Well workover, intervention and stimulation
After operating for some time, a well may become less productive or faulty
due to residue buildup, sand erosion, corrosion or reservoir clogging.
Well workover is the process of performing major maintenance on an oil or
gas well. This might include replacement of the tubing, a cleanup or new
completions, new perforations and various other maintenance works such as
the installation of gas lift mandrels, new packing, etc.
Through-tubing workover operation is work performed with special tools that
do not require the time-consuming full workover procedure involving
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.
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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 hydrochloric acid (HCL)
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.
Hydraulic fracturing is an essential technology for unconventional
shale gas and liquids extraction.
• Explosive fracturing uses explosives to fracture a formation. At the
moment of detonation, the explosion furnishes a source of highpressure 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.
