LWD/MWD FORMATION EVALUATION

Recent advances in well logging and formation evaluation

 The new developments include enhancements to wireline and while-drilling resistivity logging, pulsed-neutron logging, open- and cased-hole acoustic logging, formation testing, rig site and core evaluation, and online borehole databases. 

Stephen Prensky, Consultant, Silver Spring, Maryland

Advancements in logging and formation evaluation during the past year included conceptual designs for new logging tools, field testing of new or modified logging devices and improved characterization of previously introduced logging tools gained through modeling, field experience or new processing and interpretation methods.

WIRELINE LOGGING

Logging platform. Halliburton introduced its new PC-based LOGIQ* logging platform. Comprised of a surface system and a new �quad-combo� downhole toolstring, this system is configured to run all open- and cased-hole log-acqusition and interpretation services. The new quad-combo string is lighter and shorter than the previous generation and is rated to 350° F and 20,000 psi. The ethernet-based communication protocol allows a fivefold increase in uphole data capacity. ¹ 

Resistivity logging. New commercial logging tools undergoing field tests include array-resistivity and microresistivity devices. Also, the conceptual design for an azimuthal laterolog capable of resistivity imaging was proposed.

Array resistivity. Halliburton's ACRt* array-resistivity device uses new coil configurations, transmitter-receiver spacing, and new techniques to correct for borehole, environmental and skin effects. The multi-frequency tool consists of one transmitter and six sub-arrays (each consisting of a receiver pair) that are all placed on one side of the transmitter (asymmetrically) to allow tool length optimization, Fig. 1.² Similarly to other array-induction devices, it provides five radial depths of investigation, ranging from 10 to 90 in., and sets of matched vertical resolutions at 1, 2 and 4 ft. This design provides more accurate resistivity measurements and improved estimates of invasion depth.

Fig. 1. Schematic of the ACRt array-resistivity tool showing the relative spacing of the receiver sub-arrays. This arrangement assures complete acquisition of radial resistivity information. The shortest sub-array, S.6, is used for borehole correction.2

Microresistivity. Baker Atlas has developed a new microresistivity pad tool that simultaneously provides microlaterolog and microlog-equivalent data for improved flushed-zone characterization. 3 Microloaterolog measurements are obtained using three current-measuring buttons embedded in a metallic pad. The metallic pad surface is held at a known potential and current return is through the mandrel and metal linkage supporting the pad. A motorized adjustable standoff ensures the pad is pressed against the borehole wall. Fast, non-linear inversion methods are used to derive mudcake resistivity (R _mc ), mud thickness (h _mc ), and flushed-zone resistivity (R _xo ) corrected for mudcake. Forward modeling of the inverted microlaterolog values is used to synthetically derive the microlog measurements.

Azimuthal-electrode device. The objective of the new azimuthal tool proposed by researchers at the University of Houston is to measure resistivity anisotropy in highly deviated and horizontal wells drilled with conductive mud. The tool consists of 14 electrodes symmetrically arranged, relative to the main electrode, along the tool axis. The main electrode itself consists of 12 azimuthal electrodes. This arrangement provides six depths of investigation that allow 3D, i.e., radial, imaging of formation resistivity. ⁴ 

Acoustic logging. In addition to formation-evaluation and geomechanical applications, data from multipole array-acoustic logging devices can also provide near-well reflection surveys used to create seismic images of small-scale geological structures and features, such as bed boundaries, thin beds (stringers), fractures and faults up to 10�15 m from the borehole. In effect, these logging tools conduct microscale 2D and 3D seismic reflection surveys in vertical and high-angle wells, Fig. 2. These surveys bridge the resolution gap between surface seismic and well logs. The high-resolution (0.5 m) of these images is two-to-three orders of magnitude greater than images obtained using conventional surface or borehole-seismic techniques.

Fig. 2. Schematic of a borehole seismic reflection survey using a crossed-dipole acoustic logging tool.6

Near-well images are generated using standard seismic processing methods, including filtering, stacking, wave separation and migration. These techniques extract the background compressional- and shear-reflection data from the overwhelming direct-wave (borehole) signal. Continuing interest in acoustic-log near-well imaging is indicated by development of new techniques for optimizing data processing.^5�9

Ultrasonic refracted-velocity imaging. A new imaging technique under investigation uses refracted-wave velocity (compressional or shear), rather than reflected-wave velocity (used by existing borehole imaging devices), to image formation features and to visualize borehole stresses and stress concentrations in the near-borehole region. ¹⁰ Referring to Fig. 3, horizontal bedding (alternating light and dark banding) is seen in the unstressed example (left). Stress concentrations induced near the borehole (right) produce vertical light and dark banding that mirror the stress-induced velocity variations. Stresses are low at 0° and 180° (dark bands) while high compressive stresses at 90° and 270° produce higher velocities (lighter bands). The very bright dipping bands are caused by an iron cementation streak. ¹⁰ 

Fig. 3. Compressional-wave velocity images of a borehole in sandstone subjected to external uniaxial horizontal stress. Horizontal bedding (alternating light and dark banding) is seen in the unstressed example (left). Stress concentrations induced near the borehole (right) produce vertical light and dark banding that mirror the stress-induced velocity variations. The very bright dipping bands are caused by an iron cementation streak.10

The sensitivity of this technique to borehole stresses, and relatively high-resolution images of fractures and other borehole features, may exceed those currently available from crossed-dipole logging devices. The very shallow investigation depths obtained using high operating frequencies, e.g, 100�500 kHz, are the depths of strongest stress concentration. By comparison, conventional acoustic logging tools typically operate at frequencies =30 kHz.

Nuclear logging. Advancements were made in density measurement, pulsed neutron and detector design for nuclear logging.

Cased-hole density logging data acquisition, modeling, and processing. Reliable density data can be acquired through casing, using openhole density tools, when operational considerations prevent openhole density logging. However, because steel casing, casing collars, cement and quality of the cement bond contribute to a reduction in the logging signal and nuclear statistics, slower logging speeds are required and special processing techniques are needed to extract reliable formation measurements.

Typically, only the high-energy windows are used and, in some circumstances, the combined casing effects may even invalidate the short-spaced detector; three-detector density devices offer a technical advantage in these situations. Cased-hole data can be integrated with other openhole or cased-hole logging data for gas detection (e.g., bypassed pay), evaluation of fluid saturation, and general formation evaluation. ^11�13

Pulsed-neutron logging. Mineral-based lithofacies analysis uses elemental concentrations (yields) derived from natural and induced gamma-ray log spectra to compute core-calibrated formation lithology and mineralogy. New interpretation models and software allow faster and more accurate lithofacies analyses and formation evaluation of siliciclastic and carbonate reservoirs. Baker Atlas' RockView* service is based on a new logging system that incorporates a 14-MeV pulsed-neutron sonde (FLS) with natural spectral gamma-ray, compensated neutron, and density instruments (Fig. 4).¹⁴ During logging, periodic adjustments of the pulsed-neutron source output and a special depth filter are used to optimize count rates and thereby enhance statistical precision.

Fig. 4. Baker Atlas� RockView logging sonde.1410

Mineral-based lithofacies can be used by itself or as a component of an integrated formation evaluation, ^15,16 e.g., Schlumberger's DecisionXpress.* Reservoir pressure testing, sampling and formation testing can be optimized through the combined application of mineralogy-based lithofacies analysis and borehole imaging data.¹⁷

GasView,* Baker Atlas' new high-resolution cased-hole reservoir-monitoring service, is made possible by a combination of pulsed-neutron hardware improvements and a new analytical model.¹⁸ Significantly greater sensitivity in the gas response, that is, gas saturation, has been achieved through a reduction in the spacing of the extra-long-spaced detector on the RPM three-detector pulsed-neutron device. Improved tool response means decreased statistical uncertainty in the inelastic and capture count rates. The analytical model, which is based on these count rates, separates the tool response due to gas from the response caused by variations in mineralogy and porosity for a more accurate computation of gas saturation.

Coaxial-detector device. Operational considerations limit the size of scintillation detectors used in through-tubing carbon/oxygen tools; this limits the accuracy of the C/O measurements. A new C/O tool design using coaxial scintillation detectors (one inside the other), coincidence counting and new data-analysis algorithms, is being investigated as a means for improving the efficiency, signal-to-noise ratio, and spectral resolution of C/O tools. ¹⁹ The arrangement of the detectors, one inside the other, is designed to trigger a counting method that uses the coincidence of nuclear events recorded by the coaxial detectors to significantly reduce background counts and thus, increase signal-to-noise. Laboratory data from experiments conducted at the North Carolina State University, using NaI and high-density BGO detectors, are in good agreement with modeled data. Field tests are planned for the experimental tools.

Wireline formation testing. Two modifications to WFT probe designs promise to reduce costs by improving sampling efficiency, i.e., reducing sampling time, especially in homogeneous formations.

Oval pad. Halliburton introduced a 10-in. oval-shaped probe that replaces the two standard probes on its WFT dual-probe sub, Fig. 5. ²⁰ This modification is designed to extend fluid sampling and pressure testing in low-mobility (thin bed or low-permeability), fractured and heterogeneous (e.g., carbonate) reservoirs. The oval shape provides the increased sampling area and vertical sealing of straddle-packer probes, while retaining the operational flexibility of conventional probes.

Fig. 5. Halliburton�s the new WFT dual-probe oval pad (upper) is compared with the conventional dual-probe pads (lower).20

A focused-sampling probe uses two sampling assemblies, i.e., intakes, flowlines and pumping systems, to separate drilling and mud contamination from virgin reservoir fluids. ²¹ The primary �sample� intake is placed near the center of the probe area, where the sample is most likely to be uncontaminated, and the secondary, or �guard� intake is placed close to the probe periphery, where contamination is more likely, Fig. 6. Fluid analyzers are placed on both the sample and guard flowlines to ensure that uncontaminated samples are collected, Fig. 7. Using these ideas, Schlumberger's Quicksilver* tool has successfully completed field tests.^22,23 

Fig. 6. Comparison of a conventional WFT probe (top) and the focused sampling probe used by Schlumberger�s Quicksilver* service (bottom).22

Fig. 7. Schlumberger�s Quicksilver* focused-sampling WFT tool.22

Acoustic logging. Advancements include ultrasonic evaluation of lightweight cement and well integrity.

Cement evaluation. Conventional sonic and ultrasonic tools use compressional-wave velocity for cement evaluation; however, tool limitations often affect the quality of the evaluation. First, the ability of the tool to differentiate low-impedance cement from drilling mud is especially critical in the newer high-performance lightweight and foam cements. Second, a shallow depth of investigation may not fully characterize the material in the annulus. Third, the ability of a particular tool to distinguish channeling from poor cement maybe limited by insufficient azimuthal resolution.

Schlumberger's Isolation Scanner* service (Imaging Behind Casing* tool) adds ultrasonic flexural-wave velocity and attenuation measurements to complement conventional pulse-echo measurements and, thereby, obtain additional information about the cement bond at the casing-cement boundary and at the cement-formation interface. Using an interpretation model that integrates both sets of measurements provides enhanced evaluation in all types of cement. The tool's rotating subassembly contains four transducers: a flexural-mode transmitter, two flexural receivers and a ultrasonic pulse-echo transducer, Fig. 8. ²⁴ 

Fig. 8. Sketch of the rotating subassembly of Schlumberger�s Isolation Scanner showing the relative positions of the pulse-echo transducer and the flexural-mode transmitter and receivers.24

The flexural-mode transmitter, operating at 200 kHz, excites a casing flexural mode that leaks into the annulus. Attenuation of the flexural first casing arrival is used to estimate the state of the material immediately behind, i.e., coupled to, the casing. Echoes from the annulus-formation interface allow improved estimates of the material between the casing and the formation. The transducers are positioned and configured to provide flexural and pulse-echo measurements having similar azimuthal and vertical resolutions. Borehole fluid properties are estimated directly from the two sets of independent measurements. The combined data are used to compute the state of the material in the annulus and are graphically presented as an azimuthal solid-liquid-gas map.

The acquired data permit visualization of the position of casing in the borehole and wellbore shape, as well as determine casing ID and thickness. Vertical resolution, varying from 0.6 to 6.0 in., is a function of the logging speed and is selectable. The tool can be run in casing sizes ranging from 4.5- to 9.675-in. diameter and casing thicknesses between 0.15 and 0.80 in.

Noise logging. Fluids flowing through restrictions in or behind casing, e.g., cement channels, perforations and casing leaks, produce distinctive acoustic signals. The pressure differential created by the restriction produces variations in fluid velocity, i.e., flow regime (turbulence), that generate characteristic acoustic signals. These acoustic signals are recorded by noise logs and analyzed to detect channeling or cement annulus, locating fluid entry through perforations, casing leaks and determining the presence of single- or multiphase flow.

Conventional noise logs typically operate in stationary mode and record only the audible acoustic frequencies. Experience with surface systems demonstrated that high-frequency, i.e., ultrasonic, acoustic signals are more sensitive to small leaks and, because they propagate shorter distances than lower-frequency audible signals, can locate these leaks with greater accuracy. WelTec's Well Leak Detector* logging device uses a piezoelectric sensor sensitive to ultrasonic frequencies, and signal processing filters out unwanted background noise and audible frequencies. ²⁵ Field tests have demonstrated sensitivity to leaks as small as 0.026 gal/min, through multiple strings of tubing or casing, with an accuracy of 3 ft. The tool can operate at conventional logging speeds or in stationary mode.

LOGGING-WHILE-DRILLING

Telemetry. Grant Prideco's wired-pipe telemetry system, reported last year, underwent further field trials, including the first commercial deployment of an LWD toolstring in the BHA. Baker Hughes provided the network-compatible LWD tools and the surface software developed to manage the high data volumes transmitted by the telemetry network. The bi-directional communication capability was successfully tested by downhole reprogramming of the surveying data. ²⁶ 

Directional-survey reliability and QC. The reliability of directional-survey data is essential for real-time well placement and achieving optimal production. However, the validity of these data depends on how well the survey tool and actual data acquisition conform to the assumptions of the survey tool's error model regarding tool quality, operating procedures and environmental conditions. Operating companies are addressing this issue individually ²⁷ and as an industry. The objective of the SPE Wellbore Positioning Technical Section (WPTS) is to improve and standardize survey-tool error modeling. The most recent WPTS report identifies sources of survey error, the means for identifying such errors, and suggests alternative QC measures to ensure data integrity and survey reliability. ²⁸ 

Depth accuracy. Improving logging depth uncertainty is a continuing concern for industry. Statoil has developed a methodology to correct both LWD and wireline depths in the North Sea for pipe and cable stretch and proposes establishing a new (additional) depth expression called �corrected depth.� ²⁹ Schlumberger introduced an LWD depth-correction methodology that calculates both the mechanical stretch and thermal expansion of the drillstring. 30 Baker Hughes continues to develop the rigorous depth-correction model, called �dynamic depth correction� ³¹ (discussed in this space last year).

Retrievable resistivity device. Wireline retrievable gamma-ray/MWD systems have been available for several years. Wireline retrievability allows probe replacement in cases of tool failure, without necessitating pipe tripping, and permits full recovery of memory data in stuck-pipe situations.

Schlumberger has added borehole-compensated propagation resistivity to the list of retrievable logging services with a 1.75-in. diameter, self-contained dual-frequency device. ³² The tool uses a balanced dual-transmitter, dual-receiver configuration (effective T-R spacing of 33 in.) operating at 400 kHz and 2 MHz, to provide measurements at four investigation depths.

The tool requires special stainless-steel drill collars that allow electromagnetic transmission through axial slots. Collar sizes of 4.75, 6.75, and 8.25 in. are available to meet a variety of drilling requirements. The tool is rated to 150° C and 20,000 psi. A large downhole memory and low power consumption allow operating times up to 300 hours. The tool is capable of 6-in. sampling at logging speeds up to 1,800 ft/hr.

Fig. 9. Ultima Labs' compensated propagation resistivity (CPR) tool.33

Compact resistivity tool. Conventional compensated LWD resistivity measurements are typically based on a single transmitter and two receivers. A new tool design uses a different configuration, two transmitters and three receivers placed in an unbalanced arrangement, Fig. 9; the raw measurements are made using two transmitters and a single receiver. ³³ Compensated-resistivity is obtained using a new depth-based compensation method. The configuration used by Ultima Labs has an antenna-array length that is nearly half that of tools using balanced T-R configurations; this results in a compact tool of approximately 13 ft. Multiple depths of investigation are obtained using two operating frequencies (400 kHz and 2 MHz) and T-R spacings of 18, 27 and 36 in.

RIG SITE EVALUATION

Schlumberger introduced a PVT characterization service to facilitate rapid turnaround of downhole reservoir-fluid samples at the rig. The modular PVT Express* system is comprised of a PVT cell, a GOR module, a dual gas chromatograph, and a digital acquisition and control module. ³⁴ The high-pressure PVT cell measures single-phase fluid compressibility, saturated fluid density and
reservoir bubble point (fluid saturation pressure) at surface conditions. The GOR module uses a single-phase flash process to separate gas and liquid then measures GOR, liquid density and viscosity.

The gas chromatograph measures pure and grouped hydrocarbon compositional components in the gas (C12+) and liquid (C36+) samples. At the completion of each measurement cycle, samples are displaced or injected into the next module. Sample analysis requires 8�12 hours. A separate and optional software application, PVT Expert,* uses a neural-network based prediction model to deliver rapid volumetric phase behavior fluid properties from a specific set of measured data.

Shale membrane efficiency, typically measured on core, is an important parameter in wellbore-stability analysis. This information is used in designing drilling muds that prevent borehole collapse by strengthening shales, i.e., restricting water movement into the shale. A rapid, rigsite-deployable electrochemical test has been developed that allows evaluation of membrane potential on shale cuttings, as well as core. 35 The test measures the voltage drop across shales in contact with fluids of different salinities. Shale membrane potential, i.e., its ability to restrict ion flow, can be determined directly from the voltage drop. This is made possible through a series of linked relationships: shale-ion selectivity is calculated from the voltage drop; ion selectivity is proportional to the Cation-Exchange-Capacity to permeability ratio (CEC/k); and membrane potential, in turn, correlates with this ratio.

IP MEASUREMENTS IN SHALY SANDS

Induced polarization refers to anomalous storage of electrical charge in rocks resulting from electrochemical effects during current flow. IP effects are typically associated with oxidation and reduction in metallic ore minerals; IP logging is used to explore for copper and sulfide ores. The significance for the petroleum industry is that the presence of clay minerals can also result in a significant polarization signal. The clays in shale act as cation-selective membranes and restrict the flow of negatively charged cations and, after sufficient time, an electrochemical gradient is established across the clay-free zones by the buildup of electrolyte concentration at the edge of the clay-rich zone.

In a manner similar to NMR relaxation, the ions relax to equilibrium positions upon termination of the electric current, creating an induced-polarization decay curve as the concentration gradient decreases. Similar to NMR, IP decay can be related to pore-size distribution and pore-throat radius (permeability) in brine-saturated shaly-sands and inversion of these data allows estimation of permeability, Fig. 10. Similar to NMR, IP measurements have the potential to provide direct in-situ measurements of shaliness, e.g., cation-exchange-capacity (Q _vv) and water saturation (S_w ). ^36,37 In contrast to NMR, however, IP can provide a statistically more robust, as well as a deeper-reading permeability measurement. 38 IP may offer faster, less expensive and a non-destructive alternative to conventional capillary-pressure measurement in core evaluation (Fig. 10). ^38,39 

Fig. 10. Plots comparing laboratory derived capillary-pressure pore-size distribution (open circles) with pore-size distribution from IP-relaxation spectra (closed circles). Sample A (upper), k = 0.075 md, ? = 11%; sample B (lower), k = 2.8 md, φ = 16%.39

ONLINE BOREHOLE DATABASES

Online access to borehole information is available through a number of databases made available by national governments and commercial services. ^40-43 The new European eEarth database, ^40,41 funded in part by the European Commission, uses a multi-lingual web-GIS interface to provide access to borehole data for 2.7 million wells from the national borehole databases of six participating European countries (Britain, the Netherlands, Germany, Czechoslovakia, Lithuania, and Poland), Fig. 11. The separate eEarth-Mobile application allows portable access to the database via PDA. These databases contain a wide spectrum of data including rock properties, core descriptions, borehole temperatures, geological descriptions, and engineering data. Costs can vary from free to a nominal fee or a commercial license.

Fig. 11. eEarth, part of the EC eContent program.

VISIONARY OR SCIENCE FICTION?

Two papers caught my attention for their creative ideas, though not necessarily for their practicality. They suggest (1) the use of nanotechnology to measure borehole-fluid and formation properties, and (2) the transformation of well log measurements to music. The first paper proposes using nanorobot sensors, placed in the mud column, to measure borehole fluid and formation properties; data retrieval takes place at the surface as the circulating mud passes over the shale shakers. Actual details of these components and how such a system might work, are lacking. ⁴⁴ 

The second, slightly more serious paper, demonstrates that well logs can be transformed into audible sounds, even to musical notation, and suggests there might be geological value derived in doing so. ⁴⁵ Research on this subject, using other physical or mathematical phenomena (e.g., earthquake activity, long-term climate oscillations, and the changing shape of fossils), indicates that evaluating the data without �directly looking at it� may lead to faster identification of some anomalies. A suite of well-log measurements from Canadian wells (e.g., SP, sonic velocity, porosity, gamma-ray, and resistivity) were mapped to a number of musical attributes and the transformed logs output as MP3 files. Changes in petrophysical properties (e.g., porosity or lithology) can be represented by variation in these attributes (e.g., tempo and volume). Maybe future log analysis will be conducted using your iPod!!   

ACKNOWLEDGEMENT

 * Trademark�these are the property of the respective companies with which they appear.

LITERATURE CITED

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⁴⁵ Stewart, R.R., and Brough, S., �Log jammin': Transforming well logs to music,� paper INT 3.1, 76 th SEG Annual Meeting (2006).

 

THE AUTHOR
	Stephen Prensky has 32 years working experience in petroleum geology and petrophysics. He previously worked for Texaco, US Geological Survey, and Minerals Management Service, and currently works as a consultant to logging service companies. He has served as SPWLA Vice-President of Technology, as editor of SPWLA's Petrophysics, and now serves on the SPWLA Technology Committee and the SPE Formation Evaluation Committee. His annual �Bibliography of Well-Log Applications� has been published by SPWLA for 20 years. He is a member of AAPG, SPE, and SPWLA.