Oil-Based Mud Imaging Tool Meets Challenges Posed by Thin Beds, Low Resistive Formations, and Shales

Evaluation of thinly-bedded formations poses unique challenges for operators. Exactly what constitutes a thin bed is argued by petrophysicists, but a general definition might be any bed whose thickness is less than vertical resolution of the standard open hole log suite (e.g. a quad combo). In most cases, this thickness is between 1 and 2 feet. As the bedding thickness decreases, accurate reservoir characterization becomes increasingly difficult. In the limit of centimeter thick bedding, mere detection of the reservoir is at risk.

A typical solution to the above problem is to augment the standard log suite with high vertical resolution electrical images. Some logging tools can generate electrical images with vertical resolution on the order of a tenth of an inch. When used in concert with the standard log suite, the electrical images facilitate a more complete characterization and evaluation of a thinly-bedded reservoir. Common imaging applications include:

High vertical resolution pay zone volumetrics (both fluids and minerals)
Pay zone detection (in extreme thin bed /"low contrast" pay zones)
Structural and stratigraphic dips
Sedimentary features and textures
Net-to-gross sand counts
Identification of faults and unconformities
Evaluation of sedimentary sequences and flow units
Lithologic unit thickness
Secondary porosity evaluation
Sequence stratigraphy analysis
Borehole stresses analysis

Oil-Based Mud Electrical Imaging Challenges
While electrical imaging technology has been available for 20 years and has seen certain advances in performance (e.g. dynamic range, borehole coverage), there has been one persistent limitation--the drilling mud must be electrically conductive. In virtually all water-based mud systems, this condition is satisfied. The electrical currents which comprise the images are able to easily pass between the imaging tool and the formation.

In oil-based mud systems, however, quite the opposite condition is present. The oil muds and mud cakes are virtually perfect insulators and current passage from conventional imaging tools to the formation is virtually impossible. Clearly, a new type of sensor technology was required to satisfy demand for electrical images in oil muds.

New Imaging Tool for Oil-Based Mud
With the investment of several years of research and development, Halliburton has met this customer challenge with the introduction of the Oil Mud Reservoir Imager (OMRI™) to the industry this year. This tool utilizes highly proprietary new sensor technology that is capable of electrical current generation in the formation--without direct coupling– to generate high resolution electrical images. The tool has no dependency on mud conductivity and the sensor performs in all types of oil mud systems, whether synthetic or natural. The tool retains the highly desirable six-arm configuration of previous imaging tools to offer the same excellent degree of borehole coverage. Each of the six arms is independent and each pad is articulated in two axes. This unique design helps the tool's pads maintain positive contact with the formation wall – an important factor for all wireline imaging technologies. Finally, a highly sophisticated signal acquisition scheme is employed that electrically compensates for those cases where good pad contact cannot be maintained due to washouts or rugosity.

Tool Operation
Operationally, the oil based imager tool is virtually no different than the eXtended Range Imager tool. Once "on bottom", the six calipers arms are opened, the pads (Figure 1) are energized, and the log is recorded in the up direction at between 10 and 30 fpm depending on borehole conditions. Voltages directly in front of each pad, from about three inches into the formation, are recorded in an array of receivers. These voltages are sensitive to the varying resistivities of the formation layers and have vertical resolution on the order of an inch. These voltages are transformed into micro resistivity curves which, in turn, are processed in real time to produce oriented color images. The application of the images and the available computed products are identical to those offered with the eXtended Range Imager tool.

Figure 1. The tool's transmitter and receiver buttons are located on the sensor pad at the end of the articulating arm (Click to enlarge image).

Physically, the tool is 27.54 feet long and 5.5 inches in diameter. It can operate in a hole as small as 6.5-in. or as large as 16 inches. It weighs 760 lbs and is rated for operations at up to 350°F and 20,000 psi. Good images are available in mudcakes up to 0.25 inches thick and in formations with resistivities ranging from 0.5 to10,000 ohm-m (Figure 2).

Figure 2. OMRI Tool (Click to enlarge image).

Summary
Numerous wells in the Gulf of Mexico and in deepwater basins throughout the world are drilled with oil-based or non-conductive muds. The use of these muds poses challenges to borehole resistivity imaging. However, a new oil-based mud reservoir imaging tool enables high quality resistivity imaging for formations with resistivities as low as 0.5 ohms when oil-based muds are used for drilling. Thus, geologists now have a tool that allows them to pinpoint structural details and features such as faults and thin beds harboring hydrocarbons that otherwise would go undetected and unproduced.

The oil-based imager tool was specifically developed, manufactured and tested for resistivity imaging in non-conductive muds. Its use in the field and continued development is proving that the challenges of the past have been met and seemingly exceeded. For operators who use these muds to meet the demands of drilling in extreme environments exhibiting high pressures, high temperatures, shale sloughing, or in thick shale formations under extreme stress, the tool and its technology represent a new era in resistivity borehole imaging.