The constant bottom-hole pressure (CBHP) method of managed pressure drilling (MPD) maintains wellbore pressure above the wellbore stability or pore pressure and below the fracture pressure. It is common practice to perform frequent dynamic formation integrity/leakoff tests (FIT/LOT) to measure the fracture pressure. Several authors addressed the uncertainty in the measured value of the fracture pressure caused by mud compressibility and thixotropy. Moreover, field evidence indicates considerable variations in the effective fracture pressures resulting from varying wellbore temperatures.
This paper presents a mathematical model, validated with field data, to estimate the effective fracture pressure (EFP) 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 EFP associated with the uncertainties in the input paramaters. The method was demonstrated with two examples from the Gulf of Mexico. 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 EFP. Moreover, a log-normal distribution of the EFP 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-minutes pumps-off period.