Stimulation refers to the treatment of geological formations to improve the recovery of hydrocarbons. Common stimulation techniques include well fracturing and acidizing operations.
Fracturing of the formation is accomplished by pumping fluids into the borehole of an oil or gas well under high pressure to create fractures in the rock formation surrounding the wellbore. The fractures radiate outwardly from the wellbore, typically from a few to hundreds of meters, and extend the surface area from which oil or gas drains into the well.
Typically, the fracturing fluid is a crosslinked polymer that has been gelled to increase its viscosity. Crosslinked gels are able to withstand the high temperature conditions commonly found in deeper oil and gas wells with little reduction in viscosity. After the fracturing fluid is injected into the formation to produce the fracture, the viscosity of the fluid is reduced by gel breakers. Such components break down the gelled fluid so that it can be easily pumped and removed from the well. Often, breaker catalysts are used to activate the gel breaker.
In certain formations, aqueous acid solutions can be used to improve the permeability of the formation, thereby increasing production. These acids are often combined with the polymer gels used in fracturing to provide an acid fracturing fluid. One of the benefits of combining the aqueous acid solutions with gelled fracturing fluids is that the gelled fluid inhibits or retards the reaction of the acid with the formation. This is beneficial in that the acid would otherwise react too quickly, depleting the acid with very little penetration of the formation. Once in place, the viscosity of the fluid is reduced so that the acid is released to react with formation damage or other skin present at the face of the newly formed fractures and improving the permeability of the producing strata.
In addition to fracturing and acidizing operations, stimulation further encompasses techniques for enhancing oil recovery as well as water control treatments. In the latter, the flow of excess water is prevented while the flow of produced hydrocarbons from the well is enhanced. For instance, U.S. Pat. No. 5,228,812 and U.S. Patent Application No. US2004/0177957A1 disclose a chemical treatment that selectively reduces water production by the use of relative permeability modifiers (RPMs). RPMs are generally water soluble polymers having a charged end which assists in binding of the RPM to the geological rock formation.
Stimulation methods often include the use of spearhead fluids that are typically composed of an aqueous base fluid and chemical additives. Spearhead fluids precede the introduction of stimulation fluids. Spearhead fluids, for instance, may precede the introduction of RPMs. The spearhead fluid thereby removes heavy oil deposits in the near wellbore and oil films in the pore spaces of the rock. The RPM is then introduced and is capable of more strongly adhering to the rock.
In contrast to stimulation, remediation refers to the removal of unwanted deposits from the wellbore and production equipment. Remediation includes hydrogen sulfide mitigation. Such unwanted deposits form and/or accumulate in the wellbore, production and recovery equipment and well casing. For example, highly toxic hydrogen sulfide is produced during the decomposition of organic matter. In addition to hydrogen sulfide, other undesirable downhole products form such as scale, paraffins, fines, pipe dope, sulfur, heavy oil tar by-products and water blocks. Such accumulated deposits affect productivity and are typically removed prior to cementing or the introduction of completion fluids into the wellbore. Remediation treatment fluids are further typically used to remove such undesired deposits prior to the introduction of stimulation fluids.
Upon the completion of drilling and cementing operations, it is often desired to displace the drilling fluid system with a completion fluid system. Completion often requires the use of a packer fluid in the annulus between the casing and the production tubing. Well completion designs and reservoir conditions govern acceptable levels of particle content in packer fluids. The required cleanliness of this fluid can vary from several hundred Nephelometric Turbidity Units (NTU) down to 10–20 NTU.
Displacement of a drilling fluid with a completion fluid may be performed in several steps. Typically, the drilling fluid is first displaced by a casing cleaning fluid. Treatment with seawater normally follows. The seawater may then be displaced by a packer fluid. Complete displacement of a fluid by another fluid by use of hydraulic force is difficult to achieve in the annulus. In order to more efficiently displace the drilling fluid with a packer fluid, cleaning fluids with a variety of chemical additives are pumped between the drilling fluid and the packer fluid. The chemical additives serve to disperse and break-up solid components within the drilling fluid. In addition, they serve to decrease surface activity between the two fluids.
Well treatment fluids presently used for stimulation and remediation as well as fluids for removing drilling muds from the wellbore, prior to cementing or introduction of a completion brine, are either not biodegradable or are less efficacious than desired. There is a continued need for more effective methods and systems for drilling fluid displacement, enhancement of oil recovery, wellbore remediation and formation stimulation. In particular, there is a need for new systems that are biodegradable.