Image – Core Integration
An important application of borehole image data is image feature validation against core from the same wellbore. Comparison of depth-resolved core with image data provides a “ground-truth” for image features. Because coring is exceptionally expensive, image logs provide a “core-like” proxy in parts of the field without core. Cores are seldom collected across the entire drilled-interval of a wellbore. Image logs, however, are readily acquired across larger intervals, yielding more comprehensive information.
In the example here, 180′ of core is compared to image log and gamma data. The lighter portions of the core, comprised of sandstone, correspond with lower gamma response and a lighter (more resistive) color on the image log. Dark-colored rock in the core, comprised of mudstone and shale, is matched up with higher gamma response and darker (more conductive) image colors.
The picture above is a zoom of the core and image log showing a bioturbated sandy interval between shales. This stacking pattern dominates the cored interval and makes it difficult to perform petrophysical evaluation because reduction of formation resistivity by shales make these zones look wet. Combining the resolution of the image log with NMR and dielectric data facilitates accurate SW calculation in laminated rock. This type of analysis, in a vertical well, can enhance accurate EUR calculation and can facilitate identification of horizontal target zones.
Image - Facies Analysis
Another important application of borehole image data is the use of textural characteristics combined with lithologic information to assign image facies. Image facies act as a proxy for depositional facies.
Facies analysis occurs in two steps. First, bulk lithology is determined based upon the response of open-hole curves (gamma, density-neutron, photoelectric effect, etc.), image log color and bedding character. Second, a visual inspection of the image data is used to assign a textural classification to specific intervals. Commonly observed textures include well-preserved tabular and tangential cross-bedding, planar lamination, mottling by bioturbating organisms, concretionary horizons, authigenic pyrite mineralization, internal soft-sediment deformation and solution-collapse brecciation. Stick and pull intervals are also identified in facies analysis.
Once image facies are identified, they are compiled into pie charts that display the relative abundance of each facies type across selected intervals. Then, changes in the relative amount of image facies can be compared with changes in depth. Variation in the abundance of facies types are also observed to change within a single stratigraphic interval in wells across the field.
Once facies analysis is complete, useful comparisons are available in both the vertical well and across the field. Analysis of image facies within an individual well permits determination of changes in depositional environments through time. Comparing changes in image facies composition across a field captures spatial variation in depositional environments. Combining vertical and lateral image facies flux yields a three-dimensional portrait of the sedimentary system.
In this example, image facies were analyzed for 3 sandstone packages. Sand intervals l & ll display a similar facies suite, dominated by cross-bedded and mottled sandstones with a smaller percentage of muddy sandstones and mudstones. The facies observed in these units indicate that they were deposited in a fluvial depositional environment.
Sand III constitutes a major change in facies suite from the underlying sands. This unit displays a large percentage of mottled and laminated mudstones, with very few cross-bedded sands and a significant amount of coal. These facies indicate that the Sand III unit was deposited in a deltaic depositional environment.
Image facies data can be scaled up to enhance the understanding of reservoir architecture across fields and sub-basins. These data can contribute important constraints to fine-tune geostatistical model development.
Facies Field Map
The map above shows image facies data excerpted from a 16 well study. Image facies appear as pie charts over a structure contour map to illustrate lateral changes in sediment composition and texture within a stratigraphic interval of interest. In this example, the pie charts reveal subtle changes in the relative abundance of sand vs. muddy sand vs. mudstone and changes in the percentage of identifiable textures.
Image facies data can be used as input to constrain depositional, petrophysical and geostatistical models within a hydrocarbon reservoir, ultimately contributing to in-place reserve and productivity calculations. These models, in turn, support development of a reservoir simulation model, to fine-tune EUR calculations, establish priority for vertical or horizontal development and, with a geomechanical model, optimal well spacing.