Page 39 - Oil and Gas Production Handbook An Introduction to Oil and Gas Production

Through-tubing workover operation is work performed with special tools that
do not require the time-consuming full workover procedure including
replacement or removal of tubing. Well maintenance without killing the well
and performing full workover is time-saving and often called well
intervention. Various operations that are performed by lowering instruments
or tools on a wire into the well are called wireline operations.

Work on the reservoir such as chemical injection, acid treatment, heating etc
is referred to as reservoir stimulation. Stimulation serves to correct various
forms of structure damage and improve flow. Damage is a generic term for
accumulation of particles and fluids that block fractures and pores and limit
reservoir permeability.

• Acids, such as HCL (Hydrochloric Acid) are used to open up
calcareous reservoirs and to treat accumulation of calcium
carbonates in the reservoir structure around the well. Several
hundred liters of acid (typically 15% solution in water) are pumped
into the well under pressure to increase permeability of the
formation. When the pressure is high enough to open the fractures,
the process is called fracture acidizing. If the pressure is lower, it is
called matrix acidizing.

• Hydraulic fracturing is an operation in which a specially blended
liquid is pumped down a well and into a formation under pressure
high enough to cause the formation to crack open, forming passages
through which oil can flow into the well bore. Sand grains, aluminum
pellets, walnut shells, glass beads, or similar materials (propping
agents) are carried in suspension by this fluid into the fractures.
When the pressure is released at the surface, the fractures partially
close on the propping agents, leaving channels for oil to flow through
to the well. The fracture channels may be up to 100 meters long.
• Explosive fracturing uses explosives to fracture a formation. At the
moment of detonation, the explosion furnishes a source of high-
pressure gas to force fluid into the formation. The rubble prevents
fracture healing, making the use of propping agents unnecessary.
• Damage removal refers to other forms of removing formation
damage, such as flushing out of drill fluids.

Flexible coiled tubing can be wound around a large diameter drum and
inserted or removed much quicker than tubing installed from rigid pipe
segments. Well workover equipment including coiled tubing is often mounted
on well workover rigs.

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