Gel Placement in Fractured Systems
- R.S. Seright
- Document ID
- Society of Petroleum Engineers
- SPE Production & Facilities
- Publication Date
- November 1995
- Document Type
- Journal Paper
- 241 - 248
- 1995. Society of Petroleum Engineers
- 5.6.5 Tracers, 5.8.6 Naturally Fractured Reservoir, 4.1.2 Separation and Treating, 1.6.9 Coring, Fishing, 1.6 Drilling Operations, 5.7.2 Recovery Factors, 3.2.3 Hydraulic Fracturing Design, Implementation and Optimisation, 5.4.1 Waterflooding
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This paper examines several factors that can have an important effect on gelant placement in fractured systems. These factors include gelant viscosity, degree of gelation, and gravity. For an effective gel treatment, the conductivity of the fracture must be reduced, and a viable flow path must remain open between the wellbore and mobile oil in the reservoir. During placement, the gelant that "leaks off" from the fracture into the rock plays an important role in determining how well a gel treatment will reduce channeling. For a given volume of gelant injected, the distance of gelant leakoff is greater for a viscous gelant than for a low-viscosity gel ant (other factors being equal).
In one method to minimize gelant leakoff, sufficient gelation is designed to occur before the gelant leaves the wellbore. We investigated this approach during numerous experiments using both fractured and unfractured cores. We studied Cr(III)acetate-HPAM, resorcinol-formaldehyde, Cr(III)-xanthan, Al-citrate-HPAM, and other gel ants and gels with various delay times between gelant preparation and injection. Our results suggest both hope and caution concerning injection of gels (rather than gelants) into fractured systems. Tracer studies indicated that some gels can effectively heal fractures under the right circumstances. However, high resistance factors and pressure gradients exhibited during placement could limit the ability to propagate certain gels deep into a fractured system.
More than one million wells have been intentionally fractured to stimulate oil and gas production. Currently, 35% to 40% of newly drilled wells are hydraulically fractured. Many other wells have been fractured unintentionally during waterflooding operations. Naturally fractured reservoirs are also common.
With the proper length and orientation, fractures can enhance productivity and/or injectivity without adversely affecting sweep efficiency. Unfortunately, in many circumstances, fractures impair oil recovery. In reservoirs with water-drive or gas-drive recovery mechanisms, fractures may aggravate production of water or gas. In waterfloods or enhanced recovery projects, fractures can allow injected fluids to channel through the reservoir.
Theoretical developments and many field results indicate that gel treatments are most effective in reservoirs where fractures constitute the source of a severe fluid channeling problem. An important factor responsible for this result is that an effective gel placement is easier to achieve in fractured wells than in unfractured wells. The "permeability" of a fracture is typically 10 to 10 times greater than that of the porous rock. Thus, a gelant can propagate a substantial distance along the length of the fracture while penetrating a small distance into the adjacent rock. However, the gelant that "leaks off" into the rock plays an important role in determining how effectively the gel treatment will reduce channeling. If the distance of gelant leakoff is too great, then both productivity and oil-recovery efficiency could be damaged.
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