Abstract
If a kick is not detected and circulated properly out of the wellbore
with heavier mud weight, it leads to blowouts. In this case, reservoir
fluids gush out of the well uncontrollably without restriction leading
to loss of control. This may lead to fractures initiating in the
post-blowout capping stages, just below the casing shoe, propagating
upwards creating a channel through which reservoir fluids can flow to
the ocean floor. Being able to model these fracture failures will help
understand wellbore integrity problems from loss of control situations
and predict the possibility of broaching preventing many ecological
disasters like the 1969 Santa Barbara oil spill from Union Oil’s A-21
well. The hypothesis tested is that fracture initiation from a wellbore
in a loss of control situation can be predicted through analysis of the
near-wellbore stress field, with knowledge of the in-situ stress state
and the properties of the formation and the borehole assembly. A 3D
numerical model is employed to assess whether a fracture will initiate.
This is done by considering the stressfield at the casing shoe; the
point most vulnerable to tensile fracture failure downhole. In-situ
stress state, wellbore pressure, casing shoe depth and the casing,
cement, and formation’s mechanical properties are independent variables
that are shown to control fracture initiation; the dependent variable. A
reservoir model is used to predict pressure build-up during capping
procedures. A case study on Gulf of Mexico is presented with input
wellbore pressure data generated using a worst case discharge model.
Wellbore pressure drop during uncontrolled discharge from a well can
cause casing collapse failures and subsequently pressure build-up in the
post-blowout capping stage, may initiate fractures which can lead fluid
leakage to the surface either through the cement or the interfaces with
the casing and the formation. The region of the in-situ stress states
where fracture initiation will occur is shown in dimensionless plots.
This is useful for drilling and wellbore integrity teams. When targeting
highly-pressured formations as in deepwater, wellbore architecture must
be made with considerations of the wellbore pressures generated from
loss of control situations like blowouts. Research reported in this
publication was supported by an Early-Career Research Fellowship from
the Gulf Research Program of the National Academies of Sciences,
Engineering, and Medicine. The content is solely the responsibility of
the authors and does not necessarily represent the official views of the
Gulf Research Program of the National Academies of Sciences,
Engineering, and Medicine.