An innovative formation-imaging system exhibits unprecedented resolution of rock fabric and fractures. This feat has been accomplished by the novel application of an optical televiewer (OPTV) to obtain, in an air-filled borehole, high-resolution color images of formations associated with the production of hydrocarbons. The technology is particularly well-suited to shale reservoir evaluation.

SWSI developed hardware and other impedance-lowering technologies for the OPTV to be fully used on standard 12,500-ft (3,813-m) multiconductor wireline units and similar technologies to enable data acquisition in horizontal wells using tractor mechanisms to convey the OPTV tool to 12,500 ft total measured depth in horizontal holes. There is no requirement to load the hole with fluid. Therefore, the operator avoids expense and operational delay, and risk of causing formation damage by fluid. Loading the borehole with fluid was previously required to obtain wireline borehole image data. In the northeastern US, where air-hole drilling is common and preferred, operators would reluctantly introduce fluids to sensitive formations, with added cost and risk of formation damage, to obtain micro-resistivity or acoustic images. This system is a game-changer for one of the largest natural gas resources in the world, the Marcellus shale. The OPTV now gives equivalent or better-quality images without the added cost or risk.

OPTV products are jpg, bmp, or pdf files that can be viewed, printed, or imported into graphics software programs. The entire logged interval or smaller specific intervals can be chosen, giving different geologic perspectives. Differentiating from acoustic or micro-resistivity imaging, all inferences made are geologic; i.e., the operator “sees” exactly what is there. (Image courtesy of Superior Well Services)

The system enables identification of geologic features (fractures, bedding, mineralization, textures, vugs, breakouts, etc.) and determines their orientation, strike, and dip. Interpretation software generates arrow (tadpole) plots, stick plots, rose diagrams, and stereograms of the identified features for both structural and stratigraphic analysis. There are some completion benefits such as stage-design development and determining minimum/maximum stresses (breakouts and induced vs. natural fractures). All the benefits of high-resolution imaging are in the package. The key difference is that the OPTV does not require any inferences from an electrical or acoustic measurement before making geologic inferences; the end user is able to interpret a direct rock image. Hydraulic fracturing of shale formations can be enhanced through more precise selection of perforations that correlate directly to lithology and micro-heterogeneity that would be missed or not obtainable via conventional logging or imaging techniques. This is especially applicable in a horizontal borehole where multistage fracturing is employed. Using the OPTV image information, geohazards, and out-of-zone sections can be avoided in subsequent fracture staging.

A large US independent operating company was using horizontal air-hole drilling technology and drilling several laterals off a single pad in multipurpose efforts to maximize production and avoid the added expense of drilling with fluid. Lateral azimuth directions were selected based on information gathered from existing vertical producing wells and geologic analysis of the fracture systems. Not wanting to load these laterals with fluid, the operator was seeking the highest-quality imaging log data to:

• Confirm geologic modeling;

• Fully interpret the fracture systems encountered;

• Distinguish open, naturally producing fracture systems from non-productive systems (comparing the images to basic open air-hole hole logging suites and spinner logs);

• Aid in completion and stimulation design; and

• Determine location and azimuth of future lateral placements.

SWSI developed the application of tractor-conveyed OPTV, the first use of this conveyance method for an air-drilled, openhole horizontal shale completion. The fact the OPTV tool is only 5 ft (1.5 m) long and weighs 11 lb — far less than other imaging technologies — figured strongly in this decision.

Before logging horizontally, the operator wished to compare the OPTV with a traditional micro-resistivity imaging device using a vertical pilot hole drilled through the target formations. Both tools were run over the same intervals and the quality of images compared. The operator was satisfied that the OPTV image quality run in air was superior to those obtained by the micro-resistivity device run in fluid, especially in identifying the fractures. The horizontal project proceeded as designed, with the horizontal section drilled and logged on air.

Job participants estimated that using the OPTV system to obtain required images from an air-filled borehole yielded savings of 48 hours of rig time, US $10,000 in fluid and removal, and $25,000 in wireline logging, conveyance, and interpretation. The method also allowed the operator to avoid formation damage and associated risk completely.

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