The ultra-deepwater fields in the Gulf of Mexico are among the largest producers discovered to date. However, the reservoirs are interbedded with highly depleted zones, with pressure differentials up to 11,000 psi. The development, testing, and application of wellbore stability modeling software that accurately characterizes fractures, determines the optimal lost circulation material (LCM) blend, and delivers reliable wellbore strengthening results in the problematic production zones are discussed.
Wellbore strengthening literature focuses on three fundamental areas: stress caging, fracture–closure stress, and resistance to fracture propagation. Aspects of these approaches were incorporated into a new modeling solution that was calibrated using historical data from nine offset wells. The modeled fracture width predictions were used to design lost circulation material (LCM) treatments with specific particle size distribution values. Each formulation underwent particle-plugging testing in the laboratory, followed by flow loop testing of the best performers for compatibility with downhole tools. The highly interactive process, which currently continues, resulted in successful field applications in similarly complex wells.
The new model allowed drilling personnel to identify the parameters most likely to induce fractures. Equivalent circulating density (ECD) had the most impact, followed by minimum horizontal stress, Young’s modulus, fracture length, and Poisson’s ratio. Using modeling outputs, LCM blends were engineered to plug fracture widths ranging from 1,500 to 2,000 microns, significantly wider than previous estimates. Field results indicated that an "extended" ECD margin could be obtained for severely depleted formations. The optimized LCM treatments were applied on two wells with narrow pore pressure/fracture gradient margins and on one well with a severely depleted reservoir (4,600 psi). All three were drilled with zero losses. On a fourth well, the modeled treatment was applied to the leak-off test at the 16-in casing shoe above the production zone. The operator expected a 0.4 to 0.5 lbm/gal increase at best; the actual increase was more than 1.0 lbm/gal. After this interval was drilled, a 14 in liner was set and cemented with zero losses. Such an increase had not been possible on offsets previously. Based on these successes under similar conditions, the operator is currently implementing the model to design wells with extreme depletion to be drilled during 2020.
Decades of deepwater experience have yielded numerous best practices for drilling in narrow margins and depleted zones. However, many wells still cannot be drilled without an assurance of effective wellbore strengthening. By removing the limitations of other wellbore strengthening approaches, the field-proven geomechanics modeling software presented in this paper creates a new standard for lost circulation prevention in depleted sands with 8,000 to 11,000 psi differentials.