Conformance improvement

Conformance is a measure of the uniformity of the flood front of the injected drive fluid during an oil recovery flooding operation and the uniformity vertically and areally of the flood front as it is being propagated through an oil reservoir. This page provides an overview of selected chemical systems and technologies that promote improved conformance during oil recovery operations. See Conformance problems for a discussion of the underlying problems creating the need for conformance improvement.

Overview

Conformance improvement systems and technologies include fluid systems for use during oil recovery flooding operations in which the fluids promote sweep improvement and mobility control (e.g., polymer waterflooding) and oilfield conformance improvement treatment systems (e.g., “small-volume” gel treatments). A conformance improvement fluid system for promoting flood sweep improvement and mobility control involves injecting a volume of an oil recovery fluid that constitutes a significant fraction of the reservoir pore volume. The volume of an oil recovery flooding system that is applied for sweep improvement is usually greater than 5% of the reservoir and/or well-pattern pore volume. Conformance improvement treatment systems normally are of a relatively small volume and usually are used to treat the near-wellbore region or a relatively small fracture volume within the reservoir.

If there were perfect conformance in a perfect regular five-spot well pattern during an oil recovery flooding operation, the flood front would reach all four of the offset producers at the same time, and the flood front would reach the entire vertical interval of all four of the producing wells at the same time. Of course, there never has been a reservoir that has exhibited perfect conformance during an oil recovery flooding operation. The issues that must be considered are how imperfect is the conformance for a given flooding operation in an oil field, and what is the economic or other beneficial rate of return if a conformance improvement flood or treatment is implemented.

Improved conformance during an oil recovery operation will result in incremental and/or accelerated oil production and/or will result in reduced oil production operating costs. Properly designed and executed conformance improvement flooding or treatments will improve the effectiveness, efficiency, and profitability of an oil recovery operation, regardless of whether the oil-recovery operation is primary production, secondary waterflooding, or tertiary-mode flooding.

As originally and purely defined, conformance improvement flooding operations and treatments involve improving the uniformity of the flood front of an injected drive fluid during an oil-recovery flooding operation. In addition to the original definition, the working definition of conformance improvement treatments now includes treatments applied to production wells to shut off excessive, deleterious, and competing co-production of water or gas coming from a source other than the producing oil formation interval. Examples of such co-production are the coning of water from an underlying aquifer and the coning of gas from an overlying gas cap. Thus, using this expanded working definition of conformance improvement treatments, conformance treatments can include water or gas shutoff treatments that are applied to production wells during primary oil recovery operations. See Conformance problems Kabir presents a high-level overview of the use of polymers, gels, foams, and resins as water and gas shutoff treatments.

The vast majority of conformance improvement treatments function by reducing the permeability and fluid-flow capacity of the offending and treated reservoir high-permeability flow paths, channels, and conduits. See also Disproportionate permeability reduction.

Enhancing sweep efficiency

Improving conformance, in its original and most limited definition, is synonymous with improving the drive-fluid sweep efficiency during an oil recovery flooding operation. Improving the conformance and/or sweep efficiency for any given oil recovery drive fluid during a reservoir flooding operation involves improving one, or both, of two components of flood sweep efficiency: vertical and areal sweep efficiency.

The volumetric sweep efficiency of a given oil recovery drive fluid during a flooding operation within a reservoir or well pattern is defined as

....................(1)

where E_A is the areal sweep efficiency, and E_I is the vertical sweep efficiency. For Eq. 1 to be strictly correct, the geological layers or strata of the reservoir must be uniform in terms of porosity, thickness, and oil saturation.

Strictly speaking for a real reservoir, volumetric sweep efficiency is more precisely defined as

....................(2)

where E_P is the pattern sweep efficiency, which is the areal sweep efficiency for a reservoir with variations in thickness, porosity, and oil saturation. Areal sweep efficiency is defined as

....................(3)

where A_f is the area contacted by the oil-recovery displacement fluid, and At is the total reservoir area under consideration.

Vertical sweep efficiency is defined as

....................(4)

where A_V is the reservoir vertical cross section contacted by the oil-recovery displacement fluid, and A_tV is the total reservoir vertical cross section.

For any given oil recovery drive/displacement fluid, poor sweep efficiency often results primarily from spatial variation and/or heterogeneity in the permeability (fluid flow capacity) of the reservoir rock. Poor vertical conformance and poor vertical sweep efficiency in matrix rock (unfractured) reservoirs usually result primarily from geological strata of differing permeability overlying one another in a reservoir. Conformance treatments to improve poor vertical sweep profiles and/or to shut off competing water or gas production, emanating from a subset of geological strata, are referred to as profile modification treatments. The Dykstra-Parsons coefficient is a widely used measure of the vertical permeability heterogeneity of an oil producing reservoir and is discussed in Lake.

For any given oil reservoir, poor sweep efficiency that results from flooding with an oil recovery drive fluid is aggravated as the viscosity of the drive fluid decreases. Within any reservoir with a given degree of permeability heterogeneity, as the viscosity of the drive/displacement fluid of an oil-recovery flooding operation decreases, the degree of viscous fingering and the associated poor sweep efficiency increases. The mathematical and engineering term that relates the viscosity of the oil-recovery drive fluid to conformance and sweep efficiency is “mobility ratio.”

Mobility ratio is defined as

....................(5)

where λ_D is the mobility of the oil-recovery displacement fluid phase, and λ_d is the mobility of displaced fluid phase. In this chapter, the mobility of the displaced fluid phase is the mobility of the reservoir oil phase. Eq. 5 holds for a piston-like oil-recovery flooding operation in which the flood front is sharp. Mobility is defined as

....................(6)

where k_i is the permeability to phase i, and μ_i is the viscosity of phase i. Thus, as the viscosity of the oil-recovery displacement/drive fluid is increased in a reservoir with a given degree of permeability heterogeneity, the sweep efficiency and the degree of the oil-recovery flood conformance are improved. See Willhite for the definition of mobility ratio in the case in which a waterflood does not exhibit piston-like displacement.

When the sweep efficiency and the degree of conformance are improved during an oil-recovery flooding operation, the rate at which the reservoir oil is recovered is increased, and the amount of oil-recovery drive fluid, which must be coproduced for a given oil recovery factor, is decreased. Reducing the amount of oil-recovery drive fluid (e.g., water) that must be coproduced for the attainment of a given oil-recovery factor reduces the operating and production costs associated with producing a given amount of oil. It also often reduces certain environmental liabilities, including:

• Production of excessive and unnecessary amounts of saline reservoir brines that can contain toxic heavy-metal ions
• Production of excessive and unnecessary volumes of possibly environmentally unfriendly secondary or tertiary flooding oil-recovery drive fluids

For the most part, conformance improvement flooding operations and treatments do not decrease residual oil saturation. However, there has been a contention made in the literature that polymer flooding can reduce residual oil saturation under certain circumstances. Also, by virtue of the fact that surfactants are incorporated into foams of foam-flooding operations, foam flooding can, in principle, reduce residual oil saturation. However, oilfield foams that are applied for mobility control are believed to function primarily by improving flood sweep efficiency. For the most part, conformance improvement treatments accelerate oil production and/or delay premature economic abandonment of wells, well patterns, and fields, and can do so while conducting normal primary, secondary, or tertiary oil production operations.

Conformance improvement floods and treatments do not normally promote reductions in residual oil saturation. Therefore, conformance improvement operations should be limited to well patterns or reservoirs with a substantial and economically viable amount of moveable oil that can be recovered as a result of conducting the conformance flood or treatment.

Implementing conformance treatments

In general, a conformance improvement treatment (e.g., a gel treatment) to improve sweep efficiency and to generate incremental oil production is applied most effectively from the injection well side. When implementing a conformance improvement treatment to reduce production operating costs by reducing the rate of competing water or gas production, these treatments usually are applied most effectively from the production well side. Treatments for both improving sweep and reducing excessive water and/or gas co-production during gas or supercritical-liquid (e.g., CO₂) flooding operations in naturally fractured reservoirs normally are applied most effectively from the injection well side.

The nature of the reservoir conformance problem to be addressed through the application of polymers, gels, foams, or resins needs to be diagnosed or deduced correctly or substantial negative consequences can occur. A few sources enumerate a number of techniques for diagnosing conformance problems and excessive water- and gas-production problems. Among the techniques discussed in these references are the use of interwell chemical and radioactive tracers, simple injectivity/productivity calculations to determine if fluid flow around a wellbore is radial or linear in nature, wellbore production and injection logs, various other logging tools, and pulse and pressure transient testing. An important element in successfully implementing a water or gas shutoff treatment is to determine at the onset, or at least hypothesize, the “plumbing” of the reservoir flow path of the excess water or gas production from its source to the production wellbore. Production water/oil ratio (WOR) diagnostic plots have also been used to help diagnose conformance problems. WOR diagnostic plots should be used in conjunction with another independent conformance-problem diagnostic tool, because many diagnostic plots can be interpreted in more than one way.

There historically has been a trend whereby petroleum engineers, when first considering the application of a conformance improvement treatment in a new field, tend to underestimate the permeability and fluid-flow capacity of the high-permeability channels and flow paths within the reservoir to be treated. This has contributed significantly to the low success rate of first-time conformance improvement treatments being applied in a new field by an inexperienced petroleum engineer.

Additional information

More detailed information about specific types of conformance improvement treatments can be found in the following pages:

• Disproportionate permeability reduction (DPR) to reduce water production
• Polymer waterflooding
• Gels used for conformance improvement
• Foams as mobility control agents
• Foams as blocking agents
• Resin treatment for conformance improvement

Noteworthy papers in OnePetro

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See also

Conformance problems

Polymers

Gels

Foams

Resin treatment for conformance improvement

PEH:Polymers,_Gels,_Foams,_and_Resins