| dc.description.abstract | As more and more wells reach the end of their production life, the focus on permanent
plug and abandonment has increased in interest. Cost-efficient abandonment of wells
with ceased production is an important economic goal for the oil and gas industry. A
dominant part of the plug and abandonment operation is the removal of steel tubular
and casing to establish a rock-to-rock cross-sectional barrier in the well. This process is
aggravated by settled barite and other mud solids accumulated at the bottom of the casing
annulus, increasing over-pull and resulting in several cut and pull runs. If the settled
barite, which is already in place behind the casing, could function as a part of a barrier
envelope, it could significantly reduce such operations.
This thesis’s primary objective is to investigate if industrial field data support the utilization
of settled barite as a feasible annulus barrier element. A total of 307 wellbores
were analysed for cut and pull operations, where attempts to circulate settled barite out
of the annulus were performed. A three-layered model was suggested using theories of
barite segregation and settling regimes in drilling mud. The model was used to calculate
the hydrostatic pressure of an annulus column of drilling mud settlements to accurately
predict the differential pressure excreted through the settled barite plug. Twenty-two of
the investigated wellbores showed potential for further analysis, and four wells displayed
plugs of settled barite that would prevent fluids from a re-pressurised reservoir to flow
unintentionally to the surface or other formations. | |
