Advances in MWD and formation evaluation for 1999
FORMATION EVALUATIONAdvances in MWD and formation evaluation for 1999Developments include new wireline tools, smaller MWD tools and better core recoveryDavid Patrick Murphy, Shell E&P Technology Co., Houston
General Internet access continues to provide improvements in accessing formation-evaluation and MWD information — four examples are shown here. Log characterization. The Log Characterization Consortium (LCC) is an independent, member-run, not-for-profit organization. Characterization of new logging technologies is carried out by member organization Special Interest Groups (SIGs).1 While SIGs will exist as separate entities, they will be encouraged to keep the LCC membership informed about their progress. A third-party data management company, QC Data, has been hired to coordinate the information exchange via the LCC homepage (www.lcc.qc-data.com) and newsletters. LCC SIGs include:
SPWLA transactions. The Society of Professional Well Log Analysts (SPWLA) has published Transactions of the SPWLA on CD (see Fig. 1), Volumes 1–10 (1960–1969), in high-resolution Adobe Acrobat PDF image file format.2 Work on volumes for the 1970s and 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 a separate, text-searchable PDF file linked to corresponding papers. The CD (works with PC, Mac or Unix) is priced at $35 for a single-user license. For information on network or corporate licensing, contact SPWLA at: http://www.spwla.org/. Oilfield glossary. Schlumberger’s Oilfield Glossary is an evergreen, on-line repository of E&P terminology that draws on extensive resources of Schlumberger publications and technical experts.3 The Oilfield Glossary, available to Schlumberger customers at www.connect.slb.com, is intended for the technical generalist and, on completion, will cover all aspects of oilfield terminology in a single reference source. Many definitions are enhanced by color illustrations or photographs. The glossary is being introduced in stages. The initial version will contain about 5,000 entries, grouped into specialized modules for placement on the Web sequentially. The geology and geophysics modules are now available and comprise nearly 1,200 terms. New modules will be added quarterly. Real-time wellsite-to-office link. Two commercially available wellsite-to-office links allow real-time sharing of drilling and formation evaluation log data. Sperry-Sun’s INtegrated System for Information Technology and Engineering (INSITE) is a networked drilling information system based on the familiar Microsoft Windows NT or 9X operating systems.4 INSITE can be scaled to support any service requirement from directional-only MWD through full-blown integrated services, including directional drilling, surface logging, formation evaluation while drilling, well planning and drilling engineering. The INSITE database is Open Database Connectivity (ODBC) compliant, which allows commercial software, such as Microsoft Office, direct access to the database to generate reports and graphs. Using standard networking tools provided with Windows NT and 9X, multiple workstations have simultaneous access to all information contained within INSITE’s database — both at the wellsite and from remote locations. Using Sperry-Sun’s INSITE-ANYWHERE Web server, a Web browser can be used to view real-time information from the wellsite through an Internet connection. GeoQuest has added to the utility of Schlumberger’s InterACT communications platform with their Client Logging Station, which allows clients to monitor logging operations in real time without being at the wellsite.5 The system can receive data from GeoQuest’s LogSafe data archival service via an Internet-accessed, drop box, data-delivery service. GeoQuest has also introduced GeoREALM, which combines remote-access, log-data-processing software for new and proprietary Schlumberger logging measurements with GeoQuest’s support services. The service works with a client’s existing forward-modeling applications and can run either on a client server or through remote server access at GeoQuest. GeoREALM includes several sensor modeling applications for resistivity, density and other sensors. Wireline Logs Wireline logs typically provide more formation evaluation wellbore data at a lower cost than other tools, as illustrated here by tool / technique advancement discussions for five types of measurements. Nuclear magnetism. Commercially available Nuclear Magnetic Resonance (NMR) tools include NUMAR’s Magnetic Resonance Imaging Log (MRIL) and Schlumberger’s Combinable Magnetic Resonance (CMR). MRIL is available from Halliburton / NUMAR, Baker Atlas and Computalog. CMR is available from Schlumberger. NUMAR has introduced a new multi-band NMR logging tool, the MRIL-Prime (sometimes called MRIL-D).6 This new tool has nine sensitive volumes, three times as many as the previous tool, Fig. 2. 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 new high-power transmitter allows for essentially uninterrupted pulsing. Some new MRIL-Prime features are: a single pass for what was obtained in separate up- or down-log operations, increased logging speeds by up to a factor of four, noise suppression by a factor of two, and expanded multi-parameter acquisition. A new gradient NMR processing technique, called Enhanced Diffusion Method (EDM), has been introduced by NUMAR, exploiting the diffusion contrast between oil and water to separate NMR signals from each fluid.7 Unlike standard NMR logs acquired with short inter-echo time (TE) spacing, EDM data is acquired using relatively long TE times to accentuate diffusion effects. EDM is sensitive only to intermediate-viscosity oils (about 1–50 cp), making EDM complementary to existing direct NMR hydrocarbon-typing methods, such as the Differential Spectrum Method, that are designed for light hydrocarbons. Schlumberger has studied NMR transverse relaxation time spectrum (T2) of brine mixed with four clays (illite, smectite, kaolinite and glauconite) as a function of compaction.8 Each clay-brine slurry and its associated compacted sample only showed a single peak in the T2 distribution spectrum. A second peak, which could be interpreted as the clay-bound water, was never observed. Schlumberger’s results have important implications for NMR interpretation in shaly sands. Effects of compaction and, to a lesser extent, iron content on a clay’s T2 position make it impossible to independently determine clay type from a characteristic T2 time. Schlumberger’s data also implies that it is not possible to determine cation exchange capacity using a single T2 cutoff value (once thought to be about 3 ms). 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. Halliburton has added a new small-diameter C/O logging tool, the Reservoir Monitor Tool (RMT), a 2-1/8-in. tool designed for logging through 2-7/8-in. or larger tubing.9 Two bismuth germanate detectors yield high gamma-ray count rates with good spectral resolution. RMT has two operating modes. In C/O (inelastic) mode, saturation and lithology information can be derived. The second operation mode replicates the pulsing and gating sequence of the company’s TMD-L tool, providing all measurements normally obtained by a PNC log, but with better lithology information from the capture spectrum. Other available PNC tools include Baker Atlas’ Pulse Decay Tool (PDK-100), Multi-parameter Spectroscopy Instrument (MSICO) and RMS (field testing); Computalog’s Pulsed Neutron Decay (PND-S); Halliburton’s Thermal Multigate Decay (TMD-L); and Schlumberger’s Thermal Decay Time (TDT-P) and Reservoir Saturation Tool (RST).10 Cased hole density. Density logs provide bulk-density information which can be used to determine porosity or (with acoustic log data) calibrate seismic. Two recent approaches obtain density information through casing. A density-type porosity measurement from inelastic scattering is being developed for Computalog’s Pulsed Neutron System (PND-S).11 Cased-hole, density-type porosity is based on the attenuation of gamma rays produced by inelastic scattering of fast neutrons. Fast-neutron reactions create a dispersed gamma-ray source in close proximity to the tool’s neutron source. Subsequent transport of these gamma rays is strongly affected by the formation’s bulk density. To optimize results in a specific well, borehole geometry should be consistent, casing should be well cemented and the borehole should be liquid-filled. To improve accuracy, some local information is required to normalize specific borehole effects. Core data, open-hole logs in other well sections or offset logs are often used. Halliburton’s design, partially funded by GRI, for its proposed cased-hole density tool more closely follows the design of conventional open-hole density tools with a gamma-ray source and detectors.12 A special lab tool was constructed to acquire data used to develop the design. The company found that for cement thicknesses greater than 1/2-in., there are four independent variables that significantly affect cased-hole, gamma-gamma density tool count rates: casing thickness, cement thickness, cement density and formation density. Gamma-ray spectrum shape does not provide much information. Therefore, four gamma-ray detectors at different distances from the gamma-ray source are required to obtain enough information to solve for the four variables. To provide reasonable statistics for accurate density determination, a cobalt-60 gamma-ray source is preferable to a traditional cesium-137 source. Halliburton’s study indicates that a four-detector tool can accurately and precisely measure formation density through casing and up to two inches of cement. Openhole resistivity. Resistivity logs are typically used to obtain true formation resistivity (Rt) for determining water saturation. Laterolog or lateral tools are used in more conductive or salty, water-based muds. Induction tools are used in non-conductive or fresher water-based muds. Array devices provide multiple depths of investigation and are the newest tool types. Schlumberger has field tested its new High-Resolution Laterolog Array tool (HRLA).13 The system supplies six depths of investigation with all measurements using bucking currents returning to the tool rather than to the surface, eliminating Groningen and drill pipe effects, Fig. 3. At the wellsite, Rt is provided by a fast, one-dimensional (1-D) inversion that takes into account only radial resistivity variations and is strictly applicable only in thick beds. In thinly bedded formations, post-process-inverting a two-dimensional (2-D) axisymmetric formation model improves interpretation by simultaneously accounting for radial and vertical resistivity variations. Inversion can handle complex invasion profiles and produce a 2-D image of formation resistivity. The High-Definition Lateral Log (HDLL) tool is a new type of Multi-Electrode Resistivity Tool (MERT) developed by Baker Atlas in cooperation with Shell International Exploration and Production.14 HDLL has a single-current injection electrode and 18 potential measurement electrodes at various distances from the injection electrode. Measurements from different spacings and depths can be combined at the well site in real time to create several synthetically focused curves, e.g., shallow, medium and deep. Post-processing can be done with 2-D inversion to correct for borehole, invasion and shoulder-bed effects, or with 3-D inversion to correct for borehole, invasion and shoulder- and dipping-bed effects. Image log analysis. Two innovative approaches to analyzing and presenting electrical borehole image logs can provide additional geologic insight. Normally, use of electrical borehole images of sedimentary features, such as cross-bedding, has been limited to a cylindrical view restricted to borehole size, lacking the lateral extension needed for a better perception of the actual 3-D setting. A new 3-D image log presentation has been proposed by Schlumberger to improve visualization of sedimentary features.15 A series of block diagrams is drawn along the borehole trajectory which correspond to individual cross-bed units (with similar morphology and orientation). The blocks are oriented along the principal directions of deposition and extend to a few meters around the wellbore. Reconstruction of bed forms is achieved by inversion of a geometric model. The result is a better understanding of geologic anisotropy (not to be mistaken for resistivity anisotropy) around the borehole. Elf Exploration Production, in conjunction with University of Bordeaux, has proposed a method for automatic high-resolution, texture analysis on electrical borehole images.16 Using a statistical texture model and 2-D self-organizing map classification, electrical borehole image textures are ordered automatically according to texture features of linearity, orientation, contrast, size and frequency. A resulting textural log displays vertical changes in image texture, enabling, in certain cases, definition of dominant lithology and bed limits. The operator’s encouraging results show the approach to be capable of significantly improving sedimentological and reservoir interpretations from electrical borehole image logs. MWD MWD formation evaluation tools continue to get smaller. Described below are some new developments offered by four service companies. Smaller tools. Directional and gamma ray MWD tools can be found on collars as small as 2-7/8-in. from several companies.17 Baker Hughes INTEQ has built a new 3-1/8-in. propagation resistivity tool for coiled tubing drilling (with wireline telemetry) and rotary drilling (with mud-pulse telemetry).18 This is the smallest-diameter MWD propagation resistivity tool to date and can be run in holes in the 4-in. range with build rates of 30° per 100 ft. Applications are in geosteering and formation evaluation in reentry and very slim-hole drilling projects. Tool design uses a symmetric, borehole-compensated antenna configuration with spacings and frequencies identical to the company’s long-spaced Multiple Propagation Resistivity (MPR) tool. Sperry-Sun and Anadrill have each introduced 4-3/4-in. diameter triple combo MWD logging tools for 6-in. boreholes. A triple combo includes resistivity, bulk density (and photoelectric factor) and neutron porosity measurements, usually along with a gamma ray. Sperry’s triple combo consists of SLIM PHASE 4 resistivity, SLIM Stabilized Lithology Density (SLD) and Compensated Thermal Neutron (CTN) with gamma ray.19 Anadrill’s VISION475 resistivity measurements provide five phase-shift and up to five attenuation measurements. VISION475’s density and photoelectric factors are azimuthal, and its neutron porosity is environmentally corrected.20 Drilling formation tester. Halliburton / PathFinder is field testing its Drilling Formation Tester (DFT), a modular component of the PathFinder LWD system, Fig. 4.21 DFT features two straddle packers that are retracted while drilling and protected from drilling damage by buffers / centralizers. Packers are inflated by mud for testing selected formation intervals without tripping out. Pressure information is transmitted by mud-pulse telemetry to confirm a successful test and is also stored in downhole memory for later analysis with greater data density. Coring And Analysis Formation cores provide the best rock-property data source for reservoir characterization. Recent developments include a telescoping core barrel for improved recovery and additional rock-property catalog capabilities. Telescoping barrels. Baker Hughes INTEQ has introduced its JamBuster anti-jam coring system, Fig. 5. Two concentric inner-core barrel sleeves automatically telescope, in turn, if a core becomes jammed in the barrel. This extended coring capability is especially effective in difficult coring situations with fractured formations, slanted faults, interbedded shales and expanding clays.22 Rock catalogs. Rock-property catalogs provide analog geological and petrophysical rock-property data when limited rock descriptions are available. Two comprehensive reservoir rock catalogs are 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 easy use.23 The Shell Rock Catalog (ShRC), by Shell E&P Technology Co. and Poro-Technology, contains analyses for 83 rock samples for Shell’s use (79 for non-Shell companies).24 Samples in these two rock catalogs do not overlap; each contains a unique rock-sample library. Rock-property data for a specific rock type (or area) can usually be taken from existing core analysis data, provided data is available and reasonably accessible. CoreFinder, from Reservoirs Inc., is a primary reference source for locating geological and petrophysical data that is available for purchase.25 This expansive index comprises rock data from more than 1,400 wells within Reservoirs’ 40 regional studies from across North America. Analog rock data can be applied anywhere, not just where the core was taken. Tabular and map spatial-search capabilities allow multiple methods to locate data of interest. The CD-ROM version of CoreFinder runs on Windows 95 or NT; the Web version is currently under construction. Advance Logging Technologies (ALT) offers organization of a company’s existing core data and photos into an HTML-format, Rock Properties Catalog on CD.26 The source media for catalogs can be slides, negatives, photos or paper prints of any size and report text (digital images can be included as well). ALT’s Rock Properties Catalog CDs are readable on Mac, PC and UNIX systems equipped with a Web browser. The data can be quickly posted on a Web site for wider accessibility. 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
 The author
Copyright © 1999 World
Oil |
he latest 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 discussed here include:
David
Patrick Murphy is petrophysical engineering and economics instructor for Shell E&P
Technology Company’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.
