Deep directional resistivity LWD measurements have been shown to be sensitive to resistive transitions over a broad range of distances around the tool from tens to hundreds of feet. These detected transitional surfaces are primarily used to detect formation resistivity boundaries and assist with mapping geological profiles. The inverted formation dip and vertical resistivities are also resolved in the same search space. While the formation dip is used in conjunction with the reservoir-mapping interpretation results, the vertical resistivity, specifically the vertical and horizontal resistivity ratio, or anisotropy, has not received the same amount of attention. Resistivity anisotropy is useful when calculating the formation resistivity in layered formations, as conventional resistivity tools measure the resistivity in one direction, which is perpendicular to the tool axis. With conventional induction and propagation resistivity tools, the electrical current preferentially transits the conductive lithologies, resulting in an apparent resistivity measurement that does not represent the true sand resistivity. The petrophysical evaluation often results in an apparent high-water saturation, which can result in incorrect decisions to abandon a prospect. To understand two new fields located onshore Alaska, three horizontal appraisal wells were drilled with deep directional resistivity LWD technology. While the primary goal was to characterize the lateral resistivity profile and bed boundaries away from the wellbore, accurate water saturation calculations along the horizontal section are critical for making appropriate development decisions. A review on how and why deep directional resistivity LWD technology is sensitive to anisotropy and how anisotropy is derived from parametric inversions is presented with a comparison between deep directional resistivity LWD measurements, 3D petrophysical modeling of propagation, and offset well triaxial induction anisotropy measurements. Integrating 3D petrophysical processing and triaxial-induction technology into deep directional resistivity LWD measurements add to the strength of the anisotropy output. The comparison shows that deep directional resistivity LWD measurements can be used independently to give accurate anisotropy results. The result of this process provides a corrected resistivity measurement of vertical and horizontal resistivity in anisotropic formations for petrophysical models. Use of the corrected resistivity as a true resistivity (Rt) input for water saturation will ultimately drive better development decisions.
Reservoirs in the Northern Gulf of Mexico (NGOM) are predominantly drilled with low angle wells. Drilling a well horizontally presents its own set of challenges.The Mississippi Canyon block 22/21 operated by ANKOR Energy embodies the significant drilling challenges sometimes found in the NGOM reservoirs. The ЉH SandЉ reservoir is a low resistivity (3 to 5 ohm.m) reservoir bounded by a northeast southwest fault. The overburden is uniform for more than 200ft TVD above the reservoir, making the landing challenging with conventional well placement techniques. The operator was planning a 2200ft lateral section to be drilled close to the Gas Oil Contact (GOC). Early water production was observed in the offset well located at the toe of the planned well, questioning the current position of the Oil Water Contact (OWC). Landing this well is challenging from both a geological and drilling point of view as a 3D trajectory is required to avoid the fault and offset wells.In light of these challenges, the operator decided to use the very-deep electromagnetic (EM) directional resistivity tool with its detection range of more than 100ft, enabling the detection of the top of the H sand reservoir long before landing. In the lateral section, the tool was used in conjunction with an integrated petrophysical platform to map the top of the reservoir, detect the OWC and identify the lithology and fluids present while drilling.While landing the well, the top of the H sand reservoir was detected 48ft TVD away -10ft deeper than expected. The very-deep directional resistivity tool enabled the well to be confidently landed despite the lack of correlation markers and depth uncertainty of the ЉH SandЉ reservoir. The OWC was detected more than 70ft below the well during the landing section even though the bit had not penetrated the H sand reservoir yet. The top of the reservoir and the OWC were mapped throughout the length of the lateral section along with the lithology and fluid content. Towards the toe of the lateral section, near a producing offset well, the OWC, still 50ft below the current trajectory, was observed to be rising up and getting closer to the well. Total Depth (TD) was called early to avoid premature water production. Water coning was confirmed as the reason behind early water production in the offset well.The use of this technology during the landing and the lateral section of the well reduced dramatically the risk associated with geological uncertainty as well as fluid contact position providing critical information for field management planning.
Reservoirs located offshore Abu Dhabi can be complex in terms of sub-seismic structural features such as faults and localized deformations. With use of high-resolution resistivity image logs, a TST (true stratigraphic thickness) technique, along with 3D structural models, uncertainties related to sub seismic structural ambiguities are resolved and well trajectory is optimized while drilling. In this case study, real-time resistivity image logs were used while drilling. The sinusoid’s shape on images provided cutting down dip or up dip information. Dip trends were analyzed using a dip vector plot and to identify zones-of-interest. Dip attribute along with the log response were compared with the pre-job model and the inclination is adjusted accordingly during drilling. Several high angle features can be characterized as stratigraphic changes, fractures, or faults. The morphology and trend change observed in the dip vector plot of these features lead to the conclusion that these are sub-seismic resolution faults and deformation is associated with the fault. The stratigraphic drilling polarity and the TST were calculated using the formation dip data. Using a TST scale and splitting the logs into stratigraphic drilling polarity domains, the fault throw displacement is estimated. The model is updated to reflect the interpreted data. The fault plunge and trend are extrapolated away from the wellbore and to nearby wells.
Historically, reservoir characterization in horizontal wells is accomplished through an integrated approach, combining geophysical evaluation and geomodelling with petrophysical assessment. However, the challenge consists of decreasing geomodels’ uncertainties to enable optimal trajectory to reach the sweet spots. Moreover, the main inputs of subsurface representation derived from measurements have their own spatial resolution and scale. For an effective multiphysics integrated approach, a technology capable to bridge from high-resolution borehole data to large scale seismic is required. This communication describes an innovative method to improve the delineation of reservoir geometry and properties surrounding high-angle wells. The novel procedure is divided into three stages. The first focuses on structural delineation using seismic attributes. The second uses advanced resistivity inversions from LWD very-deep directional resistivity tool to provide 3D mapping of the structure and fluid distribution. The third stage integrates the previous steps to build a comprehensive 3D reservoir model with high accuracy, tens of feet away from wellbore, while honoring the geological context and actual spatial resolution of measurements from borehole level up to seismic scale. The three-step methodology was successfully conducted on the horizontal section of an appraisal well. The automated seismic extraction workflow on the near and far angle-stack seismic cubes is used to interpret the main stratigraphic and tectonic events around the well vicinity. Within the drilling operation, the high-definition resistivity volumes are obtained from a special 3D interpolation of the 2D LWD EM azimuthal inversions, derived from the measurements of the very-deep directional resistivity tool. Such 3D resistivity mapping is used to determine lithological and structural features over tens of feet away from the wellbore. Then, by applying an integrated approach, key geological structures and detailed internal reservoir architectures were revealed, such as the throw and azimuth of a main fault and the spatial variations in lithologies within the reservoir zone. Finally, the respective workflow can be fully applied while drilling, enabling both the complete 3D reservoir mapping but also supporting strategic geosteering decisions to optimize the extension of the net-pay exposure.
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