At the time of discovery,
reservoir fluids are in hydraulic equilibrium, and they
are vertically distributed according to
their density at reservoir pressure and temperature .The interface between these fluids is horizontal and therefore
, if the reservoir is hydraulically connected , all the wells will encounter
these fluid contacts at the same depth . As a consequence, if
different wells drilled in the same reservoir
encounter fluid contacts at different depths , the reservoir is likely to be compartmentalized .
It should be noted
that the existence of a common fluid contact in all the wells drilled
during the appraisal phase does not
guarantee in itself reservoir
continuity . In some cases , barriers
to fluid flow may have been generated only after the hydrocarbon migration phase ,
as a consequence of diagenetic
effects relate to circulation of fluids in
the reservoir . In this case ,
reservoir barriers are normally
detected only after the beginning of the exploitation ,
observing for example different rises of the fluid contacts in
different blocks , as a result of reservoir fluids withdrawal .
In the majority of cases , however , the general rule
holds and differences in the contacts
depth can be interpreted as evidences of
a degree of reservoir compartmentalization
.
Several types of data can be used to locate fluid contacts , from wireline logs , to routine core analysis , to pressure measurements .Without going into further detail on this basic issue , it should be appreciated that WFT (Wireline Formation Tester ) pressure measurements are one of the most effective way to identify fluid contacts , at any stage of field life .
The precise knowledge
of the contact of fluids (gas-oil, gas-water or oil-water) makes it possible to
define the useful height of the reservoir as well as the surface of the
reservoir.
Methods for
determining fluid contact:
· Resistivity log
· MDT -Modular Formation Dynamics Tester
· DST- Drill stem Testing
· Sw Cut off evaluation
· Structural closure:
WOC determined by SW evaluation
WOC determined by Resistivity log
Spill point in the absence of WOC, ODT and WUT, the
surface of the deposit can be approximated by the curve of structural closure
(Spill point).This assumes a load factor of the structure of 100% (Optimistic
surface and reserves).
 |
Spilling Point (La fermeture Structurale ) |
The ODT is the base of the hydrocarbon column that
does not rest directly on an aquifer. It can be taken as a pessimistic interval of
water for estimating the surface area of the deposit (and the useful height).In
a well, there is no evidence of the existence or absence of hydrocarbons lower
than this base. ODT as a "pessimistic interval of water".
Example: HC column
resting on a compact or clay level
 |
| ODT Contact |
The WUT is the highest recognized water grade in a
well. Also, nothing proves the existence or non-existence of water higher than
this level .
The WUT can be taken as an
optimistic body of water to estimate reserves in the absence of an obvious body
of water. The WUT as an optimistic 'water interval '
 |
| WUT Contact |