Summary
It has been a challenge to understand why recovery by spontaneous imbibition
could both increase and decrease with initial water saturation. To this end,
mathematical models were developed with porosity, permeability, viscosity,
relative permeability, capillary pressure, and initial water saturation
included. These equations foresee that recovery and imbibition rate can
increase, remain unchanged, or decrease with an increase in initial water
saturation, depending on rock properties, the quantity of residual gas
saturation, the range of initial water saturation, and the units used in the
definitions of gas recovery and imbibition rate. The theoretical predictions
were verified experimentally by conducting spontaneous water imbibition at five
different initial water saturations, ranging from 0 to approximately 50%. The
effects of initial water saturation on residual saturation, relative
permeability, capillary pressure, imbibition rate, and recovery in
gas/water/rock systems by cocurrent spontaneous imbibition were investigated
both theoretically and experimentally. Water-phase relative permeabilities and
capillary pressures were calculated with the experimental data of spontaneous
imbibition. Experimental results in different rocks were compared.
Introduction
Spontaneous water imbibition is an important mechanism during water
injection. Prediction of recovery and imbibition rate by spontaneous water
imbibition is essential to evaluate the feasibility and the performance of
water injection. For example, is water injection effective in the case of high
initial water saturation in reservoirs? Answers to such a question may be found
by investigating the effect of initial water saturation on spontaneous water
imbibition.
It has been observed experimentally that initial water saturation affects
recovery and production rate significantly (Blair 1964; Zhou et al. 2000;
Viksund et al. 1998; Cil et al. 1998; Tong et al. 2001; Li and Firoozabadi
2000; Akin et al. 2000). However, the experimental observations from different
authors (Zhou et al. 2000; Cil et al. 1998; Li and Firoozabadi 2000; Akin et
al. 2000) are not consistent. On the other hand, few studies have investigated
the effect of initial water saturation on recovery and imbibition rate
theoretically, especially in gas reservoirs.
Using numerical-simulation techniques, Blair (1964) found that the quantity
and the rate of oil produced after a given period of imbibition increased with
a decrease in initial water saturation for countercurrent spontaneous
imbibition.Zhou et al. (2000) found that both imbibition rate and final oil
recovery in terms of oil originally in place (OOIP) increased with an increase
in initial water saturation, whereas oil recovery by waterflooding decreased.
Viksund et al. (1998) found that the final oil recovery (OOIP) by spontaneous
water imbibition in Berea sandstone showed little variation with a change in
initial water saturation from 0 to approximately 30%. For the chalk samples
tested by Viksund et al. (1998), the imbibition rate first increased with an
increase in initial water saturation and then decreased slightly as initial
water saturation increased above 34%.Cil et al. (1998) reported that the oil
recovery (in terms of recoverable oil reserves) for zero and 20% initial water
saturation showed insignificant differences in behavior. However, the oil
recovery for initial water saturation above 20% increased with an increase in
initial water saturation. Li and Firoozabadi (2000) found that the final gas
recovery in the units of gas originally in place (GOIP) by spontaneous
imbibition decreased with an increase in initial water saturation in both
gas/oil/rock and gas/water/rock systems. The imbibition rate (GOIP/min)
increased with an increase in initial water saturation at early time but
decreased at later time. Akin et al. (2000) found that the residual oil
saturation was unaffected significantly by initial water saturation.
In this study, equations, derived theoretically, were used to study the
effect of initial water saturation on gas recovery and imbibition rate. The
equations correlate recovery, imbibition rate, initial water saturation,
rock/fluid properties, and other parameters. Experiments of spontaneous water
imbibition in gas-saturated rocks were conducted to confirm the theoretical
predictions. The effect of rock properties on gas recovery and imbibition rate
was also studied. An X-ray CT scanner was used to monitor the distribution of
the initial water saturation to confirm that the initial distribution of the
water saturation was uniform. In this study, we only focused on cocurrent
spontaneous imbibition. It was assumed that there were no chemical reactions or
mass transfer between gas and liquid.
© 2006. Society of Petroleum Engineers
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History
- Original manuscript received:
20 July 2005
- Revised manuscript received:
23 January 2006
- Manuscript approved:
7 March 2006
- Version of record:
20 August 2006
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