Predicting collapse depth of paleocaves in carbonate reservoirs

Paleocaves in carbonate reservoirs are the excellent oil and gas storage spaces in the field of petroleum exploration. Predicting the collapse depth of paleocaves in carbonate reservoirs has essential theoretical significance and applicable value for classifying of reservoir quality and delimiting lower depth of oil and gas exploration. Paleocaves in carbonate reservoirs are acted by overburden pressure, tectonic lateral compression, and fluid pressure in caves. In this study, paleocaves with irregular boundary shapes were abstracted as spheres with different diameters. Mechanical model of surrounding rocks of paleocaves was established to compute the stress states in 3–5 times diameter of caves. Through analyzing stress states in surrounding rocks, the destruction and collapse of caves was investigated. The diameters of caves in these models range from 0.01 mm to 10 m. Relationship between diameter of cave and collapse depth was obtained by fitting collapse depth of caves with different diameters. Above the collapse depth of caves with diameter 5 m is good reservoir zone. Between the collapse depth of caves with diameter 5 m and 2 m is effective reservoir zone. The interval of collapse depth of caves with diameter 2 m and 0.5 m is general reservoir zone. Below 11000 m, karst caves in carbonate reservoir nearly disappear. Numerical model of real paleocaves was established to simulate the deformation of single paleocave and paleocave group. The relationship between displacement of paleocave wall and depth of paleocave obeyed power law. The contiguous destruction and collapse occurred first between the paleocaves with small distance. The relationship can be used to classify the quality of carbonate reservoir and predict the lower depth of petroleum exploration.