Summary
We quantify the influence of oil-based mud (OBM)-filtrate invasion and
formation-fluid properties on the spatial distribution of fluid saturation and
electrical resistivity in the near-wellbore region. The objective is to
appraise the sensitivity of borehole resistivity measurements to the spatial
distribution of fluid saturation resulting from the compositional mixing of OBM
and in-situ hydrocarbons.
First, we consider a simple two-component formulation for the oil phase (OBM
and reservoir oil) wherein the components are first-contact miscible. A second
approach consists of adding water and surfactant to a multicomponent OBM
invading a formation saturated with multiple hydrocarbon components.
Simulations also include presence of irreducible, capillary-bound, and movable
water. The dynamic process of OBM invasion causes component concentrations to
vary with space and time. In addition, the relative mobility of the oil phase
varies during the process of invasion because oil viscosity and oil density are
both dependent on component concentrations.
Presence of surfactants in the OBM is simulated with a commercial adaptive
implicit compositional formulation that models the flow of three-phase
multicomponent fluids in porous media. Simulations of the process of OBM
invasion yield 2D spatial distributions of water and oil saturation that are
transformed into spatial distributions of electrical resistivity. Subsequently,
we simulate the corresponding array-induction measurements assuming
axial-symmetric variations of electrical resistivity.
We perform sensitivity analyses on field measurements acquired in a well
that penetrates a clastic formation and that includes different values of
density and viscosity for mud filtrate and formation hydrocarbon. These
analyses provide evidence of the presence of a high-resistivity region near the
borehole wall followed by a low-resistivity annulus close to the noninvaded
resistivity region. Such an abnormal resistivity annulus is predominantly
caused by high viscosity contrasts between mud filtrate and formation oil. The
combined simulation of invasion and array-induction logs in the presence of OBM
invasion provides a more reliable estimate of water saturation, which improves
the assessment of in-place hydrocarbon reserves.
© 2011. Society of Petroleum Engineers
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History
- Original manuscript received:
27 July 2007
- Meeting paper published:
11 November 2007
- Revised manuscript received:
31 August 2010
- Manuscript approved:
8 September 2010
- Published online:
29 March 2011
- Version of record:
15 September 2011
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