The present disclosure relates to gellable treatment fluids, and, more specifically, to gellable treatment fluids and methods associated therewith, where the gel times and gel strengths of the treatment fluids may be modified by using a polymer mixture containing two or more polymers having disparate average molecular weights.
Water can often undesirably accompany the production of oil and gas from a well penetrating a subterranean formation. The unwanted production of water from hydrocarbon-producing wells can constitute a considerable technical problem and expense in oilfield operations. If the ratio of produced-water to produced-oil and gas becomes sufficiently large, the cost of separating the water and disposing of it can become a financial barrier to continued production. This can lead to abandonment of a well penetrating a subterranean formation, even when significant amounts of hydrocarbons remain therein.
In a subterranean formation, water's high mobility often allows it to flow to or from a wellbore by way of natural and manmade fractures, high permeability zones, and the like. In such cases, less permeable zones in the formation can be bypassed. The bypassing of less permeable zones can be especially problematic when an aqueous fluid is introduced into the subterranean formation, particularly for the purpose of treating the subterranean formation. For example, in enhanced oil recovery techniques, an aqueous fluid can be introduced into a subterranean formation during water flooding operations. When less permeable zones are present in the subterranean formation, lower oil and gas production can occur from these zones due to a less effective water flooding operation being realized. Likewise, the presence of natural and manmade fractures, high permeability zones, and the like can also pose problems when an aqueous fluid needs to be introduced into a low permeability zone for purposes other than flooding operations. Illustrative situations in which permeation of a lower permeability zone by an aqueous fluid may be desirable include, but are not limited to, stimulation treatments and near-wellbore cleanup operations. In such cases, an aqueous fluid can preferentially enter high permeability zones and bypass the intended target low permeability zone, thereby leading to fluid loss.
One way in which the foregoing problems can be addressed is through conformance control treatments, whereby high permeability zones become fully or partially blocked to fluid flow. In the case of unwanted water production, blockage of water-producing zones can slow or stop the production of water. In the case of water flooding operations, blockage of high permeability zones can enable oil and gas production to take place from low permeability zones that might otherwise be bypassed. In the case of stimulation and near-wellbore cleanup operations, blockage of high permeability zones can allow diversion of a stimulation fluid (e.g., an acid) or well bore cleanup fluid to a low permeability zone.
Conformance control treatments can involve introducing a gellable polymer system into a subterranean formation via an aqueous treatment fluid. The gellable polymer system can form a gel through crosslinking a water-soluble polymer with a crosslinking agent. A number of different crosslinking agents can be used to crosslink water-soluble polymers in a gellable polymer system. Chromium and other transition metal ions can be used to crosslink acrylate polymers and copolymers (e.g., polyacrylamides, partially hydrolyzed polyacrylamides, and acrylamide/acrylate copolymers). Generally, gels formed using such crosslinking agents have proven unsuitable at higher formation temperatures (e.g., above about 175° F.) due to uncontrolled crosslinking rates (e.g., short gel times), crosslinking agent precipitation, polymer degradation, and the like. In addition, chromium and certain other transition metal ions can have an undesirable environmental impact. Acrylamide-containing polymers, copolymers, and partially hydrolyzed variants thereof can also be gelled with polyalkyleneimines and polyalkylenepolyamines.
The gel time and the gel strength of a gellable polymer system are among the factors that can determine the effectiveness of a conformance control treatment. As used herein, the term “gel time” will refer to the time required for a gellable polymer system to convert from a free flowing polymer fluid into a semi-solid substance that has viscoelastic properties. These viscoelastic properties can be determined using standard rheological characterization techniques that will be well known to one having ordinary skill in the art. As used herein, the term “gel strength” refers to the rheology of the gel. If the gel time is too short, introduction or placement of the gellable polymer system into a subterranean formation can prove problematic. Conversely, if the gel time is too long, the gellable polymer system may not form a gel in the desired portion of the subterranean formation, or long periods of downtime may be required before further treatment or production operations can be carried out.
The gel time of a gellable polymer system can generally be modified by changing the amount of the gellable polymer and/or the crosslinking agent. As the concentration of either component increases, the gel time can oftentimes be reduced. Although a shorter gel time can be desirable in many instances, as noted above, a difficulty with this approach is that treatment fluids having higher concentrations of polymer may be overly viscous and difficult to pump into a subterranean formation. Gel time modifiers can be used, if desired, to increase or decrease the gel time if the gellable polymer system's native gel time is unsuitable for a given application.
The molecular weight of the polymer can also have an impact on the gel strength and the viscosity of the treatment fluid. In some downhole applications, it may be more desirable to form stiff, ringing gels having a high gel strength upon crosslinking. In other instances, it may be more desirable to form deformable, lipping gels having a lower gel strength. Properties of such gels will be familiar to one having ordinary skill in the art and are described in more detail below. Lower molecular weight polymers may be preferable for the formation of stiff, ringing gels, albeit at higher polymer loadings. At higher polymer loadings, the viscosity of the treatment fluid can become undesirably high, and the cost of goods can become prohibitive. Higher molecular weight polymers can be gelled at lower concentrations, but the treatment fluid viscosity can again be undesirably high, and the gel strength can be reduced due to formation of a deformable, lipping gel by the higher molecular weight polymer. In addition to influencing the gel strength, the molecular weight of the polymer may also alter the gel time of a gellable polymer system.
In addition, the gel time of a gellable polymer system can be a function of temperature. Generally, at higher formation temperatures, the gel time can be reduced. The polymer concentration in a treatment fluid can be changed in response to the formation temperature, but the gel strength may become poor if the polymer concentration has to be lowered too much. The polymer itself may be replaced with another polymer in order to modify the gel time and/or gel stability, possibly in response to the formation temperature. None of the foregoing approaches, however, offer the opportunity to readily modify the gel strength without undesirably affecting the gel time or other property of a treatment fluid, thereby impacting process efficiency.
In addition to the conformance treatments described above, gellable polymer systems can also find use in other types of treatment operations, particularly those that utilize a particle suspension in performing the treatment operation. Among the treatment operations that can utilize a gellable polymer system in conjunction with the treatment include, for example, fracturing operations, gravel packing operations, and stimulation operations, such as acidizing operations. Again, using presently available approaches, there is no simple way to modify the gel strength of the gellable polymer system without undesirably affecting the gel time or other related property of the treatment fluid.