When operators develop shale gas reservoirs in the southern Sichuan basin in China, they encountered numerous occurrences of casing deformations (CD) and even failures. The high frequency and severity of CD have led to significant financial loss. Since then, a considerable amount of research has been conducted with some field trials. Some research findings have been implemented in fields. The purpose of this paper is to present what we know and the trial results.
We observed that casing deformation/failure were mainly in shearing failure and collapse modes. In the early stage of the development, most of the failure was due to shearing deformation caused by pre-existing geological features such as faults and weak interfaces. With the depletion of the reservoir and pressure decrease, casing collapses during the hydraulic fracture with extended length have become more and more popular in the later development stage. Laboratory tests on casing material and cementing material have shad lights on possible solutions. Increasing the casing wall thickness and cement thickness seems a viable solution for casing collapse, but the application of these recommendations yielded little effectiveness in mitigating casing deformation. Current operators redesigned a cementing material with high-strength beads which would collapse when stresses are above the designed threshold, which would "absorb" the formation displacement and reduce the severity of casing deformation caused by the aforementioned mechanisms.
This paper summarizes the main research results from implementing numerical modeling and simulation. Based on that, we designed a special cementing with hollow high-strength particles in the cement slurry. In the later stage of fracturing, when the stress is above a threshold, the particles would burst and allow the casing to nudge slightly so that the deformation severity would be much less and more graduate. We implemented the new technology on 14 wells, and so far eight wells have been successfully completed without losses of horizontal segments. This new technology certainly brings hope for future study and provides field cases for future simulation work and laboratory studies for improvement.