Probabilistic Assessment of the Temperature-Induced Effective Fracture Pressures

Summary The constant-bottomhole-pressure (CBHP) method of managed-pressure drilling (MPD) maintains wellbore pressure above the wellbore stability or pore pressure and below the fracture pressure (FP). It is common practice to perform frequent dynamic formation-integrity/leakoff tests (FIT/LOTs) to measure the FP. Several authors addressed the uncertainty in the measured value of the FP caused by mud compressibility and thixotropy. Moreover, field evidence indicates considerable variations in the effective FPs resulting from varying wellbore temperatures. This paper presents a mathematical model, validated with field data, to estimate the true FP from the leakoff-test data. The model accounts for the effect of mud compressibility and thixotropy, and considers the effect of transient wellbore temperatures on the geomechanical rock stresses. The study also presents application of quantitative risk assessment (QRA) to represent the probability-density distribution of FP associated with the uncertainties in the input parameters. The method was demonstrated with two examples from the Gulf of Mexico (GOM). The study shows that the operational parameter “pumps-off” time and two formation properties—Young's modulus of elasticity (E) and thermal-expansion coefficient (αT)—contribute most to the uncertainty in FP. Moreover, a log-normal distribution of the FP indicated a strong effect of temperature variation. It is also concluded that the uncertainty resulting from the temperature effect could be minimized by conducting the test after a characteristic 60-minute pumps-off period.