Laminated Sand-Shale Formation Evaluation Using Azimuthal LWD Resistivity

Abstract Intervals of thinly bedded hydrocarbon-bearing sands and laminar clay cannot be fully evaluated with conventional log measurements and interpretation techniques. This is because the laminar nature of the clay component suppresses the logged deep resistivity measurements and results in calculated values of water saturation that are artificially high. This formation evaluation challenge is already widely recognized by the industry and addressed by an interpretation solution that uses simultaneous measurements of horizontal and vertical resistivity, available from the new generation of wireline induction-resistivity tools. A newly deployed logging-while-drilling (LWD) sensor now provides equivalent measurements of azimuthal resistivity; this technology was deployed for the first time for the formation evaluation of thinly bedded sands in Trinidad's Dolphin Field. This tool provides multiple depths of investigation and 32 azimuthal resistivity measurements. Horizontal and vertical resistivity are extracted from the multi-spacing and multi-frequency azimuthal data using fast and robust inversion algorithms. The results were validated with a back-to-back run with an equivalent wireline service. An interpretation methodology is presented that sets out the equations used for the mathematical solution of these data. The Thomas-Stieber method is used to partition the calculated clay volume into its laminated, structural, and dispersed components and to calculate the effective and total porosity of the sand fraction of the reservoir. The corresponding true resistivity of the laminated sand fraction is derived from the quadratic solution of horizontal and vertical resistivity and the laminar clay volume. The water saturation of the sand fraction is calculated using the Archie Equation and subsequently volumetrically weighted to provide the total water saturation. The Thomas-Stieber parameters are also used to calculate the formation averages. The high angle, high step-out wells drilled in this phase of field development have calculated hydrocarbon pore volumes of up to 33% greater than historic, near-vertical wells logged with conventional resistivity tools only. The results of the thin bed interpretation are observed to be sensitive to both borehole deviation and the stratigraphic interval.