More than 600 petroleum wells in the State have undergone stimulation activities of various types since 1958, with no observed or reportable adverse consequences.
In many rock formations, biological, chemical and/or physical processes alter the reservoir rock over geologic time. In some cases, the process partially or completely fills the rock’s openings, or pores, and reduces the ability of oil and gas to flow. These diagenetic processes can determine the commercial viability of a petroleum reservoir, because they control the volume and rate at which petroleum will flow up a wellbore. In circumstances where pore space is very restricted, some form of reservoir stimulation, defined as a treatment performed to restore or enhance the productivity of a well, may be beneficial in allowing hydrocarbons to flow.
Hydraulic fracturing, also known as hydraulic fracture stimulation, is the fracturing, or breaking, of a reservoir rock with pressurised liquid. The treatment creates artificial openings and cracks in the rock that help to increase its permeability, or ability to flow, and allow more petroleum to enter the wellbore. The process involves pumping fluid down a wellbore at a pressure that is greater than the rock strength of the formation. When the rock breaks or fractures, the injected fluid flows through it, continuing the fracture propagation. It is possible that multiple fractures will be created and developed in a tree root-like pattern, with increasing numbers of branches away from the injection point.
Fluid is injected until the fracture is wide enough to accept a solid, or propping agent, known as proppant. The purpose of the proppant is to keep the fracture open once the injection operation ceases. Either sand or man-made ceramic beads are used to hold or “prop” open the fractures.
In general, hydraulic fracture treatments are used to increase the rate at which oil or gas can be produced from a well. Since the first treatment in the US in 1947, hydraulic fracturing has become a common treatment for stimulating the productivity of oil and gas wells.
It is common for a Diagnostic Fracture Injection Test, or DFIT, to be conducted prior to a hydraulic fracture stimulation. This aids in the design of the stimulation and can help determine if a stimulation is viable.
Matrix acidising refers to one of two types of low volume stimulation processes where acid is injected into the well at pressures below formation fracture pressure. This process stimulates wells to improve flow or to remove damage. During matrix acidising, these acids dissolve part of the near-wellbore reservoir formation, as well as the solids within the pores that are inhibiting the permeability of the rock. A matrix treatment can also restore permeability by removing damage around the wellbore caused by drilling, from completing a well or by workover fluids and solids formed from produced water or oil (i.e., scale or paraffin). Matrix acidising can also improve productivity in both sandstone and carbonate reservoirs by enlarging natural pores, which stimulates the flow of hydrocarbons. The permeability increase is much larger in carbonate rock than in sandstone, because carbonates are made of sediments that completely dissolve in hydrochloric acid.
The second type of low volume stimulation is acid fracturing, where acid is pumped down a wellbore under pressures greater than formation fracture pressure. This process “etch”es channels in the rock walls of naturally occurring fractures to then flow back up the wellbore. Rock must be at least partially soluble in acid for channels to be etched, and acid fracturing is thus confined to carbonate reservoirs and never used to stimulate sandstones or shales. Acid fracturing is localised, with long etched fractures difficult to achieve because of rapid acid reaction with the formation. The most commonly used fluid in acid fracturing is 15% hydrochloric acid (HCl).
Carbon dioxide (CO2) may be used as an additive in acid fracturing because it is highly soluble in both oil and water. When carbon dioxide dissolves in water, it forms carbonic acid (H2CO3), which dissolves the matrix in carbonate rocks and causes a great reduction in viscosity, or resistance to flow.
The well list summarises wells drilled in Western Australia that have been subjected to some form of stimulation (data from petroleum well records held by the Department of Energy, Mines, Industry Regulation and Safety (DEMIRS)).
Relevant information
Find more information on recent activities involving high pressure, high volume hydraulic fracture stimulation.
