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If these wireline images and other supporting data suggestthat hydrocarbons are present in the trap, then a core might be acquired over thesmall interval of interest for detailed analysi

Image Databases: Search and Retrieval of Digital Imagery

Edited by Vittorio Castelli, Lawrence D Bergman Copyright  2002 John Wiley & Sons, Inc ISBNs: 0-471-32116-8 (Hardback); 0-471-22463-4 (Electronic)

(hydro-at both how hydrocarbons are formed and how we explore for them

Oil and gas (hydrocarbons) are generally found in the pores of sedimentaryrocks, such as sandstone or limestone These rocks are formed by the burial of

sediment over millions of years and its subsequent chemical alteration

(diagen-esis) In addition to the sediment, organic material is also buried and subjected to

the same high pressures and temperatures that turn the sediment into rock Thisorganic material eventually becomes oil and gas

Over time, the oil and gas migrates upward through porous and permeablerock or fractures because it is less dense than the surrounding groundwater.Most of these hydrocarbons reach the surface and either evaporate or dissipate.However, a small fraction of these migrating hydrocarbons become trapped inthe subsurface

A hydrocarbon trap forms when an impermeable rock, such as shale, liesabove a porous rock, such as sandstone or limestone Traps are often associatedwith faults or folds in the rock layers The exploration for hydrocarbons generallybegins with the search for these traps

Oil exploration may begin with the acquisition of two-dimensional (2D)

seismic data in an area of interest These data may be thought of as

two-dimensional images vertically slicing through the Earth, each slice being tens

of kilometers long and several kilometers deep If a candidate area is located

on these images, then a three-dimensional (3D) seismic survey may be acquired

over the region This survey yields a 3D image of the subsurface

107

The 3D seismic images are then carefully analyzed and interpreted If a trap

is identified, and enough supporting evidence suggests that economical deposits

of hydrocarbons are present, then the decision to drill a well might be made.After the well is drilled, wireline logs are acquired to image the rock stratapenetrated by the well If these wireline images and other supporting data suggestthat hydrocarbons are present in the trap, then a core might be acquired over thesmall interval of interest for detailed analysis of the rock

The depicted scenario is just one possible use of imagery in the hunt forhydrocarbons There are, however, many other steps involved in exploration andproduction, some of which are discussed later in this chapter

To interpret and manage these data, the petroleum industry relies on largesoftware systems and databases Through the 1980s, oil companies developedmuch of this software in-house for interpreting and managing oil fields Mostoil companies have traditionally had a heterogeneous mix of software tools thatinclude vendor-supplied products and homegrown applications Communicationbetween these products typically involved exporting the data as ASCII text filesand importing the data into another application

Just as they have long outsourced the acquisition of data, during the 1990s theoil companies increasingly outsourced the development of software Numerousvendors now produce specialized applications that manage specific aspects ofoil field development To address the resulting interoperability nightmare, themajor oil companies invested substantial effort to standardize data storage andexchange formats In particular, the Petrotechnical Open Software Corporation(POSC) was created as a nonprofit organization whose purpose is to produce

open specifications (called Energy eStandards) for leveraging and integrating

information technologies

The late 1990s also saw the explosion of the Internet and the associatedevolution of tools and standards for business-to-business e-commerce POSCand the rest of the oil industry are embracing these new opportunities to buildeven more open data exchange standards

This chapter introduces some of the types of image data acquired duringthe hydrocarbon exploration and production task This is followed first by adiscussion of how these data are processed and integrated with each other and ananalysis of data management issues Finally, an overview of some of the mostwell-known interpretation and analysis systems is presented

5.2 DATA CHARACTERISTICS

A wide variety of image data is acquired from the subsurface during the carbon exploration task Some of the principal technologies involved in imageacquisition are discussed in this section

hydro-5.2.1 Wireline Logs

Wireline logging is the most common means for analyzing the rocks intersected

by a well (Section 5.2.2) A well is “logged” after an interval has been drilled

commu-• The hoisting equipment used to raise and lower the tool in the well.The drill and drill pipe are first removed from the well, leaving the newly drilledwell full of a high-density fluid (the drilling mud) The tool assembly is thenlowered to the bottom of the well and slowly pulled to the surface, making variousmeasurements (electrical, acoustic, and nuclear) of the surrounding rock and fluids

as it passes up through the different geologic strata These measurements generate

a continuous stream of data up the “wireline” to the data acquisition system onthe surface These data are displayed on a “log” that presents the measurementsabout the rocks and fluids as a function of depth The data are also recordeddigitally for further processing and analysis

The tool assembly is composed of numerous instruments, each of whichmeasures a different physical property of the rock and the fluid contained in thepore spaces Depending on the complexity of the rock and fluid being analyzed,and the clients’ budget, 10 or more types of measurements may be required toobtain the desired information

Some measurements examine the natural nuclear radiation emitted by therocks; others measure the formation’s response to bombardment by gamma rays

or neutrons There are yet other measurements that observe how induced tional (acoustic) waves are transmitted through the rock Electrical measurementsobserve the conductivity of the surrounding rocks and fluids: salt water is conduc-tive, whereas oil and gas are nonconductive

vibra-The typical wireline logging tool resembles a long thin pipe vibra-The Schlumbergercombined magnetic resonance (CMR) tool is typical The tool is 14 ft long with adiameter of 5.3 in It can operate in holes with a diameter as small as 5.875 in Onthe CMR tool, the sensor is a 6-in-long pad, which presses against the rock wall.The remaining 13.5 ft of the tool contain the power supply, computer hardware,and telemetry equipment needed to support the sensor

As hostile environmental conditions exist in the well, all components of thelogging tool are engineered to operate under extreme conditions Temperaturescan exceed 400◦F and pressures can exceed 20,000 psi Pulling the tools throughthe well can subject them to high shock and vibration Chemicals in the well areoften extremely corrosive

The FMS (Formation MicroScanner) and FMI (Formation MicroImager) toolsare used to image the circumference of the borehole Both these tools have

very closely spaced electrodes As such, they produce and measure electricalcurrent that flows near the well bore surface, rather than deep in the rock strata.Therefore, they measure localized electrical properties of the rock formations andyield high-resolution images.

Figure 5.1 illustrates an FMS tool The FMS consists of four orthogonal

imaging pads, each containing 16 microelectrodes or buttons (Fig 5.2), which

Figure 5.1 Formation MicroScanner (FMS) sonde (http://www.ldeo.columbia.edu/BRG/

ODP/LOGGING/MANUAL/MENU/contents.html, ODP Logging Manual).

Figure 5.2 Detailed view of the 16 electrodes on one of the four FMS pads

(http://www.ldeo.columbia.edu/BRG/ODP/LOGGING/MANUAL/MENU/contents.html,

ODP Logging Manual).

DATA CHARACTERISTICS 111

are in direct contact with the borehole wall during the recording After a portion

of the well has been drilled, the FMS sonde is lowered into the deepest part

of the interval of interest The sonde is then slowly pulled up the well with thebutton current intensity being sampled every 2.5 mm The tool works by emitting

a focused current from the four pads into the formation The current intensityvariations are measured by the array of buttons on each of the pads The FMI tool

is very similar to the FMS tool It has eight pads instead of four, and produces

a more continuous image around the circumference of the borehole An example

of an FMI image is illustrated in Figure 5.3

Despite the power of 2D imaging tools such as FMI and FMS, the majority oflogging tools are single channel, that is, for a given depth only one measurement

is made for a particular physical property Thus, as the tool is being pulled upthe hole, it is taking “snapshots” of the surrounding rock at regular intervals The

Figure 5.3 Sub-horizontal stylolites(wide dark bands) and inclined fractures (narrow dark

lines) in a Middle East carbonate formation [Akbar et al., Classic interpretation problems:

evaluating carbonates, Oilfield Rev., Winter, 38 – 57 (1995)] A color version of this figure can be downloaded from ftp://wiley.com/public/sci tech med/image databases.

typical depth-interval spacing for the single-channel logging tools is 6 inches Themeasurements taken at a specific depth are termed frames Other tools, such asthe CMR, acquire multiple measurements at each frame For example, the CMRtool measures the magnetic resonance relaxation time at each frame, which hasvarying signal intensity as a function of time.

A relatively standard presentation for wireline logging data has evolved overthe years In this presentation, the vertical axis of the cartesian plot is the indepen-dent (depth) variable, whereas the horizontal axis is the dependent (measurement)variable Some measurements are scaled linearly, while others are scaled logarith-mically, resulting in parallel plots Imagery from FMI and FMS tools is typicallydisplayed in an unwrapped format in which the vertical axis is depth and thehorizontal axis is the azimuth around the borehole This format presents theentire circumference of the borehole, although certain visual distortions result(Fig 5.3)

5.2.2 Logging While Drilling

The vast majority of vertical or deviated oil and gas wells are “logged” withwireline technology “Horizontal wells” that are steered to follow the geologic

strata at depth instead use a specialized technology called logging while drilling

or LWD

LWD is the measurement of the petrophysical properties of the rock penetrated

by a well during the drilling of the hole LWD is very similar to wireline logging

in that physical measurements are made on the rock but differs greatly in that themeasurements are made during the drilling of wells rather than after With LWD,

the logging tools are integrated into the bottom hole assembly (BHA) of the

drill string Although expensive, and sometimes risky, LWD has the advantage

of measuring properties of the rock before the drilling fluids invade deeply.Further, many well bores prove to be difficult or even impossible to measurewith conventional wireline logging tools, especially highly deviated wells Inthese situations, the LWD measurement ensures that some measurement of thesubsurface is captured in the event that wireline operations are not possible.The BHA is located at the end of a continuous section of coiled tubing Drillingmud is pumped down the center of the coiled tubing so that the hydraulic force

of the mud drives the mud motor, which in turn drives the drill bit at the end ofthe BHA The logging tools are located within the BHA but behind the drill bit.The resistivity at the bit (RAB) tool makes resistivity measurements around thecircumference of the borehole The RAB tool also contains a gamma ray detector,which supplies a total gamma ray measurement An azimuthal positioning systemallows the gamma ray measurement and certain resistivity measurements to beacquired around the borehole, thereby generating a borehole image The RABtool may be connected directly behind the bit or further back in the BHA

In LWD, the acquired logging data is delivered to the surface through mudpulse telemetry: positive and negative pressure waves are sent up through the mudcolumn The bandwidth of this telemetry system (less than 10 bits per second) is

DATA CHARACTERISTICS 113

LWD image Wireline image

Figure 5.4 Comparison of LWD (RAB) image with an FMI image in a deviated well.

Note the characteristic sinusoidal pattern caused by the intersection of the rock strata with

the cylindrical borehole (http://www.ldeo.columbia.edu/BRG/ODP/LOGGING/MANUAL/

MENU/contents.html, ODP Logging Manual) A color version of this figure can be

downloaded from ftp://wiley.com/public/sci tech med/image databases.

much lower than that supported by the conventional wireline telemetry Becausedrilling speeds are typically very low (less than 100 feet per hour), a lot of datacan be delivered to the surface even with the low bandwidth Thus, many ofthe same measurements that can be made with wireline logging can be madewith LWD

Figure 5.4 illustrates a comparison of LWD RAB tool and wireline electricalimaging FMI tool measurements of dense fracturing in consolidated sediments.Both images of the interior of the borehole wall are oriented to the top andbottom of the deviated (nonvertical) well Note that the RAB tool has inferiorbed resolution (by a factor of 30) than the FMI, although it provides completecircumferential coverage

As noted earlier, LWD is generally used in highly deviated or horizontal wellswhere it is not possible to lower a wireline tool into the hole Highly deviated andhorizontal wells are generally geosteered, that is, the driller can control in realtime the direction of the drill Geosteering requires an understanding of where thedrill bit is relative to the surrounding rock LWD is well suited to this purpose.Because the well is being “logged” as it is passing through the rock formations,the driller knows when the drill has entered or left the zone of interest, therebyallowing the geosteering activity to be controlled in near real time

5.2.3 Core Images

A core is a cylindrical sample of rock collected from a well Conventionally,when a well is drilled, the diamond drill bit pulverizes the rock To retrieve a

consolidated section of core, a coring tool is required A coring tool is essentially

a hollow pipe that cuts out a cylinder of the rock without pulverizing it The rock

is then preserved inside the pipe and brought to the surface

The first coring tool appeared in 1908 in Holland The first one used in theUnited States appeared some years later (1915) and was a piece of modified drillpipe with a saw-toothed edge for cutting — much like a milling shoe [3]

AOGC WILLIAMS #3 STANTON.K.S TOP 5661

Figure 5.5 Slabbed core in boxes Note the holes in the core where rock samples have

been removed for further analysis (http://crude2.kgs.ukans.edu/DPA/BigBow/CoreDesc,

Kansa Geological Survey, Big Bow Field) A color version of this figure can be

down-loaded from ftp://wiley.com/public/sci tech med/image databases.

DATA CHARACTERISTICS 115

Once collected, the cores are placed in core boxes The boxes are labeled withthe well identification information and marked with the measured depths of eachpiece In many cases the core is sliced down the axis of the cylinder (“slabbed”)

so that a flat surface of the rock is exposed for visual inspection (Fig 5.5) Thecore is then typically stored in large core “warehouses.” Traditionally, geologistsand technicians would then visually inspect the core and have samples extractedfor further analysis Increasingly, these “slabbed” (and “unslabbed”) cores aredigitally photographed

After the core has been boxed and possibly slabbed, small samples are takenfor higher-resolution analysis (Figs 5.5 and 5.6) The data obtained from thecore include photographic images, measurements of physical properties, such asporosity and permeability, and microphotographs of thin sections Quite often,even higher-resolution imaging is required to fully understand the properties ofthe rock In these cases, scanning electron microscopy (SEM) may be necessary.There are no standards for core photographs Only recently have laboratoriesbegun capturing the images digitally Those cores that were photographed arenow being “scanned” at varying resolutions

Figure 5.6 Slabbed core from a single well, illustrating variability in rock color, layering,

and texture Note the holes in the core where rock samples have been removed for

further analysis (http://crude2.kgs.ukans.edu/DPA/BigBow/CoreDesc, Kansa Geological

Survey, Big Bow Field) A color version of this figure can be downloaded from ftp://wiley.com/public/sci tech med/image databases.

A common technique when photographing core is to take two photographs:one in white light, the other in ultraviolet light Ultraviolet light is useful becauseoil becomes luminescent, and the oil-saturated rock then becomes easily distin-guishable from the oil-free rock.

5.2.4 Seismic Data

Seismic imaging is the process through which acoustic waves reflected fromrock layers and structures are observed and integrated to form one-, two-, andthree-dimensional images (1D, 2D, and 3D) of the Earth’s subsurface Theresulting images allow us to interpret the geometric and material properties ofthe subsurface

Figure 5.7 VSP data in a horizontal well (red trace) The data show three important

features; two faults marked A and B, which appear as anticipated in the reflected image, together with evidence of dipping The apparent formation dips seem to be parallel to the borehole until very near total depth This turned out to be entirely consistent with the Formation MicroScanner (FMS)– computed dips [Christie et al., Borehole seismic data

sharpen the reservoir image, Oilfield Rev., Winter, 18 – 31 (1995)] A color version of this figure can be downloaded from ftp://wiley.com/public/sci tech med/image databases.

SELECTED APPLICATION SCENARIOS 117

At the scale of reflection seismic imaging, the Earth is, to a first-order imation, a vertically stratified medium These stratifications have resulted fromthe slow, constant deposition of sediments, sands, ash and so forth As a result ofcompaction, erosion, change of sea level, and many other factors, the geologic,and hence the seismic character of these layers varies with the depth and age ofthe rock

approx-Seismic data acquisition uses low-frequency sound waves generated by sives or mechanical means on the Earth or ocean surface As these waves traveldownward, they cross rock layers; some of their energy is reflected back to the

explo-surface and detected by sensors called geophones (on land) or hydrophones (in

the ocean)

Modern digital recording systems allow the recording of data from morethan 10,000 geophones simultaneously Sophisticated seismic-processing soft-ware then integrates these data using the physics of wave propagation to yield3D images of the subsurface

It should be noted that not all seismic data is acquired with both the reflectionsource and the receivers being on the surface of the Earth Often, the receiversare located within the borehole, a practice commonly known as vertical seismicprofiling (VSP) This approach can often yield very high-resolution images at thedepth of the reservoir (Fig 5.7)

Seismic data from modern 3D surveys is typically represented as 3D arrays of1-, 2-, or 4-byte floats Seismic interpretation applications present the user with

a wide spectrum of tools to visualize the data in 3D voxelated volumes or as2D slices through the volumes In addition to storing the volumes as differentresolution float arrays, each volume may be stored three times — once for everydimension This permits the data to be rapidly accessed in each of the primarydimensions of the volume

5.3 SELECTED APPLICATION SCENARIOS

This section provides an overview of a number of typical applications of imagery

in the exploration and production of oil and gas

Structural interpretation involves the analysis and modeling of the geometry

of the geologic strata, which may be deformed by folding and disrupted byfaulting (Fig 5.8)

Figure 5.8 Horizontal section through a 3D seismic volume illustrating folded (curved)

rock layers being terminated by faults (A.R Brown, Interpretation of three-dimensional

seismic data, 5th ed., AAPG and SEG, 1999, p 514) A color version of this figure can

be downloaded from ftp://wiley.com/public/sci tech med/image databases.

Stratigraphic interpretation involves the analysis of how the rock varies

spatially as a function of different depositional environments For example,seismic data may be used to identify a meandering river (Fig 5.9)

By calibrating seismic attributes with other data, such as well log–derived mation, maps of horizon (layer boundary) or formation (layer volume) materialproperties may be made (Fig 5.10)

infor-5.3.1.1 Identification of “Bright Spots.” The term bright spot is used in the

industry when it is believed that pools of gas or oil have been directly observed byseismic imagery Bright spots are very high-amplitude signals that often resultfrom large changes in acoustic impedance, for example, when a gas-saturated

SELECTED APPLICATION SCENARIOS 119

Figure 5.9 Horizontal slice through a 3D seismic volume illustrating a meandering

river (A.R Brown, Interpretation of three-dimensional seismic data, 5th ed., AAPG

and SEG, 1999, p 514) A color version of this figure can be downloaded from

ftp://wiley.com/public/sci tech med/image databases.

sand underlies a shale layer, but can also be caused by phenomena other thanthe presence of hydrocarbons, such as a change in rock type

Figure 5.11 is a vertical section through a 3D seismic volume that illustrates

a bright spot caused by two gas filled sand layers The layers are dipping down

to the right Note the “flat spot” at the lower right termination of the brightspots A flat spot is caused by the horizontal (due to gravity) boundary between

Figure 5.10 Porosity map of geologic horizon based on seismic attributes calibrated

against well data [Ariffin et al., Seismic tools for reservoir management, Oilfield

Rev., Winter, 4 – 17 (1995)] A color version of this figure can be downloaded from ftp://wiley.com/public/sci tech med/image databases.

Figure 5.11 Bright spots caused by two dipping and layers filled with gas.

Horizontal flat spots caused by the gas (above) giving way to water (below)

(A.R Brown, Interpretation of three-dimensional seismic data, 5th ed., AAPG and

SEG, 1999, p 514) A color version of this figure can be downloaded from

ftp://wiley.com/public/sci tech med/image databases.

SELECTED APPLICATION SCENARIOS 121

the gas-filled sandstone above and the water-filled sandstone below As notedearlier, bright spots can be caused by changes in rock type, but the presence of

a horizontal flat spot strongly supports the premise that the bright spot is caused

by a fluid boundary

5.3.1.2 Identification of Stratigraphic Features Generally, large volumes of

rock are very similar in their composition and internal geometry This is becauseuniform depositional environments often have large spatial extents For example,the sediments blanketing the outer continental shelves are often relatively uniformlayers of shale Other environments, such as near-shore deltas or coral reefs, varyrapidly in a spatial sense Seismic imagery provides a very powerful tool forinterpreting and identifying these features

Figure 5.9 is a horizontal section through a three-dimensional seismic volumeillustrating a meandering river system This image is analogous to the view from

an airplane of the modern Mississippi river From an understanding of the physics

of river behavior, the exploration geologist can then make inferences about whichparts of the river system have sand deposits and therefore potential hydrocarbonaccumulations

5.3.2 Formation Evaluation with Wireline Images

Wireline logging is the basis of an entire science called petrophysics or rock

physics Professional organizations exist for this science, such as the Society of

Professional Well Log Analysts (http://www.spwla.org/ ).

Although the single-channel or one-dimensional logging tools are typicallyfocused at measuring the petrophysical properties of the rocks, applications ofthe imaging tools (FMS and FMI) are more varied and include

• Mapping of the internal geometry of the rock: the orientation of the strata,the frequency of the layering, and the orientation and frequency of fractures

• Detailed correlation of coring and logging depths Depths measured by line length (because of cable stretch) differ from the measured depth of core,which is derived from drill pipe length FMI, FMS, and core images are ofcomparable resolution and may therefore be used to correlate with eachother

wire-• Precise positioning of core sections in which core recovery is less than 100percent

• Analysis of depositional environments The internal geometry of the rocklayers may be indicative of the ancient environment in which they weredeposited

5.3.2.1 Bedding Geometry A great deal of information about the rock

pene-trated by a well can be inferred from conventional well logs These measurements,combined with an understanding of the geologic environment will generally yieldinformation about the rock types and their fluid content Of comparable importance