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During loss of well control events, fracture initiation occurring during the post-blowout capping stage following uncontrolled discharge, can lead to reservoir fluids broaching to the seafloor. A classic example is Union Oil's 1969 oil spill in Santa Barbara channel, where fracture initiation at various locations caused thousands of gallons per hour to broach in the ocean floor over a period of a month before it could be controlled (Mullineaux, 1970; Easton, 1972). The impacts on California's oil industry are still felt strongly today. Disasters as such could be prevented if the effects of the post-blowout loss of well control stages (uncontrolled discharge and capping) are incorporated into the shut-n procedures and the wellbore architecture. Analytical models are used to simulate the loads on the wellbore in the different stages during loss of control and predict capping pressure build-up during the shut-in to indicate fracture initiation during the capping stage. Using these models, critical capping pressure and subsequently critical discharge flowrates is calculated for a well below which fracture initiation would occur. A hypothetical case study with typical deepwater Gulf of Mexico parameters is performed demonstrating the likelihood of fracture initiation during different discharge flowrates, discharge periods and shut-in methods (abrupt/"hard" or multi-stage/"soft"). Further discussion addresses reservoir depletion during the discharge stage preceding the capping, as well as the conditions necessary for upward propagation of these fractures towards the seafloor. Through these fractures fluids from the reservoir ultimately broach into the seawater. The ability to model these fracture failures will enhance the understanding of wellbore integrity problems induced during loss of control situations and create workflows for predicting possible broaching happening during the post-blowout capping stage early on. Dimensionless plots are used to present fracture initiation for different scenarios useful for drilling and wellbore integrity engineers when making contingency plans for dealing with loss of well control situations.
During loss of well control events, fracture initiation occurring during the post-blowout capping stage following uncontrolled discharge, can lead to reservoir fluids broaching to the seafloor. A classic example is Union Oil's 1969 oil spill in Santa Barbara channel, where fracture initiation at various locations caused thousands of gallons per hour to broach in the ocean floor over a period of a month before it could be controlled (Mullineaux, 1970; Easton, 1972). The impacts on California's oil industry are still felt strongly today. Disasters as such could be prevented if the effects of the post-blowout loss of well control stages (uncontrolled discharge and capping) are incorporated into the shut-n procedures and the wellbore architecture. Analytical models are used to simulate the loads on the wellbore in the different stages during loss of control and predict capping pressure build-up during the shut-in to indicate fracture initiation during the capping stage. Using these models, critical capping pressure and subsequently critical discharge flowrates is calculated for a well below which fracture initiation would occur. A hypothetical case study with typical deepwater Gulf of Mexico parameters is performed demonstrating the likelihood of fracture initiation during different discharge flowrates, discharge periods and shut-in methods (abrupt/"hard" or multi-stage/"soft"). Further discussion addresses reservoir depletion during the discharge stage preceding the capping, as well as the conditions necessary for upward propagation of these fractures towards the seafloor. Through these fractures fluids from the reservoir ultimately broach into the seawater. The ability to model these fracture failures will enhance the understanding of wellbore integrity problems induced during loss of control situations and create workflows for predicting possible broaching happening during the post-blowout capping stage early on. Dimensionless plots are used to present fracture initiation for different scenarios useful for drilling and wellbore integrity engineers when making contingency plans for dealing with loss of well control situations.
Summary Following uncontrolled discharge during loss of well control events, fracture initiation occurring during the post-blowout capping stage can lead to reservoir fluids broaching to the seafloor. A classic example is Union Oil's 1969 oil spill in Santa Barbara Channel, where fracture initiation at various locations caused thousands of gallons per hour to broach onto the ocean floor over a month before it could be controlled (Mullineaux 1970; Easton 1972). Disasters such as these could be prevented if the effects of the post-blowout loss of well control stages (uncontrolled discharge and capping) are incorporated into the shut-in procedures, and the wellbore architectures are modified accordingly. In this study, analytical models are used to simulate the loads on the wellbore during the different stages of loss of control. Capping pressure buildup during the shut-in is modeled to indicate fracture initiation points during the capping stage. Using these models, the critical capping pressure for a well is determined, and subsequent critical discharge flow rates are calculated. Fracture initiation would occur if the actual discharge flow rate is below the calculated critical discharge flow rate. A hypothetical case study using typical deepwater Gulf of Mexico (GOM) parameters is performed demonstrating the likelihood of fracture initiation during different discharge flow rates, discharge periods, and capping stack shut-in methods (single-step/“abrupt” or multistep/“incremental”). An abrupt shut-in for this case study leads to fracture initiation at approximately 8 hours after shut-in, while a five-step incremental shut-in is shown to prevent any fracture initiation during the 48 hours after the beginning of the shut-in. Reservoir depletion through longer discharge periods or higher discharge flow rates, despite the adverse environmental effect, can delay or even prevent fracture initiations during post-blowoutcapping. The ability to model these fracture failures enhances the understanding of wellbore integrity problems induced during loss of control situations and helps create workflows for predicting possible broaching scenarios during the post-blowout capping stage. Dimensionless plots are used to present fracture initiation for different cases—this is useful for drilling and wellbore integrity engineers for making contingency plans for dealing with loss of well control situations.
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.
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