Well stimulation
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Well stimulation is a well intervention performed on an oil or gas well to increase production by improving the flow of hydrocarbons from the drainage area into the well bore.
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[edit] Cleaning the formation
The assortment of chemicals pumped down the well during drilling and completion can often cause damage to the surrounding formation by entering the reservoir rock and blocking the pore throats (the channels in the rock throughout which the reservoir fluids flow). Similarly, the act of perforating can have a similar effect by jetting debris into the perforation channels. Both these situations reduce the permeability in the near well bore area and so reduce the flow of fluids into the well bore.
A simple and cheap solution is to pump chemicals from surface into the well to dissolve the offending material. Once dissolved, permeability should be restored and the reservoir fluids will flow into the well bore, cleaning up what is left of the damaging material. After initial completion, it is common to use concentrated formic acid to clean up any mud and skin damage. In this situation, the process is called acidising.
In more serious cases, pumping from surface is insufficient as it does not target any particular location downhole and reduces the chances of the chemical retaining its effectiveness when it gets there. In these cases, it is necessary to spot the chemical directly at its target through the use of coiled tubing. Coiled tubing is run in hole with a jetting tool on the end. When the tool is at its target, the chemical is pumping through the pipe and it jetted directly onto the damaged area. This can be more effective than pumping from surface, though it is much more expensive, and accuracy is dependent on knowing the location of the damage.
[edit] Extending the perforation tunnels and fractures
In cased hole completions, perforations are intended to increase the surface area for reservoir fluids to enter the well bore by creating small tunnels extending out from the well into the reservoir rock (similar to villi in the small intestine). In many cases, the tunnels created by the perforation guns do not provide optimal surface area and it becomes desirable to extend them.
This could be because damage caused by drilling and completion operations was so severe it irreperably reduced the permeability of the surrounding rock. This means that the ability of fluids to flow into the existing tunnels is too limited. The only way to get around this is to extend the perforation tunnels further beyond the damaged zone where fluids have easier access.
Alternatively, this could be because natural decline in a tight reservoir after many months or years of production. If permeability is naturally low, then as fluid is drained from the immediate area, replacement fluid may not flow into the void sufficiently quickly to make up for the voidage and so the pressure drops. If the pressure drops, then the well cannot flow at such a high rate. In this case, extending the perforation tunnels deeper into the reservoir will allow them to reach less drained parts.
The act of extending perforation tunnels is called fracturing or commonly 'fracing'. It is performed by releasing high pressure fluids into the perforation tunnels to cause the formation to fracture. This can either be done by injecting hydraulic fluid from surface, a process called hydraulic fracturing or using an explosive to generate a high speed gas flow, a process called propellent stimulation.
[edit] Lifting the well
Some stimulation techniques do not necessarily mean altering the permeability outside the well bore. Sometimes they involve making it easier for fluids to flow up the well bore having already entered. Gas lift is sometimes considered a form of stimulation, particularly when it is only used for starting up the well and shut off during steady state operation. More commonly though, lifting as a stimulation refers to trying to lift out heavy liquids that have accumulated at the bottom, either through water entry from the formation or through chemicals injected from surface such as scale inhibitors and methanol (hydrate inhibitor). These liquids sit at the bottom of the well as can act as a weight holding back the flow of reservoir fluids, essentially acting to kill the well. They can be removed by circulating nitrogen using coiled tubing.
