Research Status and Prospect of Casing Deformation Mechanism and Control Methods in Shale Gas Wells in Sichuan Basin
Authors: Liu Yang , Tang Qi , Wu Hao , Ma Tianshou 
Published: September 2023

ABSTRACT
Casing deformation is a common problem of shale gas development in Sichuan Basin, which makes it difficult to run fracturing strings. It has become a bottleneck of restricting the development of shale gas resources. Through the investigation and the reference of many related documents, the main progress in the studies of casing deformation failure is summarized. The results show that many factors have influence on casing deformation, including the shear slip of fault or fracture evoked by hydraulic fracturing; the stress concentration in the vicinity of borehole caused by asymmetric fracturing; shale swelling caused by liquid phase invasion; annulus pressure build-up induced by the coupling effect of temperature and pressure, etc. Even so, the mechanical mechanism of casing deformation is still dominated by strain control and displacement control. Finally, the technical measures solving the problem of casing deformation are reviewed and discussed, raising casing rigidity, improving the displacement absorption capacity of cement sheath, optimizing fracturing parameters, adopting "temporary plugging fractures and multi cluster perforation" process, expansion tube technology and double layer casing technology, etc. It is hoped that this review can provide basic reference for further research and improvement of casing deformation control methods in shale gas well.

INTRODUCTION
Sichuan Basin is the earliest and the most successful area for shale gas exploration in China. Its exploitable shale gas reserves account for 30% of the whole country, and it contribute more than half of the total shale gas production. Sichuan Basin has become and will continue to be the main battleground for shale gas exploration and development in China (Zhang et al., 2021). However, compared with shale plays in North America region, the shale gas reservoir in Sichuan Basin has very different petrophysical and mechanical properties, and its geological complexity is much higher than that of the former. Meanwhile, the shale gas reservoir in the Sichuan Basin is buried more deeply at 2500∼6500m, and the in-situ stress anisotropy is much more significant than that in North American region (He et al., 2022). These differences present many challenges to the effective development of shale gas in Sichuan Basin. Fortunately, the breakthroughs in horizontal drilling and multistage fracturing technology provide the possibility to address this question (Liu et al., 2017), but it also introduces several new problems. Casing deformation is one of the bottleneck problems in the shale gas industry. The reason is that the pressure and temperature of the casing change dramatically during the multistage fracturing process, which will result in a complex and extreme mechanical environment of the horizontal casing. According to statistics, the casing deformation rate of shale gas wells in some areas of Sichuan Basin is nearly 30% (Chen et al., 2021). Casing deformation in horizontal shale gas wells has many negative impacts on shale gas production, such as making it difficult to run bridge plugs and mill shoes or even unable to reach the intended depth, which increases non-productive time and significantly increases the cost of resource extraction. More seriously, if the deformation of the production casing becomes too severe, the fractured section will be abandoned, and the capacity of the shale gas well will be significantly reduced. Field measurement data shows that the production of wells with casing deformation is 27.3% lower than that of wells without casing deformation (Huang et al., 2020). As a result, the large number and frequency of casing deformation has become a major bottleneck restricting the development of shale gas in the Sichuan Basin, China. The problem of casing deformation has attracted extensive attention from many scholars, and a lot of research has been conducted. Scholars have put forward various hypotheses and theories on the mechanical mechanism of casing deformation, and have used various schemes and ideas to prevent and control casing deformation. Nevertheless, the casing deformation phenomenon has not been completely solved so far. For this purpose, this paper briefly reviewed the progress of studies on this aspect to provide the interesting information for casing deformation prevention and control.