Mechanical and control measures on casing damage due to annular absence of cement sheath

If cementing is of poor quality, a small section of cement sheath will be absent in a certain part of oil and gas well in the subsequent operation. As a result, clearance occurs between the casing and the cement sheath, so casing stress changes and the casing is ultimately damaged. A symmetric fnite element model of casing-absent cement sheath-strata was established based on the feld data of Zaonandong Oilfeld in Dagang Oilfeld and the principle of plasto elasticity. By virtue of this model, the effects of the height of complete cement sheath absence, the size of clearance between the casing and the cement sheath after the absence of cement sheath and the internal pressure on the casing stress were investigated, and the control measures for complete absence of cement sheath were proposed correspondingly. It is shown that the stress on the casing at the interface between complete cement sheath and incomplete cement sheathis the maximum value. The stress in the section where cement sheath is absent is higher and stable. The higher the internal pressure is, the higher the maximum stress on the casing in the annular absence section is. The stress on the sections with absent cement sheath is equal no matter what the absence height of cement sheath is. Casing stress is less affected by the clearance at the frst interface. It is demonstrated that the casing damage in the section with absent cement sheath cannot be controlled effectively only by increasing the wall thickness, and the effective method is to increase the steel grade. These researches and analysis can be used as the benefcial reference to study casing damage control while cement sheath is completely absent.