Disproportionate permeability reduction

Disproportionate permeability reduction (DPR) is a phenomenon whereby many water-soluble polymers and many polymer gels reduce the permeability to water flow to a greater extent than to oil or gas flow. DPR is also referred to as relative permeability modification (RPM). However, some practitioners of this technology make the following subtle distinction. They tend to reserve the term DPR for relatively strong polymer gels that impart a large degree of disproportionate permeability reduction and a relatively large reduction in water permeability. These practitioners reserve the term RPM for systems, such as solutions of water-soluble polymers or relatively weak gels, that impart more subtle disproportionate permeability reduction and more subtle reductions in water permeability. As used in this page when referring to water-shutoff treatments, the terms DPR and RPM are synonymous.

Gels used with fractures

Most of the early work on, and application of, DPR involved fluid flow in reservoir matrix rock. More recently, water-shutoff chromium(III)-carboxylate/acrylamide-polymer (CC/AP) gels for use within fractures have been reported to impart DPR in gel-filled fractures. However, because these relatively strong fracture-problem gels significantly reduce simultaneously the permeability to oil flow in fractures, these gels are better characterized as total-fluid-flow-shutoff gels and not DPR water-shutoff gels.

Alternatively, DPR conformance improvement treatments, which involve relatively strong gels, can be successfully applied in hydraulically or naturally fractured reservoirs. In this case, the gel is placed and functions within the matrix rock that is adjacent to the fractures.

DPR application

DPR has the most value when used in water-shutoff/reduction treatments that are applied to production wells. DPR has little or no value for use during sweep-improvement treatments that are applied from the injection-well side.

A distinction needs to be made regarding classical and relatively strong (not DPR) polymer gel water shutoff (total fluid flow shutoff) treatments that are placed within fractures that surround production wells. Such gel treatments tend to be placed, or place themselves, so that they will selectively reside in the water producing fractures where the gels effectively block water flow. Such gel treatments (when properly designed and executed) tend to reduce water production without substantially reducing oil production. Here, the selective water shutoff results from selective placement of the gel in the water-producing fractures and not by the DPR mechanism.

The ability of acrylamide polymers to impart DPR to water flow in porous media was recognized as early as 1964 by Sandiford and in 1973 by White et al. The mechanism by which polymers and gels impart DPR and RPM effects has been the subject of a number of investigations. There is a lot of literature that is representative of these investigations. More recently, a set of plausible mechanisms have been proposed that explain how CC/AP polymer gels impart DPR.

The application of DPR conformance-improvement technologies for water-shutoff (and/or water-reduction) purposes is not a panacea. The successful application of bullheaded DPR long-term water-shutoff/reduction treatments, which involve radial flow in matrix reservoir (unfractured) rock and where the drawdown pressure on the producing interval is not increased after the gel treatment, is limited to when the following conditions are met:

• A conformance problem exists in a matrix rock reservoir involving differing geological strata.
• No fluid crossflow can occur within the reservoir between the water and the oil or gas producing geological strata.
• The water strata is producing at an undesirably high water cut, and the oil or gas strata will produce for the economic life of the water-shutoff treatment at 100% oil or gas cut.

Possible exceptions to these limitations are as follows. First, if the DPR treatment induces an increase in the drawdown pressure on the producing interval, the DPR treatment may promote increased oil production. Second, if the DPR treatment material in the presence of oil flow breaks down, or is otherwise inactivated (with respect to its water-blocking ability), then selective water shutoff can occur over a wider range of excessive water-production problems.

For applications in radial-flow matrix-reservoir rock, commercially available DPR water-shutoff/reduction treatments usually attempt to reduce the permeability to oil or gas flow by a factor of two or less in the treated reservoir volume and reduce the permeability to water flow by a factor on the order of ten or more. Variability in performance of these systems has often led to erratic results when trying to accomplish this objective. To date, commercially available DPR water-shutoff/reduction treatments for application in radial-flow matrix-rock reservoirs have been based, almost exclusively, on the use of either solutions of water-soluble polymers or relatively “weak” gels.

For application to fractured wells, a DPR scheme for treating excess water production is discussed in Gels. This scheme relies on placing a relatively strong DPR water-shutoff gel in the matrix rock that is adjacent to the fractures.

Issues with DPR water-shutoff treatments

DPR water-shutoff treatments are of no practical value [in terms of providing long-term (i.e., years) water shutoff] when applied to a single zone reservoir that is producing at a high water cut in the radial flow mode from a matrix rock reservoir. This is because a relative-permeability water block will form just beyond the outermost radial penetration of the DPR water-shutoff treatment. What is less obvious is that for the same basic reason when producing from matrix rock reservoirs in the radial flow mode, DPR water-shutoff treatments are not effective at promoting long-term water shutoff/reduction anytime the oil-producing zones are producing at a finite water cut or crossflow exists between the oil- and water-producing zones.

Another issue with DPR water-shutoff treatments is that for such treatments, which are based on the use of solutions of water-soluble polymers or relatively “weak” gels, their water-shutoff performance is erratic and not highly reproducible in both the laboratory and field settings. For DPR water-shutoff treatments, the restoration (or near restoration) of the oil permeability following the placement of the treatment in matrix reservoir rock can be quite slow. An important point regarding DPR water-shutoff treatments is that DPR imparted in the treated volume of the matrix reservoir rock does not necessarily correspond in the field to a proportionate reduction in the water production rate.

An additional important distinction that needs to be made is to whether the DPR treatment is being applied for long-term (i.e., years) or short-term (i.e., hours to months) water-shutoff purposes. For the relative-permeability water-block reason discussed previously, many DPR water-shutoff treatments will render short-term or transient water-shutoff/reduction during treatments that are applied in the field. The water-block problem goes a long way in explaining why so many DPR water-shutoff treatments have failed to provide long-term water shutoff and have tended to render only short-term water shutoff. There are scenarios for limited reservoir conditions, where DPR treatments that render only transient water shutoff, can be engineered to be economically attractive and profitable.

The reason that bullheadable DPR water-shutoff treatments have created such a great interest in the oil industry is that they do not require the use of mechanical zone isolation when applied to layered matrix-rock reservoirs. Mechanical zone isolation often requires costly workover operations. Use of mechanical zone isolation during water-shutoff-treatment placement is not normally feasible when the well possesses a slotted-liner or gravel-pack completion or when the well involves a subsea tieback flow line.

Historically, a large number of ineffective DPR (RPM) water-shutoff treatments have been conducted. The high failure rate of DPR water-shutoff/reduction treatments has resulted from a combination of overexpectations by operators regarding DPR water-shutoff treatments, overselling of DPR water-shutoff treatments by oilfield service companies, and failure to recognize the constraints to the successful application of DPR water-shutoff treatments within matrix-rock (unfractured) reservoirs. However, DPR treatments remain one of the few options available to successfully treat excessive water-production problems in matrix rock reservoirs for which mechanical zone isolation is not possible or practical during treatment fluid placement. Investigation, development, and exploitation of DPR conformance-improvement technologies has been actively pursued by petroleum industry R&D efforts.

Noteworthy papers in OnePetro

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External links

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See also

Conformance improvement

Polymer impact on permeability

PEH:Polymers,_Gels,_Foams,_and_Resins