As China's largest oil and gas bearing basin, the Tarim Basin has tremendous exploration and development potential. Piedmont area is the main producing area of the Tarim Basin, and single well production in this area is high. Multiple salt-gypsum formation, which consists of shale, salt, gypsum-mudstone, gypsum and other complex layers, is widely distributed in the piedmont area, which will cause drilling and completion engineering great challenges. Distinct from the character of low creep rate, simple deformation composition, high salt purity and low burying depth in North Sea, Gulf of Mexico, Brazil offshore, the salt-gypsum formation in Tarim Basin is buried in high depth of 7000m, the in-situ stress is at a high level of 150 MPa, and the formation temperature exceeds 160° C. In order to efficiently develop sub-salt reservoir, promoting sub-salt horizontal wells is necessary, but due to the characteristics of multiple salt-gypsum formation, previous sub-salt horizontal wells have low engineering success rate, restricting the development process of sub-salt reservoir.

In-situ stress prediction in multiple salt-gypsum formation is a world class problem, based on the DRA-Kaiser experiment with field crop salt core, this paper established an innovative 3-D multi-layer complex geological model. It solved the problem that, the previous model during simulating process was oversimplified and could not truly reflect the actual geological structure. Based on the finite difference method and nonlinear prediction theory, a numerical method of in-situ stress field for whole regional composite salt-gypsum formation was developed. It overcame the problem that laboratory experiment and theoretical calculation were difficult to get in-situ stress of ultra-deep composite salt-gypsum formation, on this basis a 3-D bending casing-cement mantle-salt formation geological model is established, and further researched the effect on casing strain of borehole curvature, as well as the impact of non-uniform stress on the curved borehole casing in salt-gypsum formation. The research method in this paper can also be utilized in other ultra-deep sub-salt horizontal wells around the world.

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