A better understanding of complex formations is being achieved using resistivity imaging to visualize the borehole wall. From unconventional tight gas to tar sands and coal exploration,

Figure 1. Figure 1 shows half API-scale log plots. The figure on the left shows the neutron density induction log of the continuous shale, and (right) is the corresponding HMI borehole image log of the same shale. (Images courtesy of Weatherford)
the technology is providing operators with insights into many critical aspects of formation evaluation, including structural determination, stratigraphic delineation and fracture identification.

In tight gas reservoirs, fracture characterization developed from resistivity imaging of natural and drilling-induced fractures helps optimize well orientation and hydraulic fracturing. Imaging also provides coring points in appraisal and development wells.

Coal exploration uses resistivity imaging to identify the cleating system, determine its orientation and analyze structural breakout. Logging in exploration boreholes as small as 4 3/4 in., the high-resolution images allow analysts to see features smaller than 0.2 in. With vertical resolutions near core-detail levels, the images also provide accurate orientation for cores.

But based on the sheer magnitude of the recoverable reserves involved, the most notable current application of resistivity imaging may be in the massive oil sands of northern Alberta. The complex fluvial depositions of the region’s McMurray formation present unique challenges to planning and drilling twin well bores used for steam-assisted gravity drainage (SAGD) production of the reservoir. Imaging provides a unique look at key aspects of the difficult formation to help plan and assess well location and orient the laterals.

Oil sands imaging
Canada’s oil sands may have as much as 175 Bbbl of economically viable oil, according to the Canadian Association of Petroleum Producers (CAPP).

Present production is nearly 1 MMb/d of oil (about 39% of Canadian production), and by 2020 is projected to be more than four times that. With current technology, these sands are second only to Saudi Arabia in global oil reserves, says CAPP, and that potential will grow as the technology improves.

One of the technologies helping improve tar sand production is openhole resistivity image logging. Introduced in a 5 5/8-in. wireline configuration several years ago, the resistivity micro-imager technology has been used successfully in several hundred mostly vertical wells. Recent development of a 4.10-in. memory tool has provided a practical option in smaller well bores (about 80 wells thus far) and set the stage for imaging in the horizontal sections.
Resistivity imaging is being used to achieve three key objectives: mapping the sands to determine where to drill the wells, identifying permeable and impermeable shales to optimize steam injection and production in the laterals, and supplementing core data that is often obscured by heavy oil saturation.

Mapping the sands
The fluvial depositional systems of the Cretaceous McMurray formation are similar to modern wetlands and tidal streams that can be observed, for instance, in the Ashe Island area of South Carolina and the Grand Bay National Estuarine Research Reserve in Mississippi.

The McMurray formation accumulated in terrestrial to marginal marine environments with reservoir sediments deposited by fluvial and estuarine processes in incised valleys and estuaries. These were subsequently transgressed by marginal marine environments during the early Cretaceous rise in sea levels. In general, the McMurray formation is subdivided into a lower fluvial unit, a middle estuarine unit and an upper marginal marine (shoreface) unit. The majority of the ore-grade bitumen occurs in the lower and middle units.

Some geologists feel the stratigraphy is exceedingly complex, while many others believe it could certainly stand to be simplified. Clarification of boundaries and disconformities are ongoing, and much of the geology remains to be sorted out.

Understanding this meandering system of ancient point bars and channels can be very important to the optimal location and orientation of SAGD wells. Imaging provides unique insights through visual clues that are otherwise unavailable, even with core.

For example, in this type of environment, fluvial to estuarine deposition is strongly influenced by river discharge rates balanced against marine processes. These factors create a complicated stratigraphy. They also determine the amount of freshwater and saltwater present, which affects the type of life found in these environments. Analysis of trace fossils visible in image logs can be very helpful in determining depositional environments and reconstructing geological models.

Imaging, combined with typical oilfield data such as conventional logs, core and seismic, provides additional and often critical information to aid in the exploitation of subsurface reservoirs. Image logs can be used to determine the orientation of the old river and identify bed boundaries, cross-bedding and lateral accretion surfaces. Dip characterization is much more detailed than achievable with dip meters because it is performed by interpreters instead of an automated software process.

Mud shale variations
The McMurray depositional environment resulted in two common types of mud interclasts – inclined heterolithic stratification (IHS) and mud breccia. The big problem is that IHS has no vertical permeability, where mud clast breccias do. If an impermeable clastic shale is located between the two SAGD well bores, it forms an impermeable barrier to the steam, causing the system to fail.

While normal openhole log signatures are not able to distinguish between the two shales,
Figure 2. Figure 2 shows the neutron density induction log (left) and HMI borehole image log (right) of the brecchiated shale. The red oval in both figures shows the thin shale beds which in Figure 1 are continuous shales and in Figure 2 are brecchiated, or broken up. The neutron density induction logs show little or no difference between continuous or brecchiated shale beds, whereas the image logs do.
image logs show a clear difference (Figure 1). The image log is typically run in the vertical well bore to assess the presence of these mud stones. However, some horizontal applications have been performed to measure the extent and variations of IHS and mud breccia as well as the oil sands. The assessment is critical because the old river channels often start and stop within the spacing of the vertical wells, which can be as close as 300 ft (91 m).

These initial vertical logs have been acquired with a standard 55¼8-in. wireline imaging tool. The results of these efforts and the data acquired have been so useful that operational flexibility is being expanded with the introduction of a slimmer, memory-based 4.10-in. imaging tool that can be run on the drill pipe or coiled tubing and, soon, through the drill pipe.

Core support
Cores from the thick, bituminous McMurray sands are often difficult to analyze because oil saturation can obscure details. Cross-bedding can be difficult to see, but resistivity imaging easily identifies cross-bedding and so provides a supplement to core data.

While not a replacement for coring in the McMurray, strongly correlated image logs can reduce coring requirements and, because image logs are oriented, provide needed core orientation data. Imaging also produces a continuous in-situ measurement, whereas cores do not. Lost, damaged or otherwise unavailable cores have a backup with image logs.

Fracture identification
The fracture characterization capabilities of image logging may provide some answers for a problem that has recently emerged during steam injection.

While the McMurray formation is not fractured, the overlaying caprock may be. These fractures may provide pathways that allow the injected steam to vent to the surface. Borehole visualization provides the ability to ascertain the integrity of the caprock and improve the success of well locations.

Conclusion
Resistivity imaging is providing valuable borehole information for formation evaluation in oil sands, unconventional tight gas, coal exploration and a host of other applications.

The technology is part of a broader borehole visualization capability that is producing insights into many critical aspects of formation evaluation, including structural determination, stratigraphic delineation, fracture identification, geosteering, and determining borehole shape and stability.

Engineers can select from resistivity, acoustical and density imaging options, with additional capabilities for oil- and water-based applications, and wireline and memory configurations, when they consider their alternatives for enhancing formation evaluation.

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