2005_BB.pdf

SPWLA 46th Annual Logging Symposium, June 26-29, 2005

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HIGH QUALITY ELECTRICAL BOREHOLE IMAGES WHILE-DRILLING PROVIDES FASTER GEOLOGICAL-PETROPHYSICAL

INTERPRETATION, WITH INCREASED CONFIDENCE

Jeremy (Jez) Lofts, Stephen Morris, Ren N. Ritter, Roland Chemali, Christian Fulda; Baker Hughes INTEQ

Copyright 2005, held jointly by the Society of Petrophysicists and Well Log Analysts (SPWLA) and the submitting authors. This paper was prepared for presentation at the SPWLA 46th Annual Logging Symposium held in New Orleans, USA, June 2629, 2005.

ABSTRACT

The drilling environment surprisingly, offers an ideal platform for electrical borehole imaging. At the time of drilling, the borehole wall rugosity is often minimal and electrical images generated by sensors that rotate with the drill string provide a full coverage of the borehole (when compared to the pad coverage observed on conventional wireline borehole images). Field tests and characterization in a laboratory of a new high resolution electrical imaging tool deployed while-drilling confirm its field worthiness as well as accuracy and repeatability of the images.

We show further design improvements and conclude that with a properly dimensioned imaging sensor, and use of advanced focusing techniques, a nominal image resolution comparable to that of wireline borehole resistivity imagers may be achieved. With a pixel resolution comparable to that of wireline, we advocate a more confident understanding of the geological features, achieved because of the complete borehole wall coverage.

Electrical images from various wells recorded while-drilling show a broad range of high resolution sedimentary features including laminated and disturbed injected/dewatered sands/mud rock, and cross-bedding within laminated and bioturbated sandstones, as well as composite fractures, fracture clusters and faulting. With full borehole coverage textural and facies discrimination is clear and can be extended to the real-time environment and to consider the concept of Sedimentary Steering.

INTRODUCTION

High resolution electrical imaging is now possible while drilling. A recently developed LWD tool, capable of producing such image logs in conductive water based mud was described by Ritter et al. 2004. The design and deployment of the Electrical Imaging Tool While

Drilling called StarTrak1 is part of the ongoing migration of wireline technology to LWD. This migration is driven for the most part by expectations of rig time saving and justified in high cost drilling environment. One salient feature of the new device is the continuous full-bore coverage giving a higher level of confidence in the interpretation of the images.

The resolution of the electrical images enables detailed geological evaluation.

Further image interpretation discussed in this paper highlights the predictive ability of StarTrak images in terms of image fabric characterization and interpretation of sedimentary facies and structures. It also highlights the possibility for Sedimentary Steering in which the bit may be steered to follow particular target sedimentary facies, possible because of the high resolution nature of the sensor.

An important aspect of imaging while drilling is the real-time information about the subsurface that helps the field geologist and the drilling engineer recognize the terrain surrounding the wellbore. This article address examples of memory recorded logs. Geosteering applications will be published at a later date, but resolution here for the first time allows us to consider the concept of sedimentary steering.

IMAGE ACQUISITION

A single sensor placed on the side of the collar scans the surface of the wellbore and delivers a continuous stream of data covering the entire circumference of the borehole. A reasonable rate of penetration provides continuous scanning of the borehole wall in the axial direction (Ritter et al. 2004). The maximum pixel resolution has been calculated at 0.25 (6.4 mm).

Image Processing and Interpretation

The data presented here were processed into images using Baker Atlass RECALL software with data QC performed in following methodology of Lofts and Bourke 1999. After processing, the images were displayed with interpolation which further enhanced geological features.

1 StarTrakTM Trade Mark of Baker Hughes INTEQ

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IMAGE FABRIC CHARACTERISTICS FROM STARTRAK

Interpretation comprised three stages:

dip picking of the images using a dip scheme classified according to the lithology, bed boundary styles and dip magnitudes (DOI = 0.4 in),

automated thresholding of the gamma log to obtain log facies and

separate designation of fabrics from the images (in which lithology was a minor factor).

Test Well Data

The tool was field tested in the Mounds area of Oklahoma State, USA. Mudrock dips picked from the images allowed the accurate calculation of structural dip which can be used to rotate sedimentological dips back to their original orientation. This was not done in the test well due to its low magnitude. The structural dip was comparable to that measured from the wireline run.

Lithological Interpretation

The area is interpreted as a stack of deltaic mudrocks and sandstones with classic, repeated-upwards decline in gamma log response that reflects increasing sandstone content. The high gamma (hot shale) intervals are likely to represent maximum flooding surfaces (Figure 1 at 2424 ft), and are the deposits associated with the deepest marine water when relative sea-level was highest. The bases of large sandbodies may be sequence boundaries that formed when the relative sea level dropped rapidly and channel/deltaic sandstone then encroached basinward over previous deposits (Figure 3 at c. 2400 ft).

StarTrak images have enabled a confident sedimentological interpretation of the test well using a combination of the distribution of StarTrak image fabrics, associated dip patterns and gamma log facies (Figure 1). These were subsequently used to divide the interpreted section into genetic sedimentary packages. In the case of shallow marine (deltaic) sedimentary systems, this packaging is a strong indication of sequence-stratigraphic cyclicity (cf. Emery and Myers 1996) and is caused by relative sea-level changes. In addition, several packages have been subdivided as they are likely to correspond to single, smaller deltaic parasequences.

Discrete components of deltaic sequences are identifiable from the gamma log profile and the distribution of sedimentary structures derived from images (facies interpretations: Figure 1):

hot shale: high gamma response mudrock indicating a possible maximum flooding surface.

marine mudrock: a slightly upward decreasing gamma profile indicating slowly increasing influx of sandy material. Abundant, low angle lamination seen on the StarTrak images is indicative of low energy conditions and accumulation of hemipelagic (settling) and dilute, muddy underflows of a lower delta front environment.

upward cleaning sandstones: mudrock to sandstone transition with increasingly clean sandstones upward. Dip magnitudes tend to increase upwards indicating increased energy and bedform development. This is shown by Package 5A which is characterized by intervals of flat-lying dips, interbedded with steeper dips. This represents a delta-front deposit with localized slumping that caused the steeper dips and with possible distributary channels (infilled with thin sandstones).

channel sandstone: thick, clean sandstone with interspersed structureless and structured intervals. In Sub-package 4A, cross bedding is diagnostic of strong, unidirectional currents with deposition of sand in migrating bedforms. The mud drapes associated with these (along set surfaces and in discrete layers) may also indicate tidal influence with relaxation in flow power leading to mud deposition. This is likely to represent high energy conditions of mouth bars and distributary river channels.

coal/limestone: elsewhere in the interpreted interval there are coal and limestone beds at the tops of cycles. These facies indicate emergence with plant growth and accumulation/open, shallow marine conditions respectively and indicate maximum outbuilding of the delta front before subsidence and resumption of deep marine conditions in the following cycle.

Image Fabrics

The interpretation of the test well showed a wide range of image fabrics that can be resolved and identified with the tool, and these have been designated largely independent of the host lithology (Figure 2):

STRK-01 Structureless: uniform or slightly variable resistivity response with no discernable fabrics. In rare examples, a blotchy texture may be developed although no sinusoids can be fitted to the features. This lack of fabric may indicate a truly structureless lithology or possibly a previously structured lithology that has been modified (e.g. by bioturbation, where the new fabric is below the resolution of the tool).


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STRK-02 Weakly laminated: sandstones commonly display intervals of weak lamination in which there is poor resistivity contrast between individual layers, or the lamination is fuzzy due possibly to modification. The example shows a well-defined sedimentary surface at c. 2105.4 ft dividing a bright (resistive) response facies from weakly laminated facies below. The fabric is mostly blotchy with several poorly resolved sinusoids. Weak (relict) structures such as these may have developed in structured sandstones that do not comprise substantially differentiated material and which may therefore have poor resistivity contrast. From core, some weakly laminated sandstones contained mm-scale mudclasts along sedimentary surfaces which may account for the blotchy nature in images.

STRK-03 Strongly laminated: both sandstones and mudrocks commonly display strongly-developed lamination or other sedimentary structures where thin, high resistivity contrast alternations are seen on images. Individual lamina can be defined down to a single pixel which equates to a vertical resolution of c. 7.6 mm. Such fine lamination, as seen in the example, is likely to comprise interlayered mudrock and silty bands or sandstones containing clay-rich surfaces, both of which give good resistivity contrasts.

STRK-04 Banded: a common fabric that comprises parallel zones of similar resistivity in bands that are thicker than average lamination. The example shows two, c. 0.4 ft thick, bright (resistive) bands, the lower of which contains sparse lamination. The bright bands are interlayered with more conductive material which is variable in character (laminated, structureless to blotchy and vuggy). The bright and dark banding may be due to different lithologies or, as shown in the core example, by cement zones within the host sandstone.

STRK-05 Laminated with resistive halos: resistive halos around conductive spots are common in the test well and commonly occur along laminae. These comprise a dark (conductive) spot, commonly elongated along the lamina and surrounded by a bright (resistive) halo. The halo is an artifact of resistivity imaging devices caused by current gather into the conductive spot, in this case the pyrite nodules seen in the core. Commonly, the halo diminishes slightly along the host lamina due to its slightly more conductive nature (e.g. 1185.2 ft).

STRK-06 Irregular blotchy: intervals characterized by large (borehole-width) irregular or convoluted fabrics and irregular, non-planar, highly disrupted contacts between lithologies. The example shows a resistive lithology containing high dip magnitude, non-planar structures overlying a less structured conductive lithology with a highly convoluted and distorted

contact. The equivalent core example shows a water escape structure caused by rapid sand deposition which may also be associated with larger-scale remobilization and slumping. In addition, bioturbation and the development of cementation fronts within compacted sediments may also exaggerate the visibility of contacts.

STRK-07 Patchy: this fabric is characterized by small-scale patches of alternating resistivity with no overall organization and containing, at most, very weakly-defined (relict?) structures. In core, the fabric corresponds to intervals in sandstones that contain patchy and disorganized fabrics, distributed mudclasts and bioturbation, all of which may be exaggerated by patchy cement development.

STRK-08 Vuggy: a distinct fabric that can be differentiated clearly from the Patchy type, with well-resolved, dark (conductive), circular or slightly oblate patches. The lack of resistive halos indicates that these are vugs (hollows) in the host lithology and generally coincide with limestone. The vugs are commonly concentrated along weakly-defined lamination and are likely to indicate differential dissolution of various components of the limestone.

STRK-09 Uninterpretable: rare intervals where image quality is poor and the fabric cannot be seen, although these are usually overlapped by subsequent drilling or relog passes over which good images were acquired. Typical artifacts include poor resolution due to low tool rotational velocity during run-up and rare stick-slip effects (zig-zag distortion). Borehole spiraling is rare in the test well but its presence does not affect image interpretation to a great extent (Lofts and Bourke 1999).

APPLICATIONS OF FABRIC INTERPRETATION: TEST WELL

The high resolution of the StarTrak tool allows the interpretation of detailed sedimentary and structural history using lithological and image fabric identification. This is illustrated using examples from the test site and the North Sea (Figures 3 and 4).

A detailed interpretation of a 17 ft representative section of sandstones from the test well is shown in Figure 3 and compares the StarTrak images to the wireline STAR Imager2, image fabric interpretations, facies associations and core from the offset well. Four main sedimentary fabrics have been focused on here, although the images display additional variations. These fabrics form a larger-scale genetic facies association which constitutes the channel sandstone seen in Figure 1:

2 Mark of Baker Atlas

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1659.2-1660.6 ft: a unit of irregular (and slightly blotchy) fabric within relatively conductive and mud-rich sandstones with well-defined boundaries. The dip magnitude is greater than in surrounding fabrics although not entirely chaotic. This is likely to be a slumped unit in which unconsolidated sand may have been remobilized and flowed as a coherent unit. Truncation is seen at the upper surface, indicating partial erosion by the flow that deposited the overlying, laminated sand.

1660.6-1662.5 ft: an interval of laminated and banded fabrics that are parallel and display distinct alternations in resistivity. Hints of subtle variation in fabric may also been seen within some of the bands and may indicate smaller-scale structures such as ripples or mudclast layers.

1662.5-1664.5 ft: a spectacular example of cross bedding within laminated sandstones displaying parallel set-bounding surfaces between which steeper-dipping foresets (the remnants of accreting bedforms) are preserved at clear truncation surfaces. The cross bedding is displayed by subtle variations in resistivity response that may be a result of preservation of mud drapes, small mudclasts or different grain sizes along the bedform foresets.

1669.0-1671.0ft: a zone of laminated sandstone with variable dip magnitudes and localized blotchy fabrics. This indicates the original deposition of flat, parallel laminated sand followed by partial disruption that may be due to pore water expulsion and partial remobilization. In addition, the growth of small, in-situ, non-displacive cement zones may also disrupt the sedimentary fabric.

DRILLING-RELATED FEATURES: NORTH SEA WELL

The successful acquisition of good-quality and useful image data in the North Sea (Ritter et al. 2004) shows the tools capability in the adverse conditions of a deviated (horizontal) borehole in a highly resistive formation (Rm:Rt 2000; Figure 4). The predictive ability from this well includes the following drilling related features, which if seen in real-time can serve to mitigate drilling hazards.

Borehole Breakout

A c. 10 ft example of the borehole intersecting a sliver of mudrock is shown in Figure 4a. The two sandstone-mudrock contacts of the mudrock interval can be seen in the images, they are non-parallel and the up-hole contact is also irregular. Importantly, the StarTrak tool has imaged dark, elongate, sub-parallel strips within the mudrock which indicate slight spalling of the borehole

wall and therefore measurement of the more conductive drilling mud. The geometry of these features is consistent with the development of borehole breakout over the weaker mudrock lithology and its position in the image shows that it is lateral (at the sides of the borehole). The detection of borehole breakout in such detail can provide valuable geomechanical information that can be used to predict borehole stability and, in real-time, could be used to prompt a counteractive increase in mud weight.

Drilling-Induced Fractures

Other mudrock intervals contains a range of natural and induced fractures that are not present in the sandstone intervals (Figure 4b). Several steep, full borehole-width to partial, and variably intersecting open fractures can be seen clearly as dark, conductive sinusoids (although the width of these features is likely exaggerated by the tool due to current gather). On the bottom side of the borehole a non-linear fracture can be identified which is broadly parallel to the borehole axis but also contains an offset along a natural fracture and an abrupt termination against another. The characteristics and position of this feature (relative to the borehole breakout) indicates that this is a drilling-induced fracture and also has implications for geomechanical modeling.

3D Visualization and Real-time Images: Sedimentary Steering

The high-resolution images can be used to construct tube-type 3D projections. These can be rotated and manipulated to aid in geometric interpretations of sedimentary structures and reservoir architecture (Figure 5a).

The acquisition of image data in real-time also allows the possibility of sedimentary steering by guiding the drill bit along a preferred sedimentary fabric (Figure 5b) or within a well-constrained zone within a sandbody (Figure 5c). Sedimentary steering could be used to accurately position the well to maximize coverage within a section of a sandbody that contains the best reservoir quality. In addition, the orientation of entry and exit bounding surfaces between sandbodies and mudrocks can be used for the calculation of sandbody geometries (Figure 5d; cf. McGarva et al. 1999).

CONCLUSIONS

The high resolution of StarTrak has allowed the identification of detailed image fabric types (8 in the test well). From these image fabrics we can make confident interpretations of key sedimentary structures similar to that derived from wireline image logs (especially with field or core calibration).


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When combined with lithological variation, image fabrics form the elementary building blocks with which to characterize the sedimentological variation in a reservoir.

The tool is capable of acquiring drilling-decision information even within hostile environments (such as deviated and harsh resistivity-contrast wells). DIF, borehole breakout and natural fractures are clearly resolved by the tool. In a similar fashion to wireline tools, fracture aperture is exaggerated by current gather effects.

With 3D visualization it is possible to interpret the internal architecture of sedimentary deposits and the larger-scale sandbody geometry that is critical for reservoir development.

The interpretation of image fabrics and image facies in real-time gives the potential for sedimentological steering in which the drill bit is guided through a particular sedimentary characteristic that may have favorable properties for well positioning and reservoir development.

ACKNOWLEDGMENTS

The authors wish to thank ChevronTexaco Upstream Europe and partners for permission to use the North Sea data.

REFERENCES

EMERY, D & MYERS, K.L., 1996. Sequence stratigraphy. Blackwell Science. 297p.

LOFTS J.C., & L. B. BOURKE, 1999. The recognition of artefact images from acoustic & resistivity devices, In: Lovell, M. A., Williamson, G. & Harvey, P. K. (Eds). Borehole Imaging: applications and case histories. Geological Society, London, Special Edition, 159, 59-76.

MCGARVA, R.M., BELL, C. & BEDFORD, J. 1999. Use of Resistivity At Bit (RAB) images within an Eocene submarine channel complex, Alba Field, UKCS. In: Lovell, M. A., Williamson, G. & Harvey, P. K. (Eds). Borehole Imaging: applications and case histories. Geological Society, London, Special Edition, 159, 177-189.

RITTER, R.N., CHEMALI, R., LOFTS, J., GOREK, M., FULDA, C., MORRIS, S. & KRUEGER, V. 2004. High resolution visualization of near wellbore geology using while-drilling electrical images. SPWLA 45th Annual Logging Symposium. Later published in Petrophysics, 46, 85-95.

ABOUT THE AUTHORS

Jeremy Lofts received his PhD from Leicester, UK in 1993 and he is a Chartered Geologist. He has worked as an image interpretation specialist and he lectures externally/internally on the subjects of Borehole Image Interpretation. Jez is author of +20 papers on geological and petrophysical oilfield applications. He is currently the Marketing Director, LWD Product Line Baker Hughes INTEQ, USA Headquarters.

Stephen A. Morris received his PhD from Cardiff, UK in 1998 and has authored papers on sedimentary processes. He is a specialist in deep-water sedimentary processes (turbidite systems). He has previously worked on geological interpretation of wireline and LWD borehole images and now works as an imaging specialist at Baker Hughes INTEQ, Houston.

Ren N. Ritter received his Dipl.-Ing. in Engineering from the University of Lbeck, Germany in 1997. From 1997 to 2001 he held various Technical and engineering roles in INTEQ. He is now the Project Manager within the Strategic Technology Development department of Baker Hughes INTEQ, Celle.

Roland Chemali received his engineering degree from the Ecole Polytechnique of Paris. He has authored over 20 papers and patents in electrical and acoustic logging. He is currently Product Line Manager for Emerging Technologies at Baker Hughes INTEQ, Houston.

Christian Fulda received his M.Sc. in Physics and his Ph.D. at the University of Heidelberg, Germany in 1998. During his post doctoral work at the Leibniz-Institute for Applied Geosciences, Hanover, Germany, he was associated lead of the Research Topic Groundwater. He is currently the Project Manager for the Advanced Resistivity Measurement R&D team within the Strategic Technology Development department at Baker Hughes INTEQ, Celle.

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Figure 1. Composite StarTrak image interpretation from the test well showing static-normalized images, dips and dip summaries, image fabrics, facies interpretations, larger-scale sedimentological packaging and sequence stratigraphic interpretation.


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Figure 2. Examples of image fabrics seen in the test well. Images are shown with static- and dynamic normalization alongside representative core. The table includes fabric description, interpretation of sedimentary structure type and interpretation of sedimentary processes.

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Figure 3. Large-scale interpretation of image fabrics and sedimentary structures in a c. 17 ft example from the test well, compared to the wireline image. Four key image fabrics are exemplified in the images with example core sections, although other fabrics are also present.


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Figure 4. Interpretations of two key sections of image data from the North Sea well. 4a shows where the borehole has penetrated through (or close to) a c. 10 ft sliver of mudrock that penetrates downward into the reservoir sandstones. Lateral borehole breakout is seen in the mudrock but not in the more competent sandstone. 4b shows where the borehole penetrated a c. 20 ft long mudrock interval containing an injected sandstone bed, open fractures and an axial drilling-induced fracture terminating on a natural fracture.

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Figure 5. Example of rendering of StarTrak image data in a tube-like projection (5a). This adds greater 3-dimensional visualization of sedimentary structure geometries. This example shows discordant surfaces due to likely cross bedding and a high-angle, resistive fracture that cuts through sedimentary features. Such visualization can be used to guide a well trajectory through specific sedimentary facies: Sedimentary Steering (5b). Additionally, the bit could be steered to stay within a sandbody (5c). On a larger-scale, borehole entry and exit orientations to sandbodies can be used to calculate sandbody geometries (5d).

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