Abstract

The first multicomponent induction instrument available to the industry provides vertical and horizontal resistivity data for an improved delineation and evaluation of low resistivity, low contrast pay zones frequently encountered in offshore hydrocarbon exploration. This instrument surveys the formation in three dimensions with multicomponent transmitter-receiver induction coil arrays to derive the true horizontal and vertical formation resistivities. The horizontal and vertical resistivities obtained from the multicomponent induction measurements can be interpreted jointly with standard array induction and image log interpretation data.

Conventional array induction data assists 3DEX ? resistivity data interpretation by removing deeper invasion effects through 2D inversion methods. A realistic example derived from field data including anisotropic sands demonstrates how vertical and horizontal resistivity from a multicomponent induction instrument can be jointly interpreted with NMR data to provide additional reservoir information. A second field data example demonstrates how new petrophysical evaluation methods utilizing the new multicomponent induction instrument's resistivity anisotropy data improve the economic evaluation of low resistivity, low contrast pay zones. These areas range from marginally economic or nearly depleted regions to the capital-intensive offshore deepwater plays. For these areas the multicomponent induction data interpretation provides additional information to enhance the economic value of offshore exploration and development drilling programs. Petrophysical interpretation utilizing vertical and horizontal resistivity, nuclear, and NMR data yields a more accurate and reliable formation water saturation evaluation in anisotropic thinly bedded, laminated sand shale sequences.

Conventional induction logging instruments have the transmitter and receiver array orientation aligned parallel to the tool and borehole axis. Therefore, in vertical wells with hydrocarbon-filled sand shale sequences, conventional induction instruments measure a resistivity that is greatly biased towards the low resistivity of shales. This results from the dominant current flow for the source/receiver configuration being horizontal and occurring mainly through the low resistivity shales: thus, horizontal resistivity is measured. This can lead to an underestimation of the sand laminae oil saturation. The multicomponent transmitterreceiver induction instrument configuration provides direct measurements to derive both horizontal and vertical resistivities. These resistivities allow an improved petrophysical evaluation of hydrocarbon-bearing, thinly bedded shaly sand formations.

Field examples demonstrate the benefits of joint interpretation of multicomponent data with image logs to properly model and interpret resistivity anisotropy in zones with resistive streaks and variable laminated formation structures.

Introduction

A significant percentage of the world's estimated hydrocarbon reserves are contained in thinly laminated, low-resistivity, low-contrast, shaly sand formations typically encountered in deepwater turbidites. Evaluation of hydrocarbon reserves in these formations has been a challenge to petrophysicists. Recent statistical studies reveal that turbidites are in an immature exploration stage globally and will have an important economic role in the future of hydrocarbon exploration and production.1-3 The goals of a petrophysical reservoir analysis of a layered anisotropic reservoir are mainly for volumetric analysis and flow property determination. The volumetric reservoir description includes matrix volume fractions, porosity, and irreducible saturation. The flowproperties ar

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