Page 51 - Petroleum and Gas Field Processing

temperature conditions, and have the advantage of containing all of the
debris that is left after perforating. However, because of their rigidity, they
would not go through sharp bends. For semiexpendable guns, the charges
are fixed to a flexible metal strip. After detonation, most of the debris is
expelled into the well, but the metal strip, any unfired charges, and the
electronic detonator are retrieved. The flexibility of the guns allows
movement through bends and highly deviated holes. Fully expendable
guns have advantages similar to those of semiexpendable guns. However,
because, after detonation, the gun breaks completely into pieces that are
left in the hole, there is no way to know whether all charges have been
fired.
To obtain clean and relatively undamaged perforations, the perforating
operation should be conducted with the bottom-hole pressure much less than
the reservoir pressure. Upon perforating, the differential pressure causes
fluids to surge from the formation into the well; this effectively cleans the
perforations from any debris. This method of perforating is known as
underbalance perforating. To perforate underbalance, the production tubing
must be installed in the well and the peforating mode is, therefore, identified
as the through-tubing perforating mode.
In the early days, large through-casing hollow steel carrier guns were
always needed to obtain large-diameter perforations with deep penetration.
This required that the tubing be out of the well, and in order to have
control over the well, perforating had to be conducted with the bottom-
hole pressure being higher than the reservoir pressure by a safe margin.
This mode of perforating overbalance did not offer an effective means for
cleaning the perforations of the debris. Another mode of perforating which
offers the opportunity to use large casing guns and perforating under-
balance is known as tubing-conveyed perforating. Recent technological
developments, however, have made it possible to obtain such desired
perforations using small-size charges on a semiexpendable gun.

Well Stimulation Operations

Well stimulation operations could be classified into two main categories:
matrix acidizing and formation fracturing. The objective of matrix acidizing is
to remove near-wellbore formation damage that might have been caused by
drilling or other workover operations in order to restore or enhance well
productivity. Formation fracturing, however, is conducted on formations
having very low permeability in order to increase well productivity.
For carbonate formation, matrix acidizing is achieved by injecting
hydrochloric acid (HCl) into the formation at low pressure. As acid is
injected, it preferentially flows through more permeable passages, reacting

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