Primary and Secondary Oil Recovery From Different-Wettability Rocks by Countercurrent Diffusion and Spontaneous Imbibition
- Can U. Hatiboglu (U. of Alberta) | Tayfun Babadagli (U. of Alberta)
- Document ID
- Society of Petroleum Engineers
- SPE Reservoir Evaluation & Engineering
- Publication Date
- April 2008
- Document Type
- Journal Paper
- 418 - 428
- 2008. Society of Petroleum Engineers
- 4.3.4 Scale, 5.1.5 Geologic Modeling, 5.5.2 Core Analysis, 5.7.2 Recovery Factors, 5.8.6 Naturally Fractured Reservoir, 4.1.5 Processing Equipment, 1.6.9 Coring, Fishing, 4.1.2 Separation and Treating, 5.4.7 Chemical Flooding Methods (e.g., Polymer, Solvent, Nitrogen, Immiscible CO2, Surfactant, Vapex), 1.2.3 Rock properties, 5.1 Reservoir Characterisation, 5.2.1 Phase Behavior and PVT Measurements, 4.1.1 Process Simulation, 5.3.4 Reduction of Residual Oil Saturation, 5.4.1 Waterflooding, 5.4.2 Gas Injection Methods
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This study investigates optimum matrix-oil-recovery strategies in naturally fractured reservoirs (NFRs) for different wettabilities and rock types. We compare the recovery efficiencies of two cases: (a) primary countercurrent spontaneous imbibition followed by the diffusion of a miscible phase (secondary recovery) and (b) primary diffusion of miscible fluid without preflush of matrix by spontaneous imbibition. For these recovery strategies, the effects of the matrix shape factor, matrix wettability, and type of miscible displacing phase on the rate of recovery and development of residual-oil saturation were clarified experimentally.
Cylindrical Berea-sandstone and Indiana-limestone samples with different shape factors were obtained by cutting the plugs 1, 2.5, and 5 cm in diameter and 2.5, 5, and 10 cm in length. The external surface except one end was coated with epoxy. Static imbibition experiments were conducted on vertically situated samples in which the fractures were at the bottom and matrix/fracture interaction took place in an upward direction. Mineral oil and crude oil were used as oleic phases. Brine was selected as aqueous phase for the primary spontaneous-imbibition recovery. For primary- and secondary-miscible-displacement experiments, n-heptane was used as solvent. Wettability of water-wet Berea-sandstone samples was altered by aging to observe its effects on the dynamics of spontaneous countercurrent imbibition and diffusion.
Parametric analyses were performed for the appraisal of the secondary- and tertiary-recovery potential of NFRs by immiscible- and miscible-fluid injections. The optimal recovery strategies (recovery rate, recovery time, and ultimate recovery) for different rock properties were identified and classified. In water-wet cases, starting the recovery with capillary imbibition followed by diffusion was found to be the optimal way (i.e., both effective and efficient). For limestone or aged-sandstone samples, starting the recovery by diffusion yielded a faster recovery rate and higher ultimate recovery.
For an efficient recovery of oil from weakly water-wet NFRs, interaction between matrix and fracture needs to be accelerated by use of different enhanced-oil-recovery (EOR) techniques. Different conditions lead to different recovery mechanisms. For example, recovery is obtained by spontaneous imbibition if the matrix is water-wet and an immiscible wetting phase exists in the fracture. For oil-wet rocks, miscible displacement or heat injection is more favorable because spontaneous imbibition does not take place and recovery by gravity drainage is a relatively slow process, especially for heavy oils. It is apparent that any type of EOR other than imbibition would be costlier in terms of infrastructure and expenses. However, in certain circumstances listed above, expensive techniques are inevitable because less-expensive water or gas injection does not contribute significantly to matrix recovery.
This study investigates different EOR strategies for weakly water-wet matrix types. Typically, countercurrent-type interaction was considered, and tests were performed on Berea-sandstone and Indiana-limestone samples in which the outer surface except one end was coated with epoxy. For purposes of comparison and to establish a base case, tests from a matrix with all sides open to flow were also provided.
This study is a continuation of previous work (Hatiboglu and Babadagli 2004) that aimed to describe matrix/fracture interaction by spontaneous imbibition and diffusion for strongly water-wet matrix types. Recovery by capillary imbibition may be effective if the matrix is water-wet. It is well known that a substantial amount of oil (residual oil) might be left in the matrix even for strongly water-wet systems (Hatiboglu and Babadagli 2004; Kantzas et al. 1997; Bourbiaux and Kalaydjian 1990; Mattax and Kyte 1962; Ma et al. 1995; Schembre et al. 1998; Zhou et al. 2002; Zhang et al. 1996). Tertiary recovery of this remaining oil using heat or solvent could be very costly and inefficient because the process is uncontrollable because the high-permeability fracture network controls the flow direction of the injected fluid. Starting the process with heat or solvent injection could be more effective for the rock types that do not yield any capillary-imbibition recovery (Stubos and Poulou 1999; Morel et al. 1993; Zakirov et al. 1991; Le Romancer and Fernandes 1994; da Silva and Belery 1989; Lenormand et al. 1998; LaBolle et al. 2000; Polak et al. 2003; Rangel-German and Kovscek 2002; Hatiboglu and Babadagli 2005). All these efforts require a clear understanding of the processes, especially for countercurrent interaction.
The objective of this paper is to clarify the mechanisms of countercurrent displacement of the nonwetting phase by both capillary forces and diffusion. Different shape factors and rocks with different wettabilities were tested. We compared the cases of recovery by diffusion only to cases of diffusion preceded by capillary imbibition so that we could propose the most effective and efficient recovery strategies on the basis of the uncontrollable parameters: matrix shape factor and wettability.
|File Size||3 MB||Number of Pages||11|
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