ABSTRACT: Wellbore stability and lost circulation problems cost companies millions of dollars in drilling downtime each year. However, these are costs that can be minimized by the proper planning in the pre-drill stages. This paper presents a case history of using a geomechanical model to optimize the casing design of a deviated wellbore and prevent problems that were experienced in offset wells of West Delta block 83, Gulf of Mexico (GOM).
Casing points and mud weights were adjusted based on the geomechanical model to meet the challenges of this specific well. Uncertainties in the geomechanical model were evaluated using quantitative risk assessment to determine the confidence level in different casing plans. The operator chose an aggressive casing design in order to reach the target sands with the optimum borehole size and a minimum number of casing strings. To mitigate the risk, a 7-inch contingency liner was added to the drilling plan and AFE. The well was drilled successfully, however, the aggressive casing design could not be achieved and the 7-inch liner was needed. The geomechanical model provided information critical to making informed decisions in the planning and drilling process and the well was completed successfully with minimal wellbore stability problems.
Traditional approaches to casing design and mud weight selection have typically been based on predicted pore pressures and empirically determined fracture gradients. The resulting mud weights and casing points are then adjusted based on drilling experience in offset wells, often with little understanding of the reasoning behind the adjustments. It is now commonly acknowledged that the adjustments are necessary because the mud weights required to prevent both wellbore failure and lost circulation are highly dependent on wellbore trajectory and the regional stress state. When the risk of wellbore stability problems or lost circulation events are likely, it is important to establish the limits of a safe operating mud window by utilizing a geomechanical model that incorporates the pore pressure, the stress magnitudes and orientations, the rock strength, and the well trajectory.
This paper presents a case history of using a geomechanical model to optimize the design of a deviated wellbore in West Delta block 83, GOM to prevent similar problems to those in offset wells. Data from two offset wells was used to develop and validate the geomechanical model. Casing points and mud weights were then adjusted based on the model to meet the challenges of this specific well. The upper bound mud weight was based on the least principal stress, and the lower bound mud weight was based on both the pore pressure and the mud pressure required to keep wellbore instability (breakouts) to an acceptable level. Uncertainties in the geomechanical model were then evaluated using quantitative risk assessment to determine the confidence level in different casing plans, which allowed the operator to make a risk-based decision on the final casing design. Based on the results, the operator chose an aggressive casing design in order to reach the target sands with the optimum borehole size and a minimum number of casing strings.