Fill Removal in Wellbore Using Coiled Tubing

Coiled Tubing (CT) services are widely applied in oil field to remove fill from wellbore.
Its function is to restore the productivity of oil and gas well since fill such as sand will regularly
block the production line especially at the well bottom. Predicting the behaviors of cleaning fluid
and particle transports during the CT cleanout is a major challenge because there are many variables
which affect the cleaning operation resulting in loss of cleaning efficiency, waste of cleaning fluid
and increase in cost of well services. The objective of this study is to perform detailed analyses of
both flow pattern of cleaning fluids and particle transports in wellbore during cleaning process at
different cleaning operating conditions and well geometries. In order to achieve the objective, the
problem is broken down into two parts; namely the well string and the well bottom/annulus. The
well string, which is the coiled tubing itself, is first solved semi-analytically to obtain the exit
velocity of the flow nozzle and the accompanying surface pump operating conditions. Once the exit
velocity in the coil tubing is known, the value is used as an inlet velocity boundary condition for the
bottomhole and the well annulus for the subsequent CFD analysis. Simulating the cleaning process
along the entire well span is impractical due to limitation in computational resources. Hence, only a
limited section in the bottomhole and annulus were considered, where the calculated transport
properties there is sufficient to inform the likelihood of fill being circulated to the surface. The
present study identified that diameter ratio of CT and annulus, properties of the cleaning fluid,
design of downhole nozzle are the three most important factors influencing the cleanout. The result
of this study is a linearized CT parameters design chart that allows user to plan for cleaning
operation.