Summary
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.
Introduction
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.
© 2008. Society of Petroleum Engineers
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History
- Original manuscript received:
20 February 2006
- Meeting paper published:
22 April 2006
- Revised manuscript received:
21 June 2007
- Manuscript approved:
27 June 2007
- Version of record:
25 April 2008
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