Summary
A new waterflooding process, toe-to-heel waterflooding (TTHW), was
developed, based partly on a recently developed thermal TTH displacement
process, TTH air injection (THAI). TTHW is a novel oil-recovery process that
uses a horizontal producer (HP) and a vertical injector (VI). The HP has its
horizontal leg located at the top of formation, while its toe is close to the
VI, which is perforated at the lower part of the formation.
TTHW realizes a gravity-stable displacement, in which the water/oil mobility
ratio becomes less important and its detrimental effect on sweep efficiency is
diminished; the injected water always breaks through at the toe, after which
water cut gradually increases.
A systematic investigation of the TTHW process in a Hele-Shaw laboratory
model mimicking a simulated porous medium showed that the process substantially
improved the vertical sweep efficiency as compared to conventional
waterflooding. Following these semiquantitative tests, a more comprehensive
3D-model testing was undertaken to investigate the overall sweep efficiency of
the process. The 3D model consists of a metal box filled with glass beads and
saturated with oil at connate-water saturation. Oil was displaced with
high-salinity brine, either in a TTH configuration or in a conventional array,
using only vertical wells.
A staggered line drive was used by injecting water in two vertical wells
located at one side of the box and producing oil by using either an HP with its
toe close to the injection line or a vertical producer located at the HP’s heel
position.
Several TTHW tests were carried out at different injection rates. For a
given injection rate, the TTHW results were compared to those of
conventional-waterflooding tests. For the same amount of water injected, the
ultimate oil recovery increased by a factor of up to 2, as compared to that for
conventional waterflooding.
All in all, the results of these investigations show that the novel TTHW
process is sound and can be optimized further.
Introduction
Conventional waterfloods in heavy-gravity-oil reservoirs (oil viscosity
higher than 100 mPa.s) are limited by three main factors:
• Reservoir heterogeneity, leading to water channelling.
• Gravity segregation (caused by oil/water density contrast), leading to
underriding of the injected water.
• Highly unfavorable water/oil mobility ratio, which aggravates the adverse
effects of the first two factors.
Usually, the heterogeneity is caused by pronounced vertical stratification,
manifested by a relatively large contrast in the horizontal permeability of
different layers. On the other hand, the negative effect of gravity segregation
is felt mainly when the stratification is not very pronounced and effective
vertical permeability of the pay zone is relatively high.
The effect of gravity segregation on waterflood performance was reported in
1953 when the first mathematical model of water tonguing (underriding) was
published by Dietz (1953). Initially, Dietz’s theory was believed to be
applicable mostly to thick formations. However, subsequently, Outmans showed
that this theory was equally applicable to thin oil formations (Sandrea and
Nielsen 1974).
© 2006. Society of Petroleum Engineers
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History
- Original manuscript received:
13 February 2004
- Revised manuscript received:
21 September 2005
- Manuscript approved:
16 March 2006
- Version of record:
20 June 2006
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