True Resistivity in High-Angle and Horizontal Wells - Correction of Thin Layer Effects Using Image-Constrained Inversion

Abstract

The application of traditional petrophysical interpretation techniques for formation evaluation (FE) in high-angle/horizontal (HA/Hz) wells is limited because formation geometry affects circumferentially averaged and/or deep-reading logs such as multi-propagation resistivity (MPR). The log responses of neutron-density (N/D) or gamma ray (GR) may not correlate with MPR readings as a consequence of different depths of investigation and azimuthal variation of the lithology around the wellbore. Therefore, alternative approaches and modeling workflows are needed for quantitative petrophysical evaluation of the individual layers. This study on HA/Hz wells demonstrates the benefits of combining different types of FE data for a quantitative petrophysical evaluation in a shale-sand formation using an image-constrained resistivity inversion algorithm. It includes the calculation of true formation resistivity in sand reservoir intervals, with the expectation that intersecting shale layers affect the MPR readings. The increased shale content in the form of thin laminations was indicated on the GR log. To account for the thin and conductive shale layers in a resistivity inversion, a detailed structural earth model was created for the near wellbore. The strike and dip of boundaries were interpreted from a bulk density borehole image and projected away from the borehole within a depth-of-detection tube. The layers in the model were populated with expected property values derived from offset wells and the interpretations of the related FE logs. A resistivity inversion algorithm was run as part of the workflow, providing true formation resistivity (Rt) as output. A synthetic data set was used for demonstrating purposes. The data set consisted of a 70 degree inclined well penetrating horizontally layered sand-shale sequences. The applicability of the new approach was then confirmed by a field data set from the North Sea in which FE data were acquired in a high-angle well using logging-while-drilling (LWD). In both data sets the formation geometry affected the log responses. The workflow described in this study enabled efficient merging of detailed borehole image information into the forward modeling and inversion processes. The true formation resistivity was obtained for each layer for input for further petrophysical analysis using traditional interpretation techniques. The comparison of water saturation calculations based on apparent and on true resistivity logs, respectively, demonstrated the added value of this approach for an improved quantitative petrophysical evaluation and revealed the significant impact of formation geometry effects in HA/Hz wells.