Summary
We used a commercial reservoir simulator to study, first, the dissipation of
aqueous drilling fluid filtrate invasion around a cased observation well in an
oil-saturated formation under the action of capillary pressure and, second, the
interaction of a waterflood front with the cased well and remaining invaded
zone. Hysteretic behavior of the capillary pressure and relative permeabilities
is critically important to these processes and is taken into account by the use
of the Carlson model, with the various bounding drainage and imbibition curves
computed from a pore network model.
Filtrate invasion into a hydrocarbon formation influences the readings of
well-logging tools. Although this phenomenon has been known, and corrected for,
for many years, uncertainty remains with regard to the long-time behavior of
invasion around observation wells where no flow in or out of the formation
occurs after completion, and with regard to the influence of formation
wettability. We find that after sufficient time, the invaded zone dissipates
completely in a water-wet formation, but some invasion always remains in the
oil/mixed-wet case. Nonwetting-phase trapping, manifested through relative
permeability hysteresis, is the cause. Because trapping affects the values and
the endpoints of the relative permeability curves, a waterflood front passing
across an observation well is more distorted in the oil/mixed-wet case. The
simulation results allow us to understand how logging-tool measurements made in
cased observation wells are influenced by drilling-fluid invasion and will
therefore lead to improved interpretation. This study shows strong links
between the wettability of the formation and the persistence of invaded zone
saturation and between invaded zone saturation and the distortion of subsequent
flood fronts.
© 2012. Society of Petroleum Engineers
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History
- Original manuscript received:
28 March 2012
- Meeting paper published:
5 June 2012
- Revised manuscript received:
12 July 2012
- Manuscript approved:
1 August 2012
- Published online:
23 October 2012
- Version of record:
6 December 2012
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