Cleanup of Oil Zones After a Gel Treatment
- Randall S. Seright (New Mexico Tech)
- Document ID
- Society of Petroleum Engineers
- SPE Production & Operations
- Publication Date
- May 2006
- Document Type
- Journal Paper
- 237 - 244
- 2006. Society of Petroleum Engineers
- 3 Production and Well Operations, 1.8 Formation Damage, 5.3.1 Flow in Porous Media, 4.1.5 Processing Equipment, 5.1.2 Faults and Fracture Characterisation, 1.6.9 Coring, Fishing, 4.1.2 Separation and Treating, 4.1.3 Dehydration, 5.4.5 Conformance Improvement
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A simple mobility-ratio model was used to predict cleanup times for both fractured and unfractured production wells after a gel treatment. The time to restore productivity to a gel-treated oil zone (1) was similar for radial vs. linear flow, (2) varied approximately with the cube of distance of gel penetration, (3) varied inversely with pressure drawdown, (4) varied inversely with the kw at Sor in the gel-treated region, and (5) was not sensitive to the final ko at Swr. Although ko at Swr (after gel placement) had no effect on the cleanup time, it strongly affected how much of the original oil productivity was ultimately regained.
Utility of Disproportionate Permeability Reduction. In mature reservoirs, wells typically produce more water than hydrocarbon. In many wells, hydrocarbon productivity could be increased significantly if the water production rate could be reduced. For these cases, the water and hydrocarbon must flow to the wellbore through different pathways (i.e., some zones have high fractional hydrocarbon flow, while other zones have high fractional water flow) (Liang et al. 1993). Because of physical or economic constraints, remedial chemical treatments (e.g., gel treatments) that are intended to plug water strata are often placed without zone isolation. Consequently, the injected fluids and chemicals penetrate into both hydrocarbon and water zones, and the operator must be concerned about damage to hydrocarbon productivity (Liang et al. 1993; Seright 1988). Certain water-based gels and water-soluble polymers (after adsorption or entrapment in rock) can reduce permeability to water much more than that to hydrocarbon (Seright et al. 2006; Zaitoun and Kohler 1988). Basic engineering calculations reveal that materials that provide "relative permeability modification?? or "disproportionate permeability reduction?? are currently of far more practical use when treating linear flow features (e.g., fractures) than when treating radial matrix flow problems (e.g., wells without fractures) (Seright website; Seright et al. 1998; Marin et al. 2002). For these materials to effectively treat radial matrix flow, they should reduce permeability to water by more than a factor of 10 (and preferably by more than a factor of 20). At the same time, they must reduce permeability to oil by less than a factor of two if oil zones are not protected during placement (Seright website). In contrast, when treating fractures, a significant oil residual resistance factor (permeability reduction value for oil) can be tolerated so long as (1) the permeability to water is reduced much more (e.g., >50 times more) than that to oil and (2) the distances of gelant leakoff from the fracture faces are controlled (Seright website; Seright et al. 1998; Marin et al. 2002).
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