Selective Conformance Control in Heterogeneous Reservoirs by Use of Unstable, Reactive Displacements
- K.E. Thompson (Louisiana State U.) | Ohseong Kwon (Louisiana State U.)
- Document ID
- Society of Petroleum Engineers
- SPE Journal
- Publication Date
- June 1999
- Document Type
- Journal Paper
- 156 - 166
- 1999. Society of Petroleum Engineers
- 5.3.2 Multiphase Flow, 1.8 Formation Damage, 3.2.3 Hydraulic Fracturing Design, Implementation and Optimisation, 5.4.7 Chemical Flooding Methods (e.g., Polymer, Solvent, Nitrogen, Immiscible CO2, Surfactant, Vapex), 5.5 Reservoir Simulation, 4.1.5 Processing Equipment, 5.3.1 Flow in Porous Media, 5.6.5 Tracers, 5.4.9 Miscible Methods, 5.4.1 Waterflooding, 4.3.4 Scale, 5.1.1 Exploration, Development, Structural Geology, 3 Production and Well Operations, 1.10 Drilling Equipment, 5.1.5 Geologic Modeling, 4.1.2 Separation and Treating
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This paper examines a conformance control technique in which gel is placed into a heterogeneous porous medium by first injecting a high-viscosity uncrosslinked polymer solution and second, a low-viscosity crosslinker. The latter step causes an unstable displacement to occur and allows the crosslinker to flow selectively into high-permeability strata. A visual flow cell was used to test the procedure, and complete fluid diversion was achieved in a dual-zone medium with a 4:1 permeability ratio. In addition, experiments and numerical simulations were used to study fundamental changes in viscous fingering due to the crosslinker-polymer reaction. When the crosslinking reaction is fast enough so that it occurs during injection, the viscous instability becomes somewhat damped, resulting in less distinct fingers and a nearly plug-flow displacement in some cases. The differences are because gelation causes a large pressure gradient to form just behind the displacement front, whereas unreactive fingering is caused by a small pressure gradient within the fingering zone.
Macroscopic heterogeneity is the dominant factor that causes oil to be bypassed during flooding operations. A 1993 study estimated that bypassing due to reservoir heterogeneity or irregular frontal advance has left 10 billion barrels of unrecovered oil in Louisiana alone.1 In the U.S. as a whole, an estimated 113 billion barrels of mobile oil exist,2 much of which is unrecovered because of bypassing during primary and secondary production. One strategy to counteract the effects of reservoir heterogeneity is to use profile modification and fluid diversion techniques. In this paper, we address injection-well profile modification (or conformance control), which is the use of gelants, foams, precipitates, or other solid materials to plug thief zones and high-permeability streaks in a reservoir. Successful profile modification requires selective placement of these materials to ensure that oil-bearing zones are not damaged. It is simplest in reservoirs where the zones are distinct, and selective injection equipment can be used in the wellbore. However, the more common situation is that the location of the zones requiring plugging is not known exactly and hydraulic communication within the reservoir makes zone isolation only marginally effective. For these reasons, many profile modification techniques being used today incorporate some selectivity mechanism into the treatment design.
Fortunately, thief zones will usually act as thieves for profile modification fluids also, so that whatever inherent selectivity exists is favorable. However, a number of specialized techniques have been developed that use fluid chemistry or the physics of flow to enhance selectivity. Chemical selectivity can be achieved by identifying some component that would be found in a thief zone, and sensitizing the treatment fluid to this component. Examples applied to water control are fluids that precipitate after contacting ions in reservoir brine3 or organic fluids that react upon contact with any aqueous fluid.4 Foams are also known to exhibit a natural selectivity under many conditions.5,6 Their tendency to flow into and plug high-permeability streaks is governed by capillary effects and water saturation, which can differ significantly in tight versus more permeable rocks.
Counteracting this good "inherent" selectivity is the fact that bypassed oil is usually trapped in low-permeability, hard-to-reach strata. Hence, any penetration of a conformance fluid into these zones represents a risk of formation damage. The problem is most acute in heterogeneous reservoirs that have a significant amount of hydraulic communication between different zones (which, deferring to a simple physical model, will be called vertical communication). The problem with these reservoirs is twofold: crossflow between zones can occur at any point within the reservoir so zone isolation is not effective, and the subsequent diversion of drive fluids will occur only to the depth that the conformance treatment reached because drive fluids can also flow from low to high-permeability zones.
Various strategies have been developed to partially address these problems. Dual-injection treatments are performed by pumping a protective fluid into a low-permeability zone at the same time that a diverting agent is injected into a high-permeability zone.7 Sequential injection separates polymer and crosslinker during injection so that crosslinking does not occur until the fluids have mixed in the formation.8 Displacement of gelant away from the wellbore has been suggested as a means to stagger the depths at which gelation occurs, thus allowing drive fluids to enter a high-permeability zone and then crossflow into a low-permeability zone.9 Dispersion and viscous fingering during a water postflush have been examined as methods to dilute gelant and prevent gelation in low-permeability zones, but the results were not encouraging.10 Finally, numerous studies have been performed to better understand the effects of reservoir heterogeneity on EOR processes due to phenomena such as slug breakdown and viscous crossflow.11-13
This paper explores a strategy to provide highly selective gel placement in heterogeneous reservoirs. While the need for very large volumes of gel remains, the technique may help to ensure that they are placed correctly and that the risk of damaging producing zones is minimized. The proposed strategy is to inject a large pad of high-viscosity uncrosslinked polymer followed by a low-viscosity crosslinker solution. If there exists a sufficient viscosity difference, this sequence will initiate fingering during the crosslinker injection. The advantage gained is that during an unstable displacement in a heterogeneous reservoir, the governing physics dictate that the injected fluid will flow almost exclusively into higher-permeability strata (a more detailed explanation of this phenomenon is given below).
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