Study on the Internal Pressure Strength of Perforated Casing in High Pressure Formation

ABSTRACT Casing damage is the main form of wellbore integrity failure, severely affecting oil and gas production. The mudstone has rheological properties under the high temperature and pressure of the formation, and creep deformation of the formation leads to increased external pressure of the wellbore. At the same time, perforation operations severely reduce the strength of the casing. Therefore, it is crucial to analyses the collapse strength of perforated casing in mudstone formations. In this paper, a three-dimensional numerical model of the formation-cement sheath-perforated casing is developed for the geological conditions of the Missan oilfield. The casing damage mechanism under long-term rheological effects is revealed. The effects of different casing sizes, casing thickness, perforation diameter and perforation density on casing collapse strength were carried out. The results show that the equivalent force on the perforated casing gradually increases with increasing operating time. For the same parameters, the smaller the casing diameter and the thicker the casing thickness, the higher the residual strength of the perforated casing. The collapse strength of the perforated casing decreases with increasing perforation diameter and perforation density. It is recommended to increase the casing thickness and optimize the perforation parameters in mudstone formations. The study provides a theoretical basis for the field application of perforation technology in mudstone reservoirs and has a good application prospect for improving the casing collapse strength. INTRODUCTION The Missan oil field has frequent drilling complications, among which casing damage is a serious problem. The Asmari Formation is 3000m burial depth and has a complex geological structure with extensive mudstone. Field applications and relevant statistics show that the majority of casing damage and failure occurs in the perforation holes. Perforation is an important stage of oil exploration and development, and is closely linked to subsequent production enhancement and reconstruction techniques. During the completion of an oil and gas well, the jet produced by the injection projectile will penetrate the casing, cement sheath and part of the formation. As a result, a number of perforated holes are created in the casing along the injection spiral, resulting in a reduction in the stability of the casing. During subsequent production, the perforated casing is more susceptible to strength failure due to excessive loading, which has a non-negligible impact on the wellbore integrity. At the same time, the creeping behavior of the mudstone at high in-situ stress increases the extrusion of the casing, exacerbating the potential damage to the perforated casing. Therefore, the strength analysis of the perforated casing under mudstone creep is critical to the integrity of the wellbore.