Page 10 - Well Control for Completions and Interventions

Introduction and Well Control Fundamentals 5

1.1.2.1.3 Asphaltines
Asphaltines appear as hard deposits resembling an asphalt road surface.
They are organic solids that precipitate from crude oil systems, and are
most likely to occur at close to the oil bubble point pressure. Like scale
and wax, asphaltine precipitation causes a reduction in flow rate.
Asphaltines are difficult to remove from the wellbore, and normally
require mechanical removal or hydro-jetting using coiled tubing. They
can also be removed chemically using xylene or toluene.

1.1.2.1.4 Water and gas production
There are several sources of water ingress into oil producing wells. Water
from nearby injection wells can flow preferentially through high perme-
ability layers or fractures in the formation. Underlying aquifer water can
be drawn up towards the producing zone, a problem made worse by high
drawdown. Water will also find its way through channels in cement or
leaks in the completion. Similarly, overlying gas from the gas cap can be
drawn into the well. Water entry will reduce oil production, because of
increased hydrostatic head in the tubing and relative permeability changes
in the reservoir. Water production also brings corrosion, scaling, and in
many cases an increase in sand production.
Water breakthrough and water production are monitored by taking
flowline samples and conducting well tests. Logging can reveal where
water (or gas) is entering the well, essential information if the water is to
be isolated at source. Water and gas shut-off interventions are common,
employing several widely used techniques. These include mechanical
bridge plugs and straddles, casing patches, remedial cementing, and chem-
ical treatments.

1.1.2.2 Well stimulation
1.1.2.2.1 Hydraulic fracturing (Fracking)
Hydraulic fracturing of sandstone and shale formations can significantly
increase productivity from low permeability reservoirs. Fluid is pumped
down the wellbore at above the formation fracture pressure, and the
resulting fracture is packed with proppant to keep it open. Hydraulic frac-
turing is often performed through casing before the completion is
installed, as is normally the case for shale gas wells. However, many
through tubing fracking operations are carried out on existing wells to
improve inflow.