Advances in MWD and formation evaluation for 2000

David Patrick Murphy

Developments include new wireline, memory, MWD, mud logging and testing tools

Recent advances in formation evaluation, especially data acquisition while drilling, are improving quality and increasing the quantity of formation data while reducing costs and making operations more efficient. Some of those advances include the topics of:

* General--Calibration facility and CD-ROM/Internet information resources/tools

* Wireline--Logging systems, tools and auxiliary devices

* MWD--Formation evaluation sonic and density tools

* Memory--Slimhole memory openhole logging

* Mud logging--Hydrocarbon detection and analysis tools

* Testing--Modular tools.

These topics and the tools/techniques involved are discussed and illustrated in the following presentation.

GENERAL

A new logging tool calibration facility has evolved from calibration blocks rescued from the EUROPA facility. The Internet and CD-ROMs continue to provide improvements in using and accessing formation evaluation and MWD information.

Calibration facility. Callisto (Calibration and In Situ Tool Optimization), a new well-logging-tool, test pit facility in East Leake, England, with open industry access, utilizes calibration blocks rescued from the EUROPA facility in Aberdeen, Scotland.[1,2] Following consultations with EUROPA's original oil- and service-company sponsors, along with Aberdeen University and the UK Government's Offshore Supplies Office (OSO), an agreement was reached between AEA Technology, Leicester University and Reeves Wireline Technologies Ltd. (RWTL) which transfers ownership of the test formations to Leicester University and operatorship to RWTL at East Leake.

The facility has four tanks, like shown in Fig. 1, each with two fresh-water formations, including three sandstones, three limestones and two dolomites--all with 8.5-in. holes. Reasonable access is guaranteed to third parties through Leicester University--nominal charges for access will mean that the facility runs on a nonprofit basis. Contact should be made via Mike Lovell ([email protected]), Geology Department, Leicester University, LE1 7RH, England. Tel: 44 (0)116 252 3798/3641.

[Figure 1 ILLUSTRATION OMITTED]

SPWLA Transactions. The Society of Professional Well Log Analysts (SPWLA) has added to its published Transactions of the SPWLA on CD. Volumes 11-20 (1970-1979) are now available, in addition to previously published Volumes 1-10 (1960-1969), in high-resolution Adobe Acrobat PDF image file format.[3] Work on volumes for the 1980s is underway. Each volume has been indexed and linked to allow rapid location and viewing of any paper. Paper abstracts have been typed into separate, text-searchable PDF files linked to corresponding papers. Each CD works with PC, Mac or Unix and is priced at US$35 for a single-user license. For information on network or corporate licensing, contact SPWLA directly (www.spwla.org).

SPE TIGs. The Society of Petroleum Engineers (SPE) has reorganized its Technical Interest Groups (TIGs) to focus on the topics of most interest to its members. TIGs are groups of petroleum professionals organized around specific technical disciplines aimed at furthering the transfer of technology.[4] TIG programs and communications are electronically facilitated by SPE using a secured site on the Internet. TIGs are truly international in scope and participation and are accessible 24 hours a day, seven days a week. Twenty-five TIGs are currently in place, with a number of subtopics in some categories. These TIGs encompass the breadth of SPE's technical coverage.

Membership in an SPE TIG is open to all individuals. You do not have to be an SPE member to join a TIG, and you may join as many TIGs as you choose. SPE members receive membership in one TIG free of charge; memberships in additional TIGs for SPE members are US$15 each per year. Nonmember enrollment is $30 per year for each TIG.

You may join a TIG using the online TIG Enrollment Form. This form and other TIG information can be found at: http://www.spe.org/tigs/. Current TIGs related to formation evaluation and MWD are:

* Drilling

* Formation evaluation

* Reservoir characterization

* Well testing.

Log data toolbox, Schlumberger's Log Data Toolbox suite of software applications simplifies manipulation and use of digital log data.[5] With these freeware utilities that run on Windows NT4.0, Win95, Win98 and Win2000 operating systems, users can inspect, filter and convert digital log data files before they are integrated into an interpretation software or data-management system. Log Data Toolbox, available to Schlumberger customers at: www.connect.slb.com, contains the following six applications to help handle DLIS and LAS files:

1. DLIS to ASCII

2. LAS Certify and Repair

3. DLIS Info View

4. DLIS to LIS

5. LIS to DLIS

6. ASCII Info View.

WIRELINE LOGS

Wireline logs typically provide more formation evaluation wellbore data for lower cost than other tools, except when rig costs are very high. Nine subtopics under this general category--including three wireline, nonlogging applications--are discussed here.

Logging system. Halliburton has developed its new Polaris suite of wireline logging tools with design criteria focused on high reliability, i.e., four-year operation without failure. Sensors currently being field tested include array induction, compensated neutron, gamma ray and telemetry. The density sensor is in engineering design. All but the latter will be in limited commercial operation during 2000. Polaris tools are backward compatible and can be run in combination with Halliburton's existing DITS and imaging tools (EMI and CAST-V).

Polaris array induction is called High Resolution Array Induction (HRAI); its features include:

* 10-, 20-, 30-, 60-, 90- and 120-in. depth-of-investigation results

* Bed-thickness resolution down to 1 ft

* Invasion profiling

* Independent mud-resistivity measurement.

High-temp logging. Schlumberger has introduced its Xtreme, high-pressure, high-temperature suite of 4-in.-OD wireline logging tools for hostile environments, Fig. 2.[5] Xtreme is rated to 500 [degrees] F and 25 kpsi. Design features include state-of-the-art, low-power electronics, increased thermal mass, hostile-rated materials and internal-temperature sensors. The suite's modular design enables tool string configuration to be assembled according to individual job requirements. A typical configuration can include Array Induction Tool, digital monopole Sonic Logging Tool, Litho-Density Sonde, Array Porosity Sonde neutron and Natural Gamma Ray Spectrometry Sonde. Xtreme Array Porosity Sonde neutron features include:

[Figure 2 ILLUSTRATION OMITTED]

* Epithermal measurement, minimizing lithology and density effects

* Accelerator-based neutron source, not chemical source

* Flushed-zone, sigma-capture, cross-section measurement for use as a shale indicator to help distinguish gas from oil-beating zones and to determine flushed-zone saturation independent of resistivity results.

Nuclear magnetism. Commercially available Nuclear Magnetic Resonance (NMR) tools include Schlumberger's Combinable Magnetic Resonance (CMR) and Halliburton/NUMAR's Magnetic Resonance Imaging Log (MRIL). MRIL is available from Halliburton/NUMAR, Baker Atlas and Computalog; CMR is available from Schlumberger. A discussion of the newest generation of CMR and MRIL follows.

Schlumberger's new CMR-Plus NMR tool can acquire full NMR data at logging speeds up to 2,400 ft/hr, comparable to nuclear wireline tool speeds and three to five times faster than previous-generation CMR tools.[6] Improved logging speeds are due to the use of log prepolarization magnets and a new method of acquiring measurements. A new acquisition sequence and processing algorithm has also been developed that improves precision of the total porosity measurement and the [T.sub.2] sensitivity limit. Although it was developed in conjunction with the CMR-Plus tool, the new acquisition sequence and processing algorithm can be applied to previous-generation CMR tools.

Halliburton/NUMAR's MRIL-Prime (sometimes called MRIL-D)[7] has nine sensitive volumes, three times as many as the previous-generation MRIL. Faster initial polarization is accomplished by a change in tool design. Separate polarizing fields are used to rapidly increase and stabilize nuclear magnetism before measurements are taken. A high-power transmitter allows for essentially uninterrupted pulsing. MRIL-Prime features vs. previous-generation MRIL are: a single pass for what was obtained in separate uplog/downlog operations, logging speeds increased by up to a factor of four, noise suppression by a factor of two, and expanded multi-parameter acquisition.

Shell's recent work investigating NMR-restricted diffusion and internal field gradients provides a comprehensive study of these two complications for NMR interpretation.[8] Fluid identification, using diffusion-based NMR techniques, relies on knowledge of the applied field gradients and is complicated by internal field gradients caused by iron-rich minerals. Since knowledge of the diffusion coefficient is required to analyze NMR log data, restricted diffusion caused by small pore sizes may significantly complicate evaluation. If the rock being evaluated contains iron-rich minerals or small pore sizes, rock-lab NMR measurements can provide information that will yield more accurate NMR log evaluation results.

Openhole resistivity. Resistivity logs are typically used to obtain true formation resistivity (Rt) for determining water saturation. Induction tools are used in nonconductive, or fresher, water-based muds. Laterolog or lateral tools are used in more conductive, or salty, water-based muds. Array devices provide multiple depths of investigation and are the newest tool types.

Multi-array induction tools measure formation conductivity using multiple, three-coil arrays (one transmitter and a pair of receiver coils). Measurements from various arrays are processed through a software algorithm, referred to as "software focusing" resulting in a set of curves with predetermined depths of investigations, vertical resolution and other optimized 2-D features. Baker Atlas has developed a new focusing scheme, called "inhomogeneous background-based focusing" (IBF), which improves software-focusing performance, resulting in more accurate resistivities in high [R.sub.t]/[R.sub.shoulder] contrast situations.[9] IBF processing is applied to raw data from its High Definition Induction Logging (HDIL) tool, an array device.

Schlumberger has developed a series of enhancements to its Array Induction Imager Tool (AIIT) family of multi-array induction tools which addresses several troublesome borehole and formation environments.[10] A web-based job planner allows log analysts to order the correct tool and advises on best rigup and resolution for the expected environment. A real-time environmental log quality check (LQC) algorithm looks at the same information used in the job planner but uses actual hole size and resistivity measurements.

The LQC algorithm also detects hole rugosity that can affect the ability to obtain good, high-resolution logs. Environmental LQC flags are used to produce the optimum resolution that can be supported by the environment. An algorithm to estimate borehole magnetic permeability is used to select more robust processing if this fairly rare problem is encountered. Computer-center, "log-repair" software has been developed to remove periodic error signals associated with corkscrew boreholes. Log processing based on maximum entropy inversion handles extremely high [R.sub.t]/[R.sub.shoulder] contrast and dip effects.

Dipole sonic. Computalog has introduced a new dipole-array sonic tool for vertical and deviated wells, the Monopole Dipole Array (MDA) tool.[11] MDA's main design feature is a semi-rigid tool body which consists of a series of interlocking steel rings configured in a special pattern and individually isolated by an elastomer. The latter is designed to have high compressive and tensile strength with low shear stiffness to disrupt tool-body waves.

MDA consists of a top section with an electronics cartridge, a set of two dipole transmitters oriented 90 [degrees] from each other for cross-dipole acquisition, and a monopole transmitter. The bottom section comprises an array of eight configurable dipole, cross-dipole or monopole receivers spaced 6 in. apart, and digitizer electronics. MDA dipole transmitters have a broad bandwidth capable of exciting boreholes from near 600 Hz to well above 5 kHz. MDA is 3 3/8-in. OD, which allows logging in small liners down to 4-in ID.

Pulsed neutron capture. Cased-hole pulsed neutron capture (PNC) logs determine water saturation in high-porosity, high-water salinity zones. They also distinguish formation liquids from gas. PNC log inelastic and capture spectral measurements can improve gas detection, determine fluid oil saturation (C/O), identify lithology (Ca/Si), estimate porosity and help with horizontal production logging.

Schlumberger has introduced a new service for its Reservoir Saturation Tool (RST) called RSTPro.[11,12] One of the main applications is enhanced C/O saturation measurement designed to improve precision and accuracy of C/O logging. Key features are:

* New spectral standards which extend response in low-porosity and gas environments

* Full-spectrum calibrations which minimize tool-to-tool variability

* Expanded tool-characterization database which nearly doubles the existing database

* Smooth database interpolation and extrapolation which improves accuracy

* High-precision mode for field monitoring

* Three-phase reservoir methodology which prescribes a method for computing oil saturation in the presence of an independently measured gas saturation.

A new method to calculate a density-related trace from PNC data has been proposed and applied by Cabinda Gulf Oil.[13] Traditional bulk-density measurements are based on gamma ray attenuation between source and detectors. This new PNC method separates out and ignores PNC gamma rays unrelated to bulk density during the neutron pulse. It is limited by the fact that some of the gamma rays detected reflect borehole, rather than formation properties. Like previous approaches that derive density information from PNC logs, this one depends on having relatively constant wellbore conditions.

Results are better from PNC tools that have a large number of gamma ray count gates and consistent neutron-pulse duration. While densities calculated from a PNC tool (designed primarily to make different measurements) will probably never be as accurate as those from a specialized density tool, they are economically attractive because they come from data taken for other purposes.

Imaging. DHV International Inc. has developed a slickline (memory) down-hole video camera. The prototype tool is time programmable and captures five images per trip into the well, Fig. 3. The memory tool joins DHV's two existing downhole video systems: Fiber Optic and HawkEye II.[14] The Fiber Optic system uses fiber optic cable for live, full-motion video, which is required for surveying fluid entry. HawkEye II's portable video system is deployed on single- or multi-conductor wireline cable for viewing mechanical problems or for continuous casing/tubing inspection in wells when the fiber optic system is: unavailable, uneconomic to mobilize, or too corrosive for the fiber optic cable. The new memory system is appropriate for many of the same applications as Hawkeye II, but can be run on slickline.

[Figure 3 ILLUSTRATION OMITTED]

Wireline jar, Evans Engineering & Manufacturing, Inc. is prototype testing its Model CK Wireline Jar.[15] Originally designed for perforating, this wireline jar will work with any tool--logging, etc.--that can be run on a single-conductor wireline. During tool setup, a preset tension load is selected and set. If the wireline tool becomes stuck, any wireline pull greater than the preset load will trigger the jar after a few seconds. Slack is then applied to the wireline to close the jar, and it is immediately ready to trigger again.

Tractor. The SmarTract-WL2-1/8 tractor, developed by SensorWise, is a "crawler" type tractor used for deployment of tools into horizontal wells.[16,17] Its motion is provided by setting an anchor against the wall of the wellbore, then powering the tool forward through the anchor. Once the end of the stroke is reached, a second anchor is set. The first anchor is retracted while the tool is being powered through the second anchor. SmarTract-WL2-1/8 features include:

* 2 1/8 in.-OD allows this tool to be run through 2 7/8 -in. tubing and run in up to 7 5/8-in. casing

* Anchor pads are statically attached to casing while stroking

* Energy efficient due to the high efficiency of hydraulic pistons * Compact, surface power-supply controller

* Computer controlled to optimize running speed based on required pulling force

* Lightweight, multi-modular design provides land/air transportation flexibility.

MEMORY LOGS

Slimhole memory, openhole logging tools have been introduced for several types of conveyance, as described in these two applications.

Slimhole resistivity. ProActive Diagnostic Services, Inc. has introduced a 2 3/8-in. dia., 9.33-ft sonde-length, Downhole Memory Induction Resistivity Service, developed in conjunction with BP Exploration Alaska, as a low-cost alternative to coiled-tubing-conveyed wireline logging and slimhole MWD technologies.[18] Designed for use in a coiled-tubing drilling program, the tool string is deployed with the coiled-tubing drilling unit immediately after drilling a horizontal sidetrack and before running a completion linen The tool has been designed to traverse a 50[degrees]/100 ft build section in a 3 3/4-in. hole.

Compact logging. Reeves Wireline (formerly BPB Wireline Services) offers its 2 1/4-in. suite of Compact System lightweight logging tools in memory-logging mode.[19] The absence of wireline allows for a variety of conveyance methods: 1) via drill pipe (tools attached to bottom of drill pipe); 2) out of pipe (tools run out of bottom of drill pipe that covers trouble intervals); 3) hydraulically deployed (tools pumped down to landing sub at pipe bottom); 4) cableless coiled tubing (tools attached to bottom of cableless coiled tubing); 5) through core-drill bit (tools run through removable plug in core-drill bit); or 6) other methodologies. Surface system control is by a desktop or notebook PC. Compact System services include:

* Array Induction

* High Resolution Resistivity

* PhotoDensity/Caliper

* Dual Neutron Porosity

* Sonic/CBL

* Dual Laterolog

* Microlog

* MicroLaterolog

* Gas Detector

* Gamma Ray/CCL

* Twin Arm Caliper.

MWD

Two formation evaluation tools are providing additional sensors and more accurate versions of existing sensors.

Acoustic. Halliburton Sperry Sun Drilling Services has introduced a dual array, monopole/dipole Bi-Modal Acoustic Tool (BAT), co-developed with SensorWise.[17,20] Two opposed transmitters can be fired in in-phase (monopole) or out-of-phase (dipole) modes. BAT offers the following new and advanced features:

* Two seven-receiver arrays (opposed front and back)

* High-frequency simultaneous sampling of all receivers

* Two high-power, dual-frequency transmitters

* Fully programmable transmitter firing modes

* Operation in both monopole and dipole modes

* Tool design to virtually eliminate tool-mode arrival

* Full-waveform memory storage (256/512 MB)

* Two powerful, digital-signal processors for fast, downhole delta-t calculations

* High-speed data retrieval at surface.

Further, the system presents standard compressional wave, as well as enhanced shear wave, travel time; and it uses standard wireline-industry processing techniques supported by various commercial applications.

Density. Baker Hughes INTEQ has developed an improved method of determining/reducing the impact of borehole size and standoff on MWD formation density and neutron porosity.[21] The use of standoff-based binning in the density tool provides a simultaneous measurement of density and compensation at different standoffs. In addition to providing standoff binning for the density tool, the three-axis caliper provides a robust measurement of borehole size--as can be used to calculate both borehole size correction and standoff correction for the neutron tool.

Using three ultrasonic, acoustic standoff transducers--each firing sequentially at 200 times per second--this method segregates density measurements by ranges of standoffs between tool and formation when the density measurements are acquired; these are called standoff bins. A weighted average of all density data is used, with the weighting function selected to minimize total error from the standoff (smaller standoffs are better) and from statistics (more measurements are better).

MUD LOGGING

This technology provides nearly real-time information concerning lithology and hydrocarbon presence on the rig location or laboratory. Two applications are described here.

Mud gas measurement. Datalog has developed a method for directly measuring total gas in mud without using a gas trap.[22] Gas Wizard, Fig. 4, provides quantitative hydrocarbon gas measurements in percent by volume. Response time of the probe is up to 10 times faster than traditional gas traps. Internal probe volume is small, consequently there is no spreading of gas peaks due to large internal volumes associated with gas traps, resulting in improved depth resolution.

[Figure 4 ILLUSTRATION OMITTED]

The small probe can be mounted in a variety of locations, such as the bell nipple, so the sample is taken before gas losses occur in the towline. Measurements can be taken in the suction line to determine degassing efficiency by comparing gas entering and exiting the well. GasWizard can be used in any type of drilling fluid--oil, water, air or foam.

GasWizard has no moving parts in the drilling fluid, therefore there is nothing to wear out. The user is only required to regenerate the air drier once per week and periodically calibrate and zero the sensor. The probe has a microprocessor that provides automatic calibration and zeroing, pneumatic control, thermostat and ranging. The electronics enclosure has an active cooling mechanism, as well as heating, to maintain constant temperature in hot and cold environments. Output is standard WITS format, which can be connected to the Datalog QLOG or WellWizard system, as well as third-party drilling monitoring systems.

GasWizard detectors are latest-technology, catalytic combustion detectors. They are stable and reliable--however, there are redundant sensors in case of failure. In the event of a detector failure, the microprocessor will mm on the second detector, or the user can do this manually.

Quantitative fluorescence. Texaco's Quantitative Fluorescence Technique (QFT) mud-logging, oil-detection and analysis tool has been upgraded. Quantitative Fluorescence Technique 2 (QFT2) is both a wellsite and lab UV-fluorescence technique for detecting and analyzing extractable whole oil from drill cuttings, cores and environmental samples.[23] In mud-logging usage, cuttings are prepared for analysis by spin drying, light grinding and then extracting oil using an organic solvent. Extract emissions are measured at two wavelengths using the fluorometer.

A database of analyses of a worldwide suite of oils is used to convert emission readings to oil quantity and type. However, because of variations in oil source material, genesis and migration history, database trends exhibit scatter, so that QFT2 results should be considered estimates and not substitutes for lab analysis. QFT2's major applications are oil vs. gas discrimination and oil gravity estimation.

WIRELINE TESTING

Formation testing provides pressure and fluid information. In this review of modular tool 'renovations, Halliburton's Reservoir Description Tool (RDT), Baker Atlas' Reservoir Characterization Instrument (RCI) and Schlumberger's Modular Formation Dynamics Tester (MDT) comprise the current commercial modular tools that allow versatile configuration to meet user needs.

Halliburton's RDT became available commercially during 1999.(24) Its nominal OD is 4 3/4 in., with maximum 5-in. OD at the dual probe section, Fig. 5. RDT features include:

[Figure 5 ILLUSTRATION OMITTED]

* Multi-chamber sections, each containing three 1,000-cc, DOT-transportable sample chambers

* Real-time, flow-control pumpout sections, one configured for 4,000-psi, the other for 8,000-psi pumping pressure to test zones over a wide range of pressures in a single trip

* Rate- or pressure-controlled drawdown

* Two standard, closely spaced probes (a dual probe), providing a redundant packer seal

* Formation-fluid property monitoring during testing.

Further, pressure gradient determination can be accomplished by running two dual probes in tandem. Sample chambers are filled against hydrostatic pressure, and additional pressure can be applied to help maintain RDT's single-phase integrity against thermal gradients.

Baker Atlas' RCI now features an optional, six-tank, sample-chamber module for acquiring fluid samples. Each of the six chambers can hold 840 cc of fluid sample, and they are DOT-transportable. Schlumberger's MDT, with its Optical Fluid Analyzer (OFA), uniquely predicts formation hydrocarbon properties while testing in boreholes drilled with synthetic, oil-based muds (OBM). After local empirical correlation with actual API gravity and GOR measurements have been made, these oil properties can be estimated during MDT sample collection, as well as during pump out only (called gargling).

ACKNOWLEDGMENT

The author would like to thank Shell Technology Exploration and Production for permission to publish this article. The author gratefully appreciates technical details and insight provided by companies whose technology is discussed in this article.

LITERATURE CITED

[1] Reeves Wireline Technologies Ltd., http://www.reeveswireline.com/ callisto.htm, June 1999.

[2] Leicester University, http://www.le.ac.uk/geology/re/lubr/callisto.html, August 1999.

[3] The Log Analyst, "Transactions of the SPWLA on CD," November-December 1999.

[4] Society of Petroleum Engineers, http://www.spe.org/tigs/, January 2000.

[5] Schlumberger, www.connect.slb.com, January 2000.

[6] McKeon, D., et al., "An improved NMR tool design for faster logging," SPWLA Annual Logging Symposium, Transactions, Oslo, Norway, May-June 1999.

[7] Prammer, M. G, et al., "Theory and operation of a new, multi-volume NMR logging system," SPWLA Annual Logging Symposium, Transactions, Oslo, Norway, May-June 1999.

[8] Appel, M., et al., "Restricted diffusion and internal field gradients," SPWLA Annual Logging Symposium, Transactions, Oslo, Norway, May-June 1999.

[9] Xiao, J., et al., "Interpreting multiarray induction logs in high Rt/Rs contrast environments with an inhomogeneous background-based software focusing method," SPWLA Annual Logging Symposium, Transactions, Oslo, Norway, May-June 1999.

[10] Barber, T, et al., "Interpreting multiarray induction logs in difficult environments," SPWLA Annual Logging Symposium, Transactions, Oslo, Norway, May-June 1999.

[11] Tello, L., et al., "A dipole array sonic tool for vertical and deviated wells," SPE Annual Technical Conference and Exhibition, Transactions, Houston, Texas, October 1999.

[12] Hemingway, J., et al., "Introduction of enhanced carbon-oxygen logging for multi-well reservoir evaluation" SPWLA Annual Logging Symposium, ransactions, Oslo, Norway, May-June 1999.

[13] Neuman, C., et al., "An investigation of density derived from pulsed neutron capture measurements" SPE Annual Technical Conference and Exhibition, Transactions, Houston, Texas, October 1999.

[14] DHV International Inc., http://www.dhvi.com/, November 1999.

[15] Evans Engineering & Manufacturing, Inc., "The Model CK wireline jar," Conroe, Texas, 1999.

[16] SmarTract, Inc., http://www.smartract.com/, October 1999.

[17] SensorWise, http://www.sensorwise.com/, July 1999.

[18] ProActive Diagnostic Services, Inc., http://www.memorylog.com/, April 1999.

[19] Reeves Wireline, http://www.reeves-wireline.com/compact/compact.htm, November 1999.

[20] Halliburton, http://www.halliburton.com/sperry-sun/products/ fewd/bat.asp, January 2000.

[21] Minette, D., "Utilizing acoustic standoff measurements to improve the accuracy of density and neutron measurements," SPE Annual Technical Conference and Exhibition, Transactions, Houston, Texas, October 1999.

[22] Datalog, http://www.datalog.ab.ca/directgas.html, January 2000.

[23] Texaco Upstream Technology, "QFT2 Quantitative Fluorescence Technique 2," 1999.

[24] Halliburton, "Reservoir Description Tool (RDT)" Brochure H01961, May 1999.

David Patrick Murphy is petrophysical engineering and economics instructor for Shell Technology E&P's Professional Learning group. He is also formation evaluation lecturer for the University of Houston graduate program in petroleum engineering. Murphy is a registered professional engineer in Texas, Oklahoma, Louisiana and California.

David Patrick Murphy, Shell Technology EP, Houston

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