'Stimulation' is a generic term used to describe the process of improving a wells productivity through a specific type of operation, normally by fracturing the well or through a chemical process called 'acidizing'.
Both processes are technically challenging using large amounts of equipment, fluids and chemicals. In the case of hydraulic fracturing extreme pressures may be required (up to 15,000psi) and therefore equipment ratings must be suitable to withstand such pressures.
This method of stimulation is the process of pumping fluid (e.g. gel foam, slickwater) at sufficiently high pressures as to 'fracture' the well bore rock. The chosen fluid will also contain a 'proppant', a solid material, typically sand or man-made ceramics. The 'proppants' function is to hold the induced fracture channels open the after hydraulic pressure has been removed allowing hydrocarbons to flow through.
Wells which require hydraulic fracturing are generally of poor quality and low in permeability (tight formation) but results can be spectacular and well rates increased by a factor of 10 post 'frac' operations.
The 'proppant' used in hydraulic fracturing is usually a fine grade, ‘sized' to the reservoir conditions and post 'frac' cleanouts are normally required for the 'proppant' which did not enter the rock or, which was flowed back into the well after 'frac' pressure was removed.
The equipment used in hydraulic fracturing is largely surface mounted although 'frac' operations can be conducted through coiled tubing or a 'work string' key operating parameters for a successful 'frac' are: pressure, flow and proppant volume (quantity) To achieve this requires:
- High output pumps driven by large diesel engines (high horsepower)
- Large bore high pressure surface pumping and bleed off pipework
- Large volume storage, displacement, mixing and monitoring tanks for fluids, 'proppant' and chemicals.
- Deployed on pump trucks (land) or offshore supply vessels or specialised vessels.
This method of stimulation is generally used to improve or remove 'near wellbore' damage caused by drilling fluids, well fluids or other operational considerations. Although not as technically challenging as hydraulic fracturing work each job must still be planned and reviewed to ensure as successful outcome. The risks of further damaging the wellbore through poor deployment methods or incompatible chemicals or acid are great and therefore the reservoir conditions and the type of damage must be carefully assessed. Generally hydrochloric acid is the most commonly used chemical for this type of operation and is usually deployed as a 15% or 30% solution and designed for maximum 'contact' time to react suitably with the formation. It is particularly useful in chalk reservoirs and where calcium carbonate scale may have formed in the well bore.
The equipment used is surface mounted although acidizing operations can be conducted through coiled tubing or a 'work string'. Key operating parameters for a successful acidisation are: chemical selection & concentration, contact time (to formation). To achieve this requires:
- High volume pump units suitable for corrosive fluids
- Suitable pipework, pressure and fluid rated
- Large volume storage, displacement, mixing and monitoring tanks for fluids, and chemicals.
- Deployed on pump trucks (land) or offshore supply vessels or specialised vessels)