Casing Reliability Evaluation of HTHP Wells Via Uncertainty and Stress-Strength Interference Theories

Casing reliability is essential for the high-temperature high-pressure (HTHP) wells. The complicated downhole geological environment, high casing external load during the drilling stage, and annulus pressure in the production stage may all lead to casing collapse and wellbore integrity failure. Thus, it remains a hot topic to quantitatively evaluate the casing reliability throughout the whole life cycle (WLC) of HTHP wells. In this paper, a WLC quantitative evaluation method of casing reliability of HTHP wells in the whole borehole section is established based on the Monte Carlo simulation and the stress-strength interference theory. More specifically, WLC casing load is calculated by integrating the annulus pressure, casing load under non-uniform in situ stress, and the drilling extreme conditions. The casing strength calculation model of HTHP wells is then established by the K-T formula, while considering the temperature effect on casing strength to compute the casing reliability. The proposed method is applied in an HTHP gas well in the South China Sea. The results indicate that, under the hollowing degrees with allowable safety factors, there is still some risk in the casing running and lost circulation conditions. The failure risk at the weak points of each spud is within 0.08-0.2 in the casing running condition and within 0.1-0.5 in the lost circulation condition. For the production stage, before and after annulus pressurization, the overall trend of the casing safety factor of anti-internal-pressure remains the same and decreases with increasing well depth. The weak points exist at the inner cement surface or boundary point of different wall thickness casings. The risk of casing extrusion after annulus pressurization increases with decreasing well depth. Due to the lack of consideration of annulus pressure and uncertainty in the conventional casing-strength design methods, there is a possible failure risk at the casing weak points after annulus pressurization, where the reliability at the weak points on both side casings of annulus C is within 0.55-0.67 under annulus pressure 19MPa. The research verifies that the casing strength design should leave a margin in the HTHP environment to avoid high temperatures reducing reliability. Under the leak-prone strata condition, the casing strength at weak points should be strengthened accordingly. The casing-strength design considering annulus pressure and uncertainty will improve the casing reliability. In addition, this method can also be used to calculate the maximum allowable annulus pressure and the maximum allowable hollowing degree under the existing production and drilling plans, thus helping to optimize production and casing strength design.