Abstract
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, rock mechanic experiments and numerical simulation studies were conducted. In rock mechanic experiments, three types of rock mechanics tests (uniaxial compression, triaxial compression, and Brazilian splitting) were carried out to obtain the basic rock mechanics parameters. 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, the size and geometrical features of karst caves are the main factors influencing collapse condition. For a cave system, a combination pattern and distance between caves are two main factors affecting collapse condition. In order to facilitate the application of these research results, the authors present formulas for calculating the critical conditions of karst cave collapse by means of multivariate linear regression method. The calculation formulas and prediction chart were verified as consistent with actual field results. The research results in this paper have great theoretical and practical value for revealing the mechanism of karst cave collapse in fractured-vuggy carbonate reservoirs.
Highlights
The reserves of carbonate reservoirs account for half of the world’s total oil reserves
Compressive strength, elastic modulus, and Poisson’s ratio parameters of the rock were measured by the uniaxial compression test, and the calculation formulas are shown in Equation (5)–(7)
Rock mechanics tests and numerical simulation method were conducted to study the mechanism of karst cave collapse
Summary
The reserves of carbonate reservoirs account for half of the world’s total oil reserves. Li et al [17] studied the distribution law of karst caves in the fourth block of Tahe oil field They proposed that cavesystem spatial distribution could be divided into horizontal and vertical states. Karst caves in Tahe oil field mainly exist in the form of a cave system and are distributed near the unconformity. Using an average value for the mechanic parameters of limestone is inappropriate for calculating the critical condition of karst cave collapse. The critical condition of karst cave collapse can be studied by physical model testing and numerical simulation. Formulas were developed for quantitatively calculating the stress of a single cave and cave system They are presented here, as is a chart for judging critical collapse condition. The formulas and charts can be applied in Tahe oil field to improve the success rate of drilling and increase oil production
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