Well workover, intervention and stimulation

After operating for some time, a well may become less productive or faulty due to residue build-up, 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 clean-up 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.

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.

v  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 litres 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.

v  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, aluminium 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.

v  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.

v  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.