Page 9 - Well Control for Completions and Interventions

4 Well Control for Completions and Interventions

1.1.2.1 Production decline
If production from a well is in decline, the immediate task is to determine
the reason why. It may simply be the result of declining reservoir pressure
and in line with expectations. However, a declining rate can also be an
early indication of a problem in the reservoir or the wellbore. Diagnostic
interventions are often performed to try and establish the nature and loca-
tion of flow restrictions. There are several common production related
problems that are routinely managed by intervening in, or working over
a well.

1.1.2.1.1 Scale precipitation
Scale precipitation in the formation or perforation tunnels will increase
the skin factor and reduce productivity. Scale forming in the wellbore
will reduce the tubing ID, and consequently reduce production by chok-
ing the well. The two most common oilfield scales are calcium carbonate
(CaCO 3 ) and barium sulfate (BaSO 4 ).
Increasing temperature and reducing pressure promote the precipita-
tion of calcium carbonate scale, so it tends to form high up in the
tubing where pressure is low. Several intervention techniques are used
to manage calcium carbonate scale. It can be prevented or slowed by
bullheading a large volume of scale inhibitor into the producing forma-
tion. Where already formed, it can be dissolved using hydrochloric
acid (HCl). This is normally accomplished using coiled tubing. Small
deposits of soft scale can be removed using various wireline deployed
tools.
Sulfate scale forms when sulfates, present in some injection water, mix
with barium ions in formation water. Consequently, sulfate scale can
form anywhere in the producing system, from the reservoir to the process
facilities. Unlike carbonate scale, it cannot easily be removed using che-
micals. It is normally removed from the wellbore using coiled tubing
deployed scale mills or high-pressure jetting.

1.1.2.1.2 Wax deposits
Wax is a long chain alkane hydrocarbon that solidifies at relatively low
temperatures. Wax accumulation in the tubing causes a decline in flow
rate. It can be removed by circulating hot fluid (hot oiling), mechanical
removal using wireline or coiled tubing, or chemical solvents such as
xylene or toluene.