Hydrocarbon-bearing conventional formations, mudrock formations, and source-rock formations generally contain clays, pyrite, magnetite, graphitelike carbon, and/or other electrically conductive mineral inclusions. Under redox-inactive conditions, these inclusions give rise to perfectly polarized interfacial polarization (PPIP) when subjected to an external electric field. Effective electrical conductivity and dielectric permittivity of geomaterials containing such inclusions are frequency-dependent properties due to the electric-field-induced interfacial polarization and associated charge relaxation around host-inclusion interfaces. Existing resistivity interpretation techniques do not account for PPIP phenomena, and hence they can lead to inaccurate estimation of water saturation, total organic content, and conductivity of formation water based on subsurface galvanic resistivity, electromagnetic (EM) induction, and EM propagation measurements in the presence of conductive mineral inclusions. In the first paper of our two-part publication series, we derived a mechanistic electrochemical model, the PPIP model, and we validated a coupled model that integrates the PPIP model with a surface-conductance-assisted interfacial polarization (SCAIP) model to quantify the frequency-dependent electrical complex conductivity of geomaterials. We have used the PPIP-SCAIP model to evaluate the dependence of effective complex-valued conductivity of geologic mixtures on (1) frequency, (2) conductivity of the host medium, and (3) material, size, and the shape of inclusions. Notably, we have used the PPIP-SCAIP model to identify rock conditions that give rise to significant differences in effective conductivity and effective relative permittivity of conductive-inclusion-bearing mixtures from those of conductive-inclusion-free homogeneous media. For a mixture containing as low as a 5% volume fraction of disseminated conductive inclusions, the low-frequency effective conductivity of the mixture is in the range of [Formula: see text] to [Formula: see text] with respect to the host conductivity for frequencies between 100 Hz and 100 kHz. Further, the high-frequency effective relative permittivity of that mixture is in the range of [Formula: see text] to [Formula: see text] with respect to the host relative permittivity for frequencies between 100 kHz and 10 MHz.

Electrically conductive mineral inclusions are commonly present in organic-rich mudrock and source-rock formations such as veins, laminations, rods, grains, flakes, and beds. Laboratory and subsurface electromagnetic (EM) measurements performed on geomaterials containing electrically conductive inclusions generally exhibit frequency dispersion due to interfacial polarization phenomena at host-inclusion interfaces. In the absence of redox-active species, surfaces of electrically conductive mineral inclusions are impermeable to the transport of charge carriers, inhibit the exchange of charges and behave as perfectly polarized (PP) interfaces under the influence of an externally applied EM field. Interfacial polarization phenomena involving charge separation, migration, accumulation/depletion, and relaxation around PP interfaces is referred to as PP interfacial polarization; it influences the magnitude and direction of the electric field and charge carrier migration in the geomaterial. We have developed a mechanistic model to quantify the complex-valued electrical conductivity response of geomaterials containing electrically conductive mineral inclusions, such as pyrite and magnetite, uniformly distributed in a fluid-filled, porous matrix made of nonconductive grains possessing surface conductance, such as silica and clay grains. The model first uses a linear approximation of the Poisson-Nernst-Planck equations of dilute solution theory to determine the induced dipole moment of a single isolated conductive inclusion and that of a single isolated nonconductive grain surrounded by an electrolyte. A consistent effective-medium formulation was then implemented to determine the effective complex-valued electrical conductivity of the geomaterial. Model predictions were in good agreement with laboratory measurements of multifrequency complex-valued electrical conductivity, relaxation time, and chargeability of mixtures containing electrically conductive inclusions.

Abstract Middle East carbonates frequently are heterogeneous in nature, encompassing variable pore types, strong diagenetic overprints, variable wettability and fracture networks amongst other effects. Resistivity borehole images have long been an integral constituent to understanding their complexity and unlocking volumes. High resolution LWD resistivity images were first introduced in the 1990's, however as downhole environments became progressively more challenging, resistivity images suffered from the dynamic acquisition environment resulting in severely degraded images. The Al Shaheen field has been developed with Extended Reach Drilling (ERD) wells, and wells of 30,000 feet are commonplace. Early LWD resistivity image data suffered from excessive stick and slip, with approximately half of the wellbore suffering from poor quality image data, degrading with depth. The outer portion of the wellbore is prohibitive to impossible to access via conventional drill pipe conveyed tools, resulting in an absolute requirement for high quality LWD resistivity images. The new methodology redefines the acquisition and processing methodology, resulting in images unaffected by stick slip with a 100% success rate in the most challenging of ERD environments. This paper illustrates the improvements in logging while drilling images (LWD) and subsequent fracture network characterization as a result of implementing a new image acquisition strategy and processing algorithm. The paper explores the close collaboration necessary to drive the innovation to dramatically enhance existing technology, and demonstrates the results with comparisons of the LWD images using the old and new methodologies. The development of the Al Shaheen Field The Al Shaheen field was discovered in the 1970's in connection with appraisal drilling of the underlying North Field Khuff reservoir. It is located on the central axis of the Qatar Arch some 70 Kms N-E of the Qatar peninsula, and covers an area of approximately 2080 km2, Figure 1. The reservoirs were initially considered uneconomical due to their thin oil columns. In 1992 Qatar Petroleum entered into an Exploration and Production Sharing agreement with Maersk Oil Qatar and the Al Shaheen field has since been under development, (Thomasen et al, IPTC-10854). The field contains multiple stacked reservoirs, and the main producing targets are the Cretaceous Kharaib B and Shuaiba carbonate formations and the Nahr Umr sandstone formation, Figure 1. The field is being developed using extended reach horizontal wells (ERD), and is home to some of the longest wells in the industry. The application of ERD technology has enabled the large areal accumulation to be developed using only nine production platforms. Horizontal well technology is particularly attractive due to thin producing columns. The length of these horizontal wells has increased over time, pushing the limits of available technology to today where for example wells of 30,000 feet to 40,000 feet are geosteered within a 10-foot window (Sonowal etc al, SPE/IADC-119506). Increases in well lengths also require continuous improvement in associated well technologies from completion technology (Brink et al, SPE-134934), to new logging technology.

Logging-While-Drilling (LWD) images acquired in horizontal wells are characterized by various features that are sensitive to formation structure near the wellbore. In current commercial processing, the different features – commonly referred to as “sinusoids”, “bulls-eyes”, or “reverse bulls-eyes” – are extracted from the images manually. However, manual feature extraction is time consuming and prone to user bias. This is of particular concern in high-angle/horizontal wells, where small errors in structural dip translate to large errors in reservoir volumetrics. Here we present a new automatic method for three-dimensional structural interpretation of sinusoidal and bulls-eye features observed in LWD images. With processing time of a few seconds for hundred feet of data, the method is sufficiently fast for use in real-time analysis, or to provide constraints for physics-based inversion.

Summary The paper illustrates the improvements in logging while drilling (LWD) images and subsequent formation evaluation by using a new methodology for depth and survey measurements corrections. LWD depth measurements are often considered inaccurate and, therefore, not as reliable for well-to-well correlations, correlations with data acquired with wireline measurements and formation layer thickness determinations. The reasons for these inaccuracies generally originate from the traditional practice that LWD depth is purposely made equal to the driller's depth, which is a static pipe length measurement made by tape at the surface. There is almost always a difference between the actual measured depth (MD) of the LWD sensor downhole and this static pipe measurement, because downhole the drillpipe is subject to an environment that is not representative of the derrick (e.g., varying drilling mechanical conditions and temperature changes). Here, we demonstrate the applications of the method, which allows dynamic driller's depth correction for the effects of drillstring weight, downhole friction, weight on bit, thermal expansion, residual rig heave, and tide. Another significant inaccuracy source is a standard practice of calculating borehole position from stationary survey points typically taken every 90 feet (ft) using the minimum curvature method. Neglecting the complex borehole shape between survey stations can lead to a systematic error in determining the borehole position. We consider using continuous inclination and azimuth measurements along with stationary surveys to correct these errors. We provide comparisons of LWD images before and after the depth and survey corrections to illustrate how the measurement errors affect formation dips interpreted from the images. We demonstrate how improved accuracy allows filtering out the artifacts and provides more decisive and accurate identification of geologic features. We show how using the corrected 3D position improves accuracy of the formation thicknesses calculations and therefore improves the reservoir summation results. As a result, we propose a borehole 3D position measurement that is accurate, consistent between wells (regardless of rig type or bottomhole assembly [BHA] configuration), and independent of the drilling mode. Using this new measurement significantly improves the quality of the formation evaluation.

D048 A New Methodology for Effectively Correcting LWD Depth Measurements G.A. Bordakov* (Schlumberger) A.V. Kostin (Schlumberger) J. Rasmus (Schlumberger) & H. Laastad (Statoil) SUMMARY LWD or drilling derived logging depths can be inaccurate and therefore unreliable for well-to-well correlations correlations to offset well data and formation thickness determinations. These inaccuracies generally originate from the fact that the LWD depth measurement is purposely made to equal the driller’s depth. The actual depth of the measurements downhole is rarely equal to this static pipe measurement for the primary reason that while drilling the drillpipe is subject to an environment that is not

Summary Six horizontal wells were drilled into the Tertiary Chatt Sand reservoir of the Breitbrunn gas field in Bavaria, Germany. The purpose of this campaign was to develop part of the depleted reservoir into a gas-storage sand. A detailed geological and petro-physical study was prepared before drilling and resulted in the identification of high-quality reservoir layers that were targeted by the horizontal wells. Despite the simple anticline structure of the field, geometric drilling was ruled out because of remaining geological and directional uncertainties. The geosteering approach adopted relies on real-time resistivity-at-bit images, which were used for the first time during this drilling campaign. The image data are compressed downhole and transmitted to the acquisition computer on the rig, where they are decompressed and analyzed. The images allow the precise placement of the borehole relative to the geology. Layer heterogeneity such as tight streaks, concretions, or patchy porosity can be identified as such and is not interpreted as a different layer entered by the hole, which would lead to a wrong geosteering decision. Logging-while-drilling (LWD) azimuthal data are acquired during drilling and during washdown passes that follow a bit change. A comparison of these time-lapse data sets can provide invasion profiles through time and around the borehole.

This article, written by Technology Editor Dennis Denney, contains highlights of paper SPE 71733, "The Use of Real-Time and Time-Lapse Logging-While-Drilling Images for Geosteering and Formation Evaluation in the Breitbrunn Field, Bavaria, Germany," by Hendrik Rohler, RWE-DEA A.G., and Ted Bornemann, Alexis Darquin, and John Rasmus, Schlumberger, originally presented at the 2001 SPE Annual Technical Conference and Exhibition, New Orleans, 30 September-3 October.

Over 100 horizontal laterals averaging 4,600 feet in length have been drilled in a shallow, heavy oil reservoir in Eastern Venezuela. Measurement While Drilling (MWD) continuous directional measurements and Logging While Drilling (LWD) azimuthal measurements are used to steer the wells through the reservoir, along a specific well path chosen from vertical stratigraphic wells for each pad and 3 Dimensional (3D) seismic which covers the field area. Experience from drilling the horizontal wells has shown that this reservoir is laterally discontinuous between the vertical stratigraphic wells and that this discontinuity is not always predicted from the surface seismic. However, the LWD azimuthal measurements, presented in the form of a 3D image is able to give the orientation of the wellbore relative to the geometry of the channels. Another use of the azimuthal images is to identify the numerous shale stringers that are only a few inches thick. These stringers are not detectable on the vertical stratigraphic wells due to the limited bed resolution of the wireline logging tools run in these wells. These stringers in the laterals greatly reduce the non-azimuthal resistivity measurements and give a false indication of an approaching reservoir bed boundary.

More than 50 horizontal laterals averaging 4600 feet in length have been drilled in a shallow, heavy oil reservoir in eastern Venezuela. Precise navigation through the formations was necessary to optimize placement of the lateral drainholes and maximize the percentage of reservoir sand exposed. To meet this requirement, an optimized bottom hole assembly (BHA) design and logging while drilling (LWD) interpretation technique were developed based on field experience.Measurement while drilling (MWD) continuous directional measurements were used to steer the well through the reservoir, along a specific well path chosen from three-dimensional (3D) seismic that covers the field area. The LWD azimuthal and bit electrode measurements were integrated with the 3D seismic to allow an interactive interpretation of the stratal boundaries present within the reservoir which resulted in continual refinements to the planned well path. Using the information derived from the LWD azimuthal and bit electrode measurements, the location of shale stringers, bed boundaries, and pay sand relative to the BHA could be determined and the well then steered in the appropriate direction to maintain an optimum position within the oil reservoir.As a result, the number of sidetracks was reduced and the percent sand encountered increased. The longest horizontal lateral in Venezuela was also drilled and completed as part of this project.