Geomechanical Implications of Reservoir Depletion vis-à-vis Post-Blowout Well Capping: A Gulf of Mexico Case Study

Reservoir depletion can impose major implications on wellbore integrity following blowouts. A loss-of-well-control event can lead to prolonged post-blowout discharge from the wellbore causing considerable reservoir depletion in a well's drainage area. Fractures initiated and propagated during well capping procedures following an offshore blowout can lead to reservoir hydrocarbons broaching the seafloor. In this paper, reservoir depletion is examined for a case study on actual deepwater Gulf of Mexico (GoM) parameters, evaluating analytically its impacts on in-situ reservoir conditions, hence assessing the likelihood of longitudinal or transverse fracture initiation during post-blowout well capping. The reservoir rock is modeled as a porous-permeable medium, considering fluid infiltration from the pressurized wellbore. A novel analytical workflow is presented, which encompasses the major effects of reservoir depletion on the (i) in-situ stress state, (ii) range of in-situ stress states stable against shear fault slippage, and (iii) limits of tensile fracture initiation. The geomechanical implications of each individual effect on post-blowout well capping is discussed with the individual results illustrated and analyzed altogether on dimensionless plots. These plots are useful for engineers when making contingency plans for dealing with loss-of-well-control situations. The workflow is demonstrated on a case study on parameters taken from the M56 reservoir, where the April 20, 2010 blowout took place at the MC 252-1 "Macondo" well. A smaller post-blowout discharge flowrate is shown to increase the shut-in wellbore pressure build-up at any given time-point following well capping, whereas an increased post-blowout discharge period leads to a lower shut-in wellbore pressure build-up, hence reducing the likelihood of a fracture initiation scenario and vice versa. Assuming a robust wellbore architecture, the most likely location of fracture initiation is the top of the M56 reservoir within the openhole section of the Macondo well. The critical discharge flowrate, an established indicator for fracture initiation during well capping using information from the post-blowout discharge stage is employed, pointing that fracture initiation is highly-unlikely for the assessed parameters. Nevertheless, fracture initation during post-blowout well capping remains a real possibility in the overpressurized, stacked sequences of the GoM. Finally, the model is extended to an "incremental"/multi-step capping stack shut-in imposed over a longer time-period (e.g. 1 day than abruptly over a single-step) to suppress the wellbore pressure build-up, if necessary to avoid fracture initiation.