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Abstract A new, through-the-bit, ultra-slim wireline borehole-imaging tool for use in oil-based mud provides photorealistic images. The imager is designed to be conveyed through drill-pipe. At the desired well section, it exits the drill pipe through a portal drill bit and starts the logging. Field test measurements in several horizontal, unconventional wells in North America show images of fine detail with a large amount of geological information and high value for well development. A relatively new solution for conveying tools to the deepest point of a high angle or horizontal wells uses a drill bit with a portal hole at the bit face. As soon as the bit reaches the total depth, a string of logging tools is pumped down through the drill pipe. The tools exit the bit through the portal hole, arriving in the open hole and are ready for the up log. The tools operate on battery and store the log data in memory so that no cable is interfering as the drill pipe is tripped out of the well while the tools are acquiring data. The quality of wireline electrical borehole images in wells drilled with oil-based mud has significantly improved in recent years. Modern microresistivity imagers operate in the megahertz-frequency range, radiating the electromagnetic signal through the non-conductive mud column. A composite processing scheme produces high-resolution impedivity images. The new, ultra-slim borehole-imager tool uses these measurement principles and processing methods. Innovating beyond the existing tool designs the tool is now re-engineered to dimensions sufficiently slim to fit through drill pipes and to use through-the-bit logging techniques. The new, ultra-slim tool geometry proves highly reliable and, due to the deployment technique, highly effective in challenging hole conditions. The tool did not suffer any damage and showed only minute wear over more than twenty field test wells. The tool’s twelve-pad geometry provides 75% coverage in a six-inch diameter borehole and its image quality compares very well with existing larger tools. The field test of this borehole imaging tool covers all scenarios from vertical to deviated and to long-reach, horizontal wells. Geological structures, sedimentary heterogeneities, faults and fractures are imaged with detail matching benchmark wireline images. The interpretation answers allow operators of unconventional reservoirs to employ intelligent stimulation strategies based on geological reality and effective well development. A new high-frequency borehole imager for wells drilled with oil-based mud is introduced. Deployed through the drill pipe and its portal bit, the imager carries photorealistic microresistivity images into wells where conventional wireline conveyance techniques reach their limits in both practicality and viability.
Abstract A new 2 1/8-in. outer-diameter photorealistic imager for oil-based muds (OBM) has recently started field testing in unconventional formations in North America. To obtain the best interpretation of its measurements, a twostep quantitative inversion workflow has been developed with a performance similar to the existing inversion workflows for the regular high-definition OBM imagers. The new inversion workflow provides borehole resistivity images, borehole rugosity images, and borehole dielectric permittivity images as well as multiple quality curves. The modeling of the new borehole imager is performed with a 2D axisymmetric finite element code. An efficient forward model is developed by fitting the tool response tables into fourth-order polynomials in terms of the sensor standoff, formation, and mud impedivities for broad ranges of model parameters. The fast forward model based on the polynomial fitting is calibrated against the actual tool measurements in a laboratory setup and applied in the inversion algorithms. The inversion workflow is tested with synthetic data and the inverted model parameters are compared with their true values to study and analyze their corresponding measurement sensitivity and optimize the inversion input parameters. It is used to invert several field test datasets in unconventional wells. The results show that the inversion results provide critical added value for formation evaluation, showing geological features that would otherwise be missed, such as fracture properties. Projection-based formation impedivity images, as available for the regular high-definition OBM imagers, are ideal for conductive formations but suffer from a rollover effect in resistive formations. In comparison, the image formed from the inverted formation resistivity does not roll over and is more consistent for resistive formations. The image formed by the inverted standoff reflects surface conditions of the borehole and can be used to interpret whether the fractures and the faults are open, closed, or damaged in the drilling process. Multiple image examples are given from unconventional wells to demonstrate that the inverted standoff image can reveal fractures when there is insufficient or even no contrast in medium properties. The inverted standoff image also serves as a diagnostic tool for interpreting borehole and tool conditions during the measurements. The inverted permittivity may have a larger dynamic range than the resistivity especially for unconventional formations, thus providing an alternative and potentially clearer borehole image.
Characterization of laminations and thin beds is important for understanding sedimentary processes. Laminae are defined as layers less than 1 cm thick and beds as layers thicker than 1 cm. The frequency of laminations and beds, also called the lamination index, gives an indication of sedimentary energy and provides important clues for understanding the depositional process within a geological framework. In thinly bedded conventional reservoirs, quantification of thin beds is vital to accurately determine volumetrics. The degree of lamination in unconventional reservoirs, on the other hand, can significantly affect hydraulic fracture initiation and propagation.
Resistivity images of the borehole are high-resolution measurements that precisely capture the orientation and various properties of the laminations. Conventional analysis relies on the manual picking and counting of boundaries observed as sinusoids on the borehole image. This process is time consuming and user dependent. New image processing techniques enable automatic and fast extraction of lamination properties and statistics. A new workflow offers the possibility to perform this analysis at relevant scales for the layer thicknesses and to focus the characterization on particular layers such as ash beds.
The first step of the workflow consists of estimating the lamination orientations in a sliding window and is based on a method called the Hough transform. Resistivity curves are then extracted from the borehole image, parallel to the orientation. The algorithm is robust to gaps in the image (as in the case of wireline images) and small depth discrepancies. In addition, the extracted resistivity curve is not influenced by the presence of small scale textural features, fractures and breakouts. This curve is then decomposed in the frequency domain into different scales, corresponding to different thickness ranges. For each scale, lamination and bed boundaries are identified and their properties are determined (layer contrast and resistivity). Finally, the lamination index thicknesses are computed. The calculation uses the true stratigraphic thickness index and is thus independent of the well deviation. These properties can then be used in facies analysis or correlation.
The workflow is illustrated by examples from both conventional and unconventional reservoir settings. Beds at different scales are discriminated in a conventional sand-shale sequence. Bed thickness arrangements are readily identified, such as thickening or thinning upward sequences valuable for stratigraphic studies. A second example is from the unconventional Eagle Ford shale play. In this example, the sequence can be classified into two categories, sedimentary beds/laminations and individual conductive layers (i.e. clay rich, altered ash or pyrite rich). The density and statistics are separately computed for both categories. In addition, highly conductive beds that are thinner than the image resolution are characterized using an equation derived from tool response modelling and validated by comparison with core photographs. The final bed/lamination density is accurately calculated and can be used as potential input to hydraulic fracture models or to build stratigraphic framework (sequence or correlation based) in 3D reservoir static models.
Chen, Yong-Hua (Schlumberger-Doll Research) | Omeragic, Dzevat (Schlumberger-Doll Research) | Habashy, Tarek (Schlumberger-Doll Research) | Bloemenkamp, Richard (Etudes et Productions Schlumberger) | Zhang, Tianhua (Etudes et Productions Schlumberger) | Cheung, Phillip (Etudes et Productions Schlumberger) | Laronga, Robert (Services Techniques Schlumberger)
The high-definition oil-based-mud (OBM) imager is a pad-based microelectrical wireline tool designed to operate in wellbores filled with nonconductive mud. To complement standard composite data processing and provide quantitative interpretation, we developed a model-based parametric inversion using the Gauss-Newton algorithm that matches the measurements to an accurate and efficient forward model built by multidimensional fitting of simulated data. The inversion-based workflow allows flexible selection of model parameters to be inverted and can process logging data from multiple depths and buttons simultaneously, stabilizing the inversion, overcoming the underdetermined problem and measurement calibration limitations.
Besides producing accurate formation resistivity, the inversion improves image quality in highly resistive and fractured formations, improves consistency among the pads, and helps eliminate “blending” artifacts. The inversion also generates a tool-standoff image detailing the borehole shape and a dielectric-permittivity image, which can be valuable for standalone formation evaluation or joint interpretation with array dielectric measurements.
The inversion algorithm is applied to field data acquired in different conditions, to illustrate its potential of inversion-based workflow to further enhance interpretation of the new OBM imager. The field data examples include various complex cases with blending artifacts, large standoff, and resistive formations. The processed resistivities compare well with standard array-induction responses, and so do the computed dielectric permittivities with those derived from an array-dielectric tool. The standoff image helps characterize fractures, faults, and other natural or drilling-induced events on the borehole surface.
The application of the microelectrical imager (Luthi, 2001) has been limited to the conductive water-based fluids (WBM), conditions favorable for use of the low-frequency galvanic measurement physics. The WBM imager is able to produce high-definition images of 0.2 in. resolution and 80% circumferential coverage in an 8-in. borehole, using 192 buttons distributed over 8 pads. Conventional interpretation of the microelectrical images covers determination of structure, identification of thin beds, classification of heterogeneities, facies classification, identification of the depositional environments, fracture analysis, and use in constraining the reservoir model (Hansen and Fett, 2000; Slatt and Davis, 2010).
Chen, Yong-Hua (Schlumberger) | Omeragic, Dzevat (Schlumberger) | Habashy, Tarek (Schlumberger) | Bloemenkamp, Richard (Schlumberger) | Zhang, Tianhua (Schlumberger) | Cheung, Phillip (Schlumberger) | Laronga, Robert (Schlumberger)
The new high-definition oil-based mud (OBM) imager is a pad-based microelectrical imager operating at high frequency to establish capacitive contact with the formation in wellbores filled with nonconductive mud. From multiple modes of operation, formation resistivity-like images are generated using an efficient composite data-processing scheme that approximates formation resistivity either by filtering or applying a correction to minimize the contribution of the OBM to the measured signal. Data from the different modes are ?blended? based on estimated formation parameters to generate an optimized image. This approach requires some knowledge of mud electrical properties.
In addition to the composite processing scheme, we also developed a model-based parametric inversion for quantitative interpretation. The Gauss-Newton algorithm matches the measurements to an accurate computationally efficient approximate forward model built by multidimensional fitting of the data generated using a finite-element simulation. The workflow overcomes the underdetermined inversion problem and calibration limitations of the measurements. The inversion allows flexible model definition and parameterization, including refinement of the calibration, and can process intervals of logging data and measurements from multiple buttons simultaneously. The workflow stabilizes the inversion and improves the consistency of the processed results. To overcome the underdetermined nature of the problem and speed up the inversion, we use a sequence of inversion runs to first iteratively estimate the mud properties for a small depth section of the log; this estimate is then used to invert for the button standoff and the formation resistivity and permittivity for longer data sections.