process of the formation has experienced multiphase tectonic movement and karstification reformation that lead to great differences in cave reservoir development scale;strong reservoir heterogeneity; variable fluid properties; deep buried reservoir. Karst cave reservoir as the natural porous media mainly effected by the ancient karst cavity, dissolution and fracture, cause blowout, sticking, strata collapse and a series of complicated conditions, even catastrophic accidents. Through the analysis of logging data, rock mechanics parameters of tahe oilfield are summarized; work area caves mainly exists in the form of rock houses, large rocks, and according to drilling recognition, karst cave height between0.3～23m, about5m of the highest occurrence of karst cave.Work area cave stability discriminant method and index system have been researched. Through Analyzing the influence of geological structure, cave shape, in-situ stress and artificial factors on the stability of karst cave mining and using knowledge of rock mass mechanics, the karst cave rock mass can be divided into complete and anisotropy of rock mass, crushing of rock mass. This paper introduced geotechnical stability cave discrimination method and indicators such as rock displacement, compression, shear strength and so on to discriminate on the stability of the cave.The work area monomer caves and multilayer complex karst cave finite element mechanics model have been set up. According to the work area development situation of karst cave, the paper uses ABAQUS finite element software to simulate the stability of karst cave under different hole diameter, shape. The simulation results show that with the increase of cavity diameter, the cave surrounding rock displacement and stress are increased obviously, and the principal stress influence range is3to5times the diameter of the karst cave. Considering wellbore pressure difference change, the paper simulates the monomer caves and the stability of complex multi-layer karst, and analyzes the differential pressure change and the karst cavern surrounding rock under the relationship of stress and displacement. The results show that when the wellbore pressure difference is more than7MPa, monomer caves easily happen in the maximum horizontal principal stress direction instability, multilayer cave instability associated with the thickness of the overburden.Through the study of this paper, the cave stability analysis is one of the keys of safety drilling of the seam and preventing the reservoir damage. The results can provide certain theoretical basis to fracture-cavity carbonatite reservoirs in tahe oilfield drilling and oil recovery process design. |
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