ABSTRACT

Large differences in MWD resistivity values were observed between horizontal wells and low-angle wells in a shaly-sand reservoir located offshore Indonesia. These differences made calculated water saturation (Sw) values too low in the horizontal wells by up to 30 saturation units. These Sw discrepancies, if not understood, could have led to erroneous net pay and reserve estimates. Tool modeling software was used to invert four phase-shift resistivity measurements to produce a horizontal resistivity (Rh) and a vertical resistivity (Rv), which helped in the evaluation of this reservoir. Rh was used for Rt in the calculation of Sw with satisfactory results, matching the values obtained in adjacent low-angle wells. Constructing a detailed geologic model of discreet anisotropic units, that could be verified using forward modeling code, was impossible given the complex nature of this reservoir. Modeling results from adjacent shales and analysis of offset core data suggest that the observed macroscopic anisotropy is due to fluid stratification and not shale lamination. The shale section situated stratigraphically higher than the reservoir does not exhibit any obvious anisotropy effects, regardless of the relative dip angle (RDA). The core data from adjacent wells show the permeability to vary significantly on a sample-by-sample basis, consistent with a reservoir that is made up of thin layers of differing Sw. The study area is the Yakin Field, which is located offshore East Kalimantan, Indonesia. The sandstone reservoir is an unconsolidated, tidally influenced, distributary channel with lithic fragments, feldspars, clays, and detrital coal. A horizontal-drilling project, implemented to drain oil from this reservoir, was planned and executed using MWD tools for formation evaluation. The MWD tool string provided real-time and recorded gamma ray, neutron porosity, and formation density measurements.

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