An Image Log is Worth a Thousand Words

Borehole image logs have been utilized by the oil and gas industry since the late 70s and now more than ever can provide critical insights to subsurface characterization projects (Poppelreiter et al., 2010), either for the oil industry or CCS/Geothermal.? Borehole image logs come in many flavors, but they all measure a property at the wellbore wall to generate a map (this is your image) of the distribution of such property through the wellbore wall. Most borehole image logs measure conductivity/resistivity or acoustic properties (amplitude and travel time). Less commonly used are those tools that generate an image out of density or gamma ray measurements. They can be run as part of a conventional wireline logging suite or together with the drilling assembly to deliver images in real time. In all cases, the final product is displayed as un unwrapped view of the wellbore wall (Figure 1). But it gets better… because the images come always oriented to north (in vertical wells) or top of hole (in deviated wells) which allows for lots of quantitative analyses.

The SLB FMI (Formation MicroImagerTM) is perhaps the best known of all these tools and consists of a set of 4 mechanical arms that need to be in touch with the wellbore wall in order to collect conductivity data. As a result, the borehole image has data gaps in between the readings not providing a full borehole coverage except for slim holes where the tool pads are so close together that they cover the entire borehole circumference.? On the other hand, acoustic image logs, such as CBILTM or STARTM, provide a 360-degree coverage of the wellbore that generates two types of image logs, an amplitude image, and a travel time image (Figure 2).

Resistivity-based image logs are usually displayed using a yellow-to-black scale. The convention is that conductive features are displayed in darker colors, while resistive features appear as light. These types of tools are great to pick up features that might be open and get filled with conductive drilling mud. Fractures or bedding planes are easily imaged by resistivity-based image logs, but they can also picture sedimentological and textural features amongst others. As the tool is oriented, it provides information on the dip and dip azimuth of such features. Additionally, it highlights the presence of induced stress features, like wellbore breakouts and drilling induced fractures. Such features are critical for inferring the orientation and magnitude of the maximum horizontal stress (Figure 3).

With superior wellbore coverage, acoustic image logs provide information about the presence of fractures and bedding planes, as well as stress induced features (Figure 3). Borehole enlargements related to open features scatter energy from the acoustic beam, reduce the signal amplitude and produce recognizable features on the images (Paillet et al., 1990).? The acoustic impedance contrast between the borehole fluid and the well is indicative of the relative hardness of the borehole wall. Filled secondary porosity features might be detected even when there is no change in borehole diameter if there is sufficient acoustic contrast. The combination of travel-time and amplitude images also provides a wealth of information about geologic facies, that when properly calibrated with core, can be used to build a stratigraphic framework. This can also be done with resistivity tools, but it often poses a more challenging task. Table 1 summarizes all the different types of features that could be interpreted from image logs.

CCS applications

In our experience image logs are often underutilized, and while they are included in 90% of the logging programs, they end up not being used due to the lack of expertise in processing, loading, and interpreting the data. In fast paced CCS projects, there is an opportunity to dig up the old wells and find those borehole image logs that were collected but perhaps never studied in depth. Instead of waiting for the next well to be drilled and collect image logs, there is plenty of information that could be derived from existing datasets, even if they are not of great quality, or if all you can find by now is a PDF file of the image log from a well drilled in the 80s... There are several well-defined areas where image logs can have a substantial impact in the process of developing a CCS project and meeting the US Environmental Protection Agency (EPA) requirements for a Class VI well:

1. In-situ stress field – identification of borehole breakouts and induced fractures to infer orientation and magnitude of the maximum horizontal stress.
2. Seal integrity - identification of seal thickness and vertical heterogeneity, as well as characterization of density and openness of natural fractures in the cap rock.
3. Characterization of the injection zone – identification of image log facies suitable for injection, as well as identification of natural fractures. Natural fractures within the injection zone can impose a permeability anisotropy that impacts plume geometry and extent.

Geothermal applications

Image logs have similar applications to those described for CCS. High enthalpy geothermal systems often exploit the network of natural fractures that connect an injector-producer pair of wells. Understanding the properties of natural fractures, including fracture intensity, orientation, and aperture, is critical to optimize well placement and injection rates during field development. Image logs in geothermal projects are required to characterize the in situ stress field. The magnitude and orientation of in situ stresses are basic inputs to assess dynamic performance on stress-sensitive geothermal reservoirs, as well as to minimize the risk of induced seismicity.

Perhaps the most challenging part of running image logs in geothermal systems is the technological limitations imposed by such a hostile environment. Most image log tools are rated for low temperature wells. There is just a handful of companies that manufacture tools rated for high temperatures. These are often slim hole tools like the Acoustic Borehole Image (ABITM) manufactured by Luxemburg-based company Advanced Logic Technologies (ALT). While these tools might seem a boutique type of equipment, the amount of reservoir information provided is worth the investment. In a best-case scenario, after collecting core in few appraisal wells, core-calibrated image logs could be the ideal replacement for core during field development.

In our next couple of technical notes, we will continue exploring the value of image logs. First, by integrating core and image log, as a way of improving the characterization of fractured reservoirs. Later, we will dive into what sort of geomechanical information can be extracted from borehole image logs, and some of its most immediate applications.

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References

Gaillot, P., T. Brewer, P. Pezard, and Y. En-Chao, 2007, Borehole imaging tools—Principles and applications: Scientific Drilling, v. 5, p. 1–4, doi:10.2204/iodp.sd.5.07S1.2007.

Paillet, F. L., C. Barton, S. Luthi, F. Rambow, and J. R. Zemanek, 1990, Borehole imaging and its application in well logging—An overview, in F. L. Paillet, C. Barton, S. Luthi, F. Rambow, and J. R. Zemanek, eds., Borehole imaging: Houston, Texas, Society of Professional Well Log Analysts, p. 3–23.

Poppelreiter, M., C. Garcia-Carballido, and M. Kraaijveld, 2010, Borehole image log technology: Application across the exploration and production life cycle, in M. Poppelreiter, C. Garcia-Carballido, and abd M. Kraaijveld, eds., Dipmeter and borehole image log technology: AAPG Memoir 92, p. 1–13, doi:10.1306/13181274M923406.