Control factors and risk prediction of casing deformation in deep shale gas wells in southern Sichuan Basin

Casing deformation seriously affects the fracturing progress and stimulation effect of shale gas reservoirs. Considering casing deformations in fractured deep shale gas wells in the Luzhou block of southern Sichuan Basin, the influences of micro-structure, natural fractures, frequent layer penetration during drilling, cementing quality, borehole enlargement and fracturing operation on casing deformation were systematically analyzed. The results show that the occurrence rate of casing deformation is 51 % and 66.3 % respectively in areas with microstructure and frequent layer penetration, the overlap rate of casing deformation points and natural fracture points is only 22 %, the proportion corresponding to good cementing quality is only 36.5 %, borehole enlargement/shrinking universally exists, and the fracturing intensity and scale are generally large before casing deformation. It is concluded that casing deformation is closely related to micro-structure, layer penetration and lithologic mutation surface, and controlled by well diameter, frequent layer penetration, cementing quality, micro-structure and natural fractures in a descending order of influence degree. Accordingly, a multi-factor weighting evaluation method and the vertical well trajectory, well quality and well stimulation classification criteria were built to realize the prediction, prevention and control of casing deformation in deep shale gas wells in the Luzhou block, demonstrating effectively improvement in stimulation effect.