ABSTRACT:

The availability of multi-component induction measurements, such as data provided by the 3D Explorer SM (3DEX) instrument, has become an important interpretation tool for petrophysicists. The 3DEX delivers unique information to determine formation resistivity anisotropy and thereby allows for performing a more comprehensive petrophysical analysis than possible with conventional resistivity data. The anisotropy interpretation is typically based on either multi-component induction data alone or on a combination of multi-component and array induction data, such as the High-Definition Induction Log (HDIL). The ideal environment for this induction log combo is a non-conductive, oil-based mud (OBM). In certain hydrocarbon-bearing regions, however, the drilling industry is turning from the use of OBM to environmentally sensitive, conductive water-based mud (WBM) systems. Unfortunately, highly conductive WBM limits the effective operational range for induction tools. To overcome this challenge and to expand the operational envelope of the 3DEX technology, we propose to combine 3DEX with galvanic tools, such as Dual Laterolog (DLL), Micro Laterolog (MLL), and/or High-Definition Lateral Log (HDLL), which provide the capability to accurately resolve the near borehole environment in the presence of a highly conductive borehole fluid. We consider a two-stage data interpretation process. In the first stage, we use 2-D inversion of the galvanic data to determine the near-wellbore formation properties and the horizontal resistivity of the uncontaminated zone. In a second stage, we use 2-D inversion of the deep-reading multi-component induction data to determine the vertical resistivity of the uncontaminated zone. Evaluation of the method on a series of synthetic models allows us to define an expanded 3DEX operational environment. In the presented case study, multi-component induction and array lateral log data were acquired in a vertical offshore well in the Mediterranean region. The well was drilled into a deep marine turbidite sequence, encountering noteworthy volumes of thinly bedded, laminar silty shales and high porosity gas sands. Interpreting the induction data alone yields an unrealistically high value of anisotropy, indicating the difficulties the induction method has with the given borehole environment. Anisotropy values obtained by applying the sequential galvanic and induction data inversion procedure provide significantly more accurate formation anisotropy estimates. They are in good agreement with the expected results. The conclusion is that the combination of galvanic and induction measurements along with an inversion-based, data interpretation method can effectively extend the dynamic range of the 3DEX technology, allowing the use of this technology in wells drilled with conductive water-based mud systems.

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