Darrell Eubanks Doug Seiler
Halliburton Energy Services
HoustonBill Russell
Focus Energy
Shreveport, La.
Modern borehole imaging tools use advanced technology to produce dramatic views of the subsurface that are making the work of geologists easier. The core-like images provide quick and clear visual evidence to supplement and support what are often subjective geological interpretations.
Such geologic features as faults, fractures, cross-bedding, textural changes, and laminated beds can be identified and described. When the orientation of a feature is important, strike and dip can be easily calculated with an interactive graphics workstation.
ELECTRICAL IMAGING
The resistivity micro imaging tool introduced by Halliburton produces electrical images that are being used in a variety of geological applications.
The tool is a microresistivity-type imaging device with six independent articulating arms extending from the main body. Each arm is mounted with a pad containing a 25-electrode scanning array.
The EMI tool and all other micro imaging devices measure changes in resistivity along the borehole wall that correspond to subtle changes in rock composition, grain texture, and fluid properties.
High-end graphics software processes the data to produce highly visual 2D and 3D images of the borehole.
The images are displayed with a variety of color maps or in grayscale. Lighter hues commonly represent the more resistive features while darker hues indicate less resistive (more conductive) features.
Various image filters are used to selectively enhance the data. Such enhancements tend to highlight certain geological features, making them easier to visualize and describe.
Borehole images are best analyzed on a graphics workstation, where dips can be hand-picked quickly and accurately through the use of an interactive sine wave overlay. When a high-angle planar feature, such as a fault or fracture, intersects the borehole, it appears as a sinusoidal trace on the 2D display. The dip magnitude of the feature is related to the amplitude of the trace. The lowest point on the trace is normal to the strike. Fault analysis Faults can provide important petroleum traps. Identifying the orientation and depth at which a fault plane intersects the borehole is a key element to exploration in a fault play.
Recognition of drag patterns on dip logs is a common method used to identify and locate faults in a well.
Drag patterns, in the ideal case, are progressive changes in bedding dip just above and/or below a fault.
Dip logs can be difficult to interpret and usually require the attention of a specially trained dipmeter analyst.
Even then, the interpretation can be difficult to defend without other supporting geological evidence.
Electrical images allow geologists to identify faults more easily and to more clearly present their findings to other geologists, engineers, managers, or investors who may not be acquainted with the more specialized interpretation techniques.
The data used in the example of Fig. 1 (109656 bytes) were logged in East Texas. This example, which shows a normal fault, illustrates how borehole imaging can be used to locate a fault and determine the fault's orientation and dip magnitude. Displayed in Fig. 1 (109656 bytes), from left to right, are 1) a standard processed dip log, 2) image dips that were hand-picked with an interactive
workstation, and 3) 2D and 3D microresistivity images.
The fault is identified on the standard processed dip log by a classic drag pattern. In the image, the fault is displayed as a highly resistive (white), high-angle, discordant feature. The resistive nature of the fault indicates this zone to be highly mineralized.
Due to the complexity of the drag pattern associated with the fault, determining the direction and orientation of the fault is somewhat difficult with the processed dip log. However, the determination is easily deduced from the 2D image.
On 2D images, the low side of the sinusoidal trace of a normal fault is in the direction of throw-in this case, northwest. Thus, when analyzing faults with borehole images, the geologist does not have to rely on drag patterns, which in many cases are not present.
FRACTURE ANALYSIS
Fractured reservoirs are important hydrocarbon producers because of the additional permeability that results from fractures. Microresistivity imaging is an excellent method for studying these reservoirs, which are difficult to evaluate with standard wireline logging suites.
Characterizing fractured reservoirs using images enables geologists to locate the exact depths of the fractures, describe fracture type and density, and determine fracture orientation.
Under the assumption that open fractures are permeable and are filled with conductive drilling mud, such fractures would appear as dark features on the images. Healed fractures that are mineralized with a highly resistive mineral, such as quartz or calcite, appear as light features.
Distinguishing man-made hydraulically induced fractures from naturally occurring fractures is often difficult. Identification methods used in fractured-core analyses are helpful but are not as straightforward with images.
Hydraulically induced fractures often bisect the borehole vertically if the deviation of the well and bedding dip are low. On a 2D microresistivity image, the induced fractures normally appear as vertical cracks down the borehole wall that are 180 out of phase with each other. These fractures are commonly called bi-winged fractures.
In Fig. 2, (70074 bytes) a hydraulically induced fracture is easily identified on both the CAST and EMI images. (The CAST tool is an acoustic imaging device.) The subvertical fracture was created during a minifrac and intersects the borehole in a northeast- southwest direction.
Such information is useful in designing full-scale frac- turing treatments that might follow the minifrac. It is also useful in determining offset well locations in a drilling program where well spacing will be dense and production requires large fracturing treatments. (If an offset well is located in the direction of fracture growth, then the possibility of choking the well off during a massive frac job may be great.)
The geometry of natural fractures can appear simple or complex, depending on the rock type and past history of formation stresses. Fig. 3 (98044 bytes) was generated from data logged in the Black Warrior basin and is an excellent example of natural fractures with a simple geometric structure.
The section of borehole imaged in Fig. 3 (98044 bytes) contains well- developed, planar, high-angle, natural fractures that strike northwest-southeast. Their planarity is inferred because of their sinusoidal appearance in the 2D display. The conductive appearance of the traces (dark) indicates the fractures to be open, in which case there would be a conduit to formation fluids.
ROCK TEXTURE
When stratigraphic traps are the target of an oil and gas play, it is necessary to understand the depositional environment.
Electrical imaging helps geologists understand depositional systems by providing detailed images of internal bedding features. These features help identify stratigraphic facies that make up a particular depositional system.
Fig. 4 (91087 bytes) is a comparison of core plugs and microresistivity images. The plugs were taken from a full-diameter core retrieved from the same interval as the EMI images.
The figure illustrates a normal graded bed sequence. Graded beds are sedimentary deposits characterized by a gradation in grain size, from coarse to fine, in an upward direction. These beds can form in a variety of depositional environments.
The beds in this example were deposited in a fluvial environment. The textural changes in this graded bed sequence are evident in both the 2D and 3D images and are confirmed by the core plugs.
The 3D images look very similar to a conventional core when displayed in grayscale. This rock is a well-rounded to subrounded conglomeratic sandstone grading upward into a finely laminated sand.
The pebble-sized grains in the core consist of quartz and shale clasts. The resistive quartz pebbles appear as rounded, light- to very-light-colored objects in the microresistivity image. Shale clasts are conductive and appear as dark-colored objects.
The ability to recognize sedimentary features such as graded bedding is a definite benefit. Even though no single sedimentary feature is indicative of a particular depositional environment, recognition of these features can at least narrow the options for a given rock sequence.
STRAT DIP ANALYSIS
Cross-bedding is another type of sedimentary structure commonly recognized by geologists. Such structures are important because cross-bedding reflects the hydro-dynamic conditions at the time of deposition.
Not only is cross-bedding an important indicator of the energy of the paleocurrent that deposited the sediments, but cross-bedding orientation is related to the paleocurrent's flow direction. Stratigraphic dip analysis on cross-bedding allows geologists to recognize these paleocurrent patterns, which are vital in predicting the trend of a sedimentary deposit.
Fig. 5 (126166 bytes) displays well-defined sets of planar cross-bedding from the Mississippian St. Louis formation in Kansas. The lithology is an oolitic calcarenite (carbonate sand) that was deposited in a coastal-dune-type environment.
Each cross-bed set is terminated by a subhorizontal bounding surface. The dip magnitudes of the individual foreset laminae, which were hand-picked interactively on a graphics workstation, vary from 5-20. Direction through this zone is unimodal with a preferred orientation to the southwest.
Even though the orientation of the cross-bedding may be very obvious, as in this example, predicting the trend of a stratigraphic deposit depends very strongly on accurate interpretation of the depositional environment.
A coastal dune depositional model was used as the basis of the interpretation in this example. This selection was based on common local knowledge, and there was nothing obvious in the image to dispute the use of this model.
This stratigraphic unit developed parallel with the paleo- beach line, which is perpendicular to the direction of cross- bedding dip. This indicates that the deposit has a northwest- south-east orientation.
The ability to view internal sedimentary structures in a stratigraphic deposit, as was done in this example, is state-of- the-art technology. This information can be used to support a chosen depositional model and, when used with dip analysis, helps improve predictions of these sedimentary deposit's orientation.
THIN-BED ANALYSIS
Thinly laminated sand-shale reservoirs are important hydrocarbon producers in many sedimentary basins. These reservoirs can develop in a variety of geological settings ranging from fluvial overbank deposits to deep-water distal fan deposits.
Sedimentary facies that comprise reservoirs of this type are often unrecognized with conventional logging suites, which usually measure relatively low resistivities in these reservoirs. Hence, the reservoirs are commonly termed unconventional or low- resistivity pay. However, the very fine vertical resolution of micro-resistivity imaging tools allows detection of the poten- tially hydrocarbon-bearing laminated sand-shale sediments.
Fig. 6 (84584 bytes) shows a wireline triple combo log (resistivity, neutron, and density) obtained in a thinly bedded sand-shale reservoir. This classic thin-bed example is from the Gulf of Mexico. Reservoirs of this type are easily bypassed.
Near the top and bottom of the log are textbook examples of gas sands. Each of the two intervals displays high resistivities and the familiar neutron-density crossover that is common to Gulf Coast gas sands.
The section of log just below the upper gas sand appears to be shale. However, the microresistivity image through this same interval (Fig. 7)(172754 bytes) reveals the laminated nature of this rock, giving any oil finder evidence for further testing.
Imaging also reveals that simply perforating the laminated zones may not be an efficient completion method since there is approximately a 50% chance that perforations would penetrate a shale. Any completion method must provide a way of interconnecting the sand beds. Completion designs that call for a combination of hydraulic fracturing and gravel packing operations, such as used in Halliburton's FracPac service, have enjoyed much success in the highly permeable, laminated reservoirs of the Gulf of Mexico.
CONCLUSIONS
Microresistivity imaging of the borehole provides high- resolution, core-like pictures that greatly enhance any geological study.
Geologic features that once were difficult to recognize from other information sources are now easy to identify and analyze with borehole imaging. These features include faults, fractures, rock textures, various stratigraphic structures, and sequences of thinly laminated beds.
Perhaps one of the most important advantages of using borehole images is that imaging clarifies many geological interpretations for geologists, engineers, managers, and investors who may not be familiar with the geology of an area or may not be acquainted with the more specialized techniques of log analysis.
BIBLIOGRAPHY
Eubanks, D.L., and Seiler, D., Use of interactive graphics workstation applications and borehole imaging data in geologic studies, AAPG annual convention, Denver, June 12-15, 1994.
Eubanks, D., Applications of borehole imaging in geologic studies, West Texas Geological Society, Midland, Tex., Oct. 31- Nov. 1, 1994.
FracPac Completion Services, second edition, Halliburton Energy Services, Houston, 1995.
Seller, D., King, G., and Eubanks, D., Field test results of a six arm microresistivity borehole imaging tool, 35th SPWLA Logging Symposium, Tulsa, Okla., June 19-22, 1994.
Seiler, D., Three dimensional visualization of borehole images, First International Seminar on Improvements in Practices of Oil and Gas Exploration, Lima, Peru, Nov. 2-5, 1993.
Reineck, H.R., and Singh, 1.B, Depositional sedimentary environments, 2nd edition, Springer-Verlag, Berlin, 1980.
