Jackline Maia Rennó's Thesis
by
Jackline Maia Rennó, M.S.E.
University of Texas at Austin, 2004
Supervisors:Carlos Torres-Verdin
The determination of fluid density and the assessment of fluid contacts (oil-water and gas-oil) are two important
applications of wireline formation testers. However, in the presence of mud-filtrate invasion, true formation
pressure is not available from formation testers. This could lead to significant errors in the calculation of both
fluid contact locations and fluid density. Moreover, the existence of fluid transition zones produces errors in
the calculation of fluid density when formation tester measurements are acquired within such zones.
The effects of capillary pressure and wettability on formation tester measurements have been discussed in previous
publications. A central objective of this thesis is to assess the effect of capillary pressure arising in the
transition and invaded zones, on the determination of fluid density and on the location of fluid contacts. This is
accomplished with an extensive sensitivity analysis of multi-phase formation tester measurements. The objective is
to distinguish the predominant biasing effect, either due to capillary pressure or to fluid segregation, on the
determination of fluid density and in the location of fluid contacts for a variety of synthetic rock formations.
A commercial black-oil reservoir simulator is used to model dual-packer formation tester measurements acquired in
a vertical well subject to water-base mud-filtrate invasion. The sensitivity analyses were performed by
constructing synthetic models of petrophysical and fluid properties. Rock formation properties such as
permeability and porosity were varied, along with relative permeability and capillary pressure curves. All of the
modeled cases contained a gas cap, an oil zone, and a water zone.
Results of the sensitivity analyses show that fluid density is well defined with or without the presence of
mud-filtrate invasion provided that the duration of the draw-down and build-up tests is sufficiently long. The
same conclusion holds true for different types of rock formations, even though the high permeability/porosity
formations entail the best agreement between the simulated pressure gradient and the theoretical value. However,
fluid pressure gradients simulated in fluid transition zones yield values of fluid density that are not realistic.
The estimated locations of free-water levels also remain significantly affected by the presence of mud-filtrate
invasion.
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