Page 421 - Hydrocarbon Exploration and Production Second Edition

408 Workover Activity

scale
leaking
packer

formation
damage damaged
tubing

crossflow

perforation
damage oil
sand
water
stuck tool

Figure 17.4 Aworkover candidate.
safety reasons. Where damage is not so severe, it may be possible to use ‘through
tubing’ techniques to install a tubing patch or plug, on wireline or coiled tubing – both
cheaper options.
Formation damage is usually caused by pore throat plugging. It may be a result of
fine particles such as mud solids, cement particles or corrosion products invading the
formation. It can also be caused by emulsion blocking or chemical precipitation.
Impairment can sometimes be bypassed by deep perforating or fracturing through
the damaged layer, or removed by treatment with acids. Acid treatment can be
performed directly through production tubing or by using coiled tubing to place
the acid more carefully (Figure 17.5).
Normally acid would be allowed to soak for some time and then back-produced
if possible along with the impairing products. One of the advantages of using coiled
tubing is that it can be inserted against wellhead pressure so the well does not have
to be killed, a potentially damaging activity.
Coiled tubing can also be used to remove sand bridges and scale. Sometimes
simple jetting and washing will suffice, and in more difficult cases an acid soak may be
required. For very consolidated sand and massive scale deposits, a small fluid-driven
drilling sub can be attached to the coiled tubing. In extreme cases, the production
tubing has to be removed and the casing drilled out. Coiled tubing drilling (CTD) is
explained in Section 4.5, Chapter 4.
When only small amounts of sand, wax or scale are experienced, the situation
can often be contained using wireline bailers and scrapers, run as part of a well
maintenance programme.