What's new in MWD and formation evaluation - measurement while drilling

New developments include refined NMR logs, smaller MWD tools, better core recovery, improved wireline fluid sampling methods and internet-accessible technical information

Recent developments in formation evaluation, particularly data acquisition while drilling, are improving the quality and increasing the quantity of formation data obtained, while reducing costs and making operations more efficient. Highlights discussed below include:

General. Internet-based technical interest groups and on-line log interpretation charts.

Wireline. New array lateral log, improved NMR, shear acoustic, downhole video, new slimhole logging tools, releasable cable head and real-time wellsite-to-office link.

MWD. Smaller tools, formation dip estimation while drilling and a new formation pressure measurement system.

Coring and analysis. Additional core recovery in unconsolidated formations and web page format rock catalogs.

Wireline testing. Determination of producible hydrocarbon type and oil quality in wells drilled with synthetic oil-based muds.

GENERAL

The Internet continues to provide improved access to formation evaluation and MWD information.

FE and MWD TIGs. The Society of Petroleum Engineers (SPE) has initiated a three-year pilot program that will allow SPE members and non-members to join SPE-sponsored Technical Interest Groups (TIGs).[1] TIGs are organized around a specific discipline instead of geographic boundary to complement, not replace, SPE's traditional section structure. TIG meetings, programs, communications and events are facilitated through the Internet. For each TIG, a secured news server will be maintained to facilitate round-the-clock discussion among TIG members. Each discussion group is restricted to members of that TIG.

Current TIGs related to formation evaluation and MWD are:

* Core analysis

* Formation evaluation

* Measurement while drilling

* Openhole logging

* Petrophysics

* Reservoir characterization

* Rock mechanics

* Well testing.

A new TIG may be formed when 50 or more individuals seek to be associated in the study of a particular technical interest. Established groups that seek to affiliate with SPE as a TIG may continue as self-governed organizations, retaining the organization's name, structure, and funds. Additional details and enrollment forms are available from SPE TIG web site http://www.spe.org/ip/tigs/index.html.

On-line log interpretation charts. Schlumberger and Western Atlas now provide current log interpretation charts for their clients on-line via the Internet. Western Atlas wireline log interpretation charts can be found at http://www.waii.com/WAII/_WALS/catalogs/LgIntCht/CNT/licndx00.html. Chart pages can be printed directly from a web browser, or GIF-format files can be downloaded. Schlumberger wireline and LWD charts can be accessed through Connect Schlumberger (access limited to Schlumberger clients) at http://www.connect.slb.com. Chart pages can be downloaded in either GIF or PDF formats.

WIRELINE LOGS

Wireline logs typically provide more formation evaluation wellbore data for lower cost than other tools.

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

Western Atlas has introduced an array lateral log, the High-Definition Lateral Log (HDLL).[2] HDLL, the result of a 1994 technology development agreement between Shell International E&P and Western Atlas, is a multi-electrode device [ILLUSTRATION FOR FIGURE 1 OMITTED] that provides formation resistivity measurements at multiple depths of investigation in salty muds. Inversion allows for extracting all information contained in the raw data with reference to the resistivity distribution surrounding the borehole. Deliverables from HDLL fall into two general categories, wellsite and post-well.

Wellsite deliverables include:

* Selected raw data plots

* Three borehole-corrected synthetic focused resistivity curves

* Selected lateral and normal apparent resistivity curves

* Quality assurance curves.

Post-well deliverables include:

* Accelerometer-corrected data-corrects for uneven tool motion

* Estimated formation resistivity earth model-inversion results represented by a sequence of layers and each layer associated with a selected invasion profile

* Data fit showing match between synthetic and inversion results

* Quality assurance indicators.

For non-conductive or fresher water-base muds, commercially available array induction logs are Schlumberger's Array Induction Tool (AIT) and Western Atlas' High-Definition Induction Log (HDIL).

Nuclear magnetism. NUMAR's Magnetic Resonance Imaging Log, Series C-Total Porosity (MRIL-CTP), and Schlumberger's Combinable Magnetic Resonance (CMR-200) tools are commercial, new-generation, pulsed nuclear magnetism logs. MRIL is available from NUMAR, Western Atlas Logging Services (except in Canada), Halliburton Energy Services (except in Canada) and Computalog Wireline Services Inc. (Canada only).

Nuclear Magnetic Resonance (NMR) responds primarily to pore fluids. Three parameters - longitudinal or spin-lattice relaxation ([T.sub.1]), transverse or spin-spin relaxation ([T.sub.2]) and diffusion - an be measured by NMR. These measurements can provide information about hydrocarbon type, formation porosity, pore size distribution, permeability, oil viscosity and bound or moveable fluid saturations. NMR works equally well in clean or shaly rocks.

Schlumberger's newest pulsed NMR tool is the CMR-200.[3] With a 50% improvement in tool signal-to-noise ratio, a reduction in minimum echo spacing from 0.32 to 0.20 ms and new processing software, the CMR-200 can provide a total porosity NMR measurement. CMR-200 has the capability to see [T.sub.2] times down to about 0.3 ms, which includes clay-bound water. Differences between NMR total porosity and density log porosity can be used to infer the presence of gas. Information about clay-bound water saturation can be used to compute more accurate hydrocarbon saturations from resistivity log measurements and can also be used to obtain more robust permeability estimates.

Acoustic. Dipole-type acoustic logs provide shear velocity information even in slow formations.

Western Atlas' Cross-Multipole Array Acoustilog (XMAC) is a new full-wave monopole, dipole and cross-dipole system that provides compressional and shear velocity data in formations ranging from slow, soft, unconsolidated sandstones to fast, low-porosity, fractured carbonates.[4] XMAC's use of Acutec/Shell matched receiver technology provides high quality cross-dipole data for anisotropy analysis. A semi-rigid acoustic isolator provides effective isolation of the transmitters from the receivers while enabling XMAC to be pipe-conveyed in highly deviated or horizontal wells. It is combinable with other Western Atlas logging services.

Other commercial dipole and multipole acoustic tools include Halliburton Energy Service's Low Frequency Dipole Logging (LFD) service and Schlumberger's Dipole Shear Sonic Imager (DSI).

Downhole video. Downhole video (DHV) provides images of the wellbore environment, mainly in cased-hole situations. Applications include fluid characterization (to detect fluid and particulate entry, determine production and injection profiles, perform tracer tests, conduct pre-treatment analysis, monitor treatments and confirm treatment results), mechanical inspections (to assist fishing, detect casing or tubing leaks, detect mineral or other buildups and inspect condition and operation of downhole equipment), and gas-filled openhole characterization (to inspect storage wells, and determine size and shape of openhole fractures)[5]

Halliburton DHV services are performed with a 1 11/16-in.-OD, 10-ft-long tool rated to 12,000 psi and 225 [degrees] F. Real-time video images are transmitted up a 7/32-in. fiber-optic cable available in lengths up to 17,000 ft. Alternately, time-delayed single frames can be telemetered to the surface through single- or multi-conductor cable. A 2 1/2-in.-OD flasked DHV tool, rated to 350 [degrees] F, is now available. High resolution grayscale cameras are dominant, however, a color version with somewhat poorer resolution is available.[6]

Slimhole logging tools. Schlumberger is introducing a modern slim (2.5-in. nominal) quad-combo, which consists of gamma ray, compensated neutron, array density, monopole array sonic and array induction.[7] SlimAccess tools are designed for use in small boreholes (minimum diameter 3 in.) and high dogleg severity (60 [degrees] in 100 ft in a 4.125-in.-diameter borehole), addressing the needs of reentry and slimhole wells. The tools may be deployed on wireline, drill pipe or coiled tubing. A further innovation is systematic use of an "accessibility planner" that enables operating personnel to correctly configure the tool prior to the job to ensure passage of the entire toolstring past points of maximum severity in the borehole.

Releasable wireline cable head. Halliburton has developed a Releasable Wireline Cable Head (RWCH), a seven conductor cable head that releases the logging wireline via an electrical command from the surface.[8] A fusible material holds a shaft in a locked position, which secures a collet around the wireline anchor mechanism [ILLUSTRATION FOR FIGURE 2 OMITTED]. When the wireline is to be released, an electrical command from surface triggers heating coils that melt the fusible material, releasing the shaft and allowing the collet to open and free the wireline. Benefits of RWCH include:

* Optimized control over maximum pull on logging cable, reducing the number of fishing jobs

* Increased capacity to support heavy tool strings

* Release occurs quicker and can be terminated at any time before the melting temperature is reached

* RWCH can be redressed at the wellsite after a successful fishing operation

* Eliminates the possibility of accidentally breaking the weak point

* Allows maximum pull on wireline across shallow, depleted sands where weak points limit maximum pull

* Minimizes release shock compared to breaking a weak point.

Real-time wellsite-to-office link. Schlumberger's InterACT communications service connects the logging job to the office and provides real-time accessibility to optical and digital logging data without having to go to the wellsite.(9) Features include real-time transmission of log data to the office PC, two-way dialog with wellsite engineer and two-way transmission of all file types. InterACT requires the use of a MAXIS MCM logging unit at the wellsite. Minimum specifications for the office PC are 90-MHz Pentium, 32-MB RAM and Windows NT. InterACT software can be used with a variety of connection types including TCP/IP direct network, SLIP dial-up access or RAS on a telephone line; as long as it has 10-kbps minimum throughput.

MWD

MWD formation evaluation tools are getting smaller. Real-time formation dip can be estimated while drilling and is most accurate for high apparent dips. Annular and internal drill pipe pressure measurements have become prevalent[10] and a while drilling (almost) formation pressure system has been introduced.

Smaller tools. Schlumberger's VISION475 [ILLUSTRATION FOR FIGURE 3 OMITTED] is the first integrated triple combo 4 3/4-in. MWD/LWD system for holes as small as 5 3/4 in.[11] Resistivity measurements give five compensated phase shift and five compensated attenuation curves. Azimuthal gamma ray, density and photoelectric effect measurements can be used to generate lithology and porosity borehole wall images to assist geosteering decisions. Neutron porosity is also measured. Nuclear sources and stored data are retrievable should the tool become stuck.

MWD compressional acoustic velocity measurements provide real-time and memory formation porosity estimation, formation overpressure indications and correlations, well location correlation with seismic and information concerning rock strength and drillability. Halliburton's Compensated Long Spaced Sonic (CLSS) is available in 4 3/4, 6 3/4 and 8-in. collar sizes. Schlumberger's ISONIC is available in 6 3/4 and 8 1/4-in. collar sizes.[10]

Real time dip. Formation dip can be estimated in real time using Anadrill's Resistivity-At-the-Bit (RAB) tool.[12] A new processing method calculates the apparent dip direction, dip magnitude and a quality factor at approximately 3-min intervals. These data are transmitted to the surface and displayed as apparent and true dip. The information can be used to modify the drilling plan, if warranted.

Conventional methods of dip computation are not possible in the MWD environment while the tool is downhole. Since the tool does not know its location, and cannot use conventional multi-depth dip processing, an alternative cross-correlation approach was developed that uses the relationship between the RAB's deep and medium resistivity buttons to estimate formation dip.

Conventional dip computations have poor performance at high apparent dip angles. However, the new RAB method is most accurate at high apparent dip angles when the bedding planes are nearly parallel. to the borehole. For the RAB button spacing, apparent dips greater than 53 [degrees] provide acceptable results. While the cross-correlation approach produces inconsistent and low quality dips when the apparent dip is less than 53 [degrees], it is the only automatic processing capable of calculating dip at high apparent dip from either wireline or MWD logging.

Formation pressure system. Halliburton has recently introduced the Early Formation Pressure System (EFPS), an inflatable packer test system that can be run in a wiper trip during drilling to perform a series of quick formation pressure tests similar to those run with a wireline formation tester.[13] Conveyed by drill pipe, EFPS consists of two inflatable packers, a pump and high precision pressure gauges [ILLUSTRATION FOR FIGURE 4 OMITTED]. Drill pipe pressure is used to inflate packers, and pipe reciprocation initiates pressure drawdown. A new spherical flow well test equation is used to predict formation pressure and permeability from early to intermediate time pressure transients.

CORING AND ANALYSIS

Formation cores provide the best rock property data source for reservoir characterization. Recent developments include additional core recovery in unconsolidated formations and web-based rock catalogs.

Additional core recovery. To provide additional core recovery in unconsolidated formations, Security DBS has a new collapsible lower shoe on their Posiclose Full Closure coring system.[14] Previous full close coring systems have needed to raise the inner barrel to close the core catcher, missing the bottom few feet of core. This new method uses an additional ball to divert mud flow, pressure up and close the clam shell, damaging only the bottom few inches of core.

Rock catalogs. Two comprehensive reservoir rock catalogs are now available in HTML web page CD-ROM format. The Worldwide Rock Catalog (WWRC), by Reservoirs, Inc., has 83 rock samples cataloged with powerful search options for easier use.[15] The Shell Rock Catalog (ShRC), by Shell E&P Technology Co. and PoroTechnologies, contains analyses for 83 rock samples for Shell Companies' usage (79 for non-Shell companies).[16] The samples in these two rock catalogs do not overlap, each containing a unique rock sample library.

WIRELINE TESTING

Wireline formation test tools have many uses, including measuring formation pressures, determining reservoir free water level, locating depleted or water-flooded zones, obtaining formation fluid samples (up to gallon-sized), determining formation fluid density, permeability prediction, identifying multiple pressure regimes and determining true vertical depth.

Schlumberger's Modular Formation Dynamics Tester (MDT) and Western Atlas' Reservoir Characterization Instrument (RCI) comprise the current commercial. modular tools that allow versatile configurations to meet user needs. Halliburton's Reservoir Description Tool (RDT)[6] will be commercial in 1998. Modular wireline test tool features include dynamic sampling pressure control, multisample modules for PVT sample collection, pump-out module to purge fluids that enter tool, accurate and quick pressure gauges and fluid analysis to assess formation fluid type.

Hydrocarbon typing in OBM. Work by Shell and Schlumberger has shown that PVT-quality hydrocarbon samples can be obtained from boreholes drilled with synthetic oil-based mud (OBM) using Schlumberger's MDT Optical Fluid Analyzer (OFA).[17] Their study demonstrated that OFA can be used during the pump-out phase of operations to predict API gravity and GOR of reservoir hydrocarbons prior to securing a downhole fluid sample. And, they could predict the pump-out time required to obtain a reservoir sample with less than 10% mud filtrate contamination, if reservoir fluid viscosity and formation permeability were known or could be estimated.

This work was done in the U.S. Gulf of Mexico on unconsolidated-to-slightly consolidated, clean-to-slightly shaly sandstones, with the in situ hydrocarbons and synthetic OBM filtrate having measurable differences in their visible and/or near infrared spectra.

ACKNOWLEDGMENT

The author would like to thank Shell Exploration and Production Technology Co. 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 Society of Petroleum Engineers, "SPE establishes technical interest groups," http://www.spe.org/ip/tigs/index.html, Nov. 18, 1997.

2 Western Atlas, "High-Definition Lateral Log," Brochure L97-151-9726A-CPL, Houston, Texas, 1997.

3 Freedman, R., et al., "Measurement of total NMR porosity adds new value to NMR logging," SPWLA 38th Ann. Logging Symp. Houston Texas, June 1997.

4 Western Atlas "Crass-Multipole Array Acoustilog," Brochure L97-149-9725A-3.5M, Houston, Texas, 1997.

5 Halliburton, Downhole Video Services, Applications and Well Preparation Best Practices Series, Publication H00023, Houston, Texas, 1996.

6 Halliburton, Quarterly Technical Review, Houston Texas, Dec. 3, 1997.

7 Stoller, C., et. al., "Density logging in slim holes with a novel array density tool," SPE Paper 38650, SPE Ann. Tech. Conf., San Antonio, Texas, Oct. 1997.

8 Halliburton, "Releasable Wireline Cable Head," Brochure H00760, Houston, Texas, Nov. 1997.

9 Schlumberger, "InterACT," http://www.connect.slb.com, 1997.

10 Wiseman, K., "1997 MWD Comparison Tables," Petro. Eng. Int., May 1997, pp. 62-86.

11 Schlumberger-Anadrill, "VISION 475," Sales brochure, 1997.

12 Rosthal, R. A., et al., "Real-time formation dip from a logging-while-drilling tool," SPE Paper 38647, SPE Ann. Tech. Conf., San Antonio, Texas, Oct. 1997.

13 Skinner, N. G., et al., "New early formation pressure system field test results and advances in early time pressure buildup analysis," SPE Paper 38648, SPE Ann. Tech. Conf., San Antonio, Texas, Oct. 1997.

14 Security DBS, Posiclose, SPE Ann. Tech. Conf., Security DBS Booth, San Antonio, Texas, Oct. 1997.

15 Reservoirs, Inc., Worldwide Rock Catalog Network Version, Houston, Texas, 1997.

16 Shell E&P Technology Co. and PoroTechnologies, Shell Rock Catalog WWW Version, Houston, Texas, 1997.

17 Hashem, M. N., et al., "Determination of producible hydrocarbon type and oil quality in wells drilled with synthetic oil-based muds," SPE Paper 39093, SPE Ann. Tech. Conf., San Antonio; Texas Oct. 1997.

The author

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

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