Tight Rock Analysis of Shale Sidewall Cores from XL-Rock Rotary Sidewall Coring Service | SLB

Tight Rock Analysis of Shale Sidewall Cores from XL-Rock Rotary Sidewall Coring Service

Published: 10/16/2012

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Core material needed for analysis

An operator drilling a well in north central Pennsylvania wanted formation samples from a black shale targeted for unconventional gas. In this environment, core samples would be valuable for determining composition, texture, and physical properties of the rock. Conducting rotary sidewall coring would be more efficient than coring the full openhole interval, but uncertainty would be introduced for some analytical techniques, such as tight rock analysis, that would require combining sidewall core samples to have a sufficient volume of core material.

Large-volume sidewall core

XL-Rock large-volume rotary sidewall coring service closes the gap between core plugs from continuous conventional coring and wireline-conveyed rotary sidewall cores. Retrieving up to fifty 1.5-in-OD by 2.5-in-long sidewall core samples from a single descent, XL-Rock service delivers cores that are more than 300% of the volume of previous-generation sidewall cores. The core samples from XL-Rock service deliver a rock volume equivalent to that of conventional core plugs, matching the industry's standard sample size for analysis and enabling key answers in less time and at lower cost than conventional coring.

Operational risk is greatly reduced by using XL-Rock service. Only 37 ft long, the tool is the shortest rotary sidewall coring service in the market today. Real-time control of the physical drilling parameters such as weight on bit ensures that the tool is coring at the optimal settings for each core point. If the core bit were to become stuck in the formation, XL-Rock service incorporates an optional controlled release of the bit.

Graphic: Tight Rock Analysis of Shale Sidewall Cores from XL-Rock Rotary Sidewall Coring Service
Large-volume XL-Rock sidewall cores recover a sufficient volume of rock to extract three triaxial minicores for full analysis of completion quality, which previously required samples taken from conventional core.

Answers in context

Of the 100 core points planned for the full openhole interval, 96 of the samples were brought to surface, achieving 96% core recovery. The large-volume samples improved measurement precision by enabling analytical techniques, such as tight rock analysis, to be conducted on a single sample instead of having to combine multiple small samples.

XL-Rock service's coring operation was followed by FMI fullbore formation microimager logging. The coring leaves a 2.5-in-diameter hole in the borehole wall where the 1.5-in-diameter rock sample was extracted. These holes show up vividly on the high-resolution image acquired by the FMI imager, providing visual confirmation of the exact place on the borehole wall from which the core was taken. The core analysis program and subsequent integration with logs greatly benefit from the visual check of whether the samples taken are representative of the interval and from knowing their precise context with respect to the log measurements.

Based on the wealth of information delivered, the operator has made XL-Rock service with a confirmatory FMI imager log an integral part of the standard exploration logging program.

Graph: Tight Rock Analysis of Shale Sidewall Cores from XL-Rock Rotary Sidewall Coring Service
United States, North America, Onshore

Challenge: Collect sidewall cores that are representative of specific shale intervals in a north central Pennsylvania well.

Solution: Use XL-Rock service to obtain large-volume rotary sidewall cores with a sample volume more than 300% of that of previous-generation sidewall coring service, which is sufficiently large that tight rock analysis can be conducted on a single sample instead of combining multiple small samples.

Results: Determined composition, texture, and rock physical properties from 96% recovery of 100 planned core points with a visual confirmation of sample locations from FMI fullbore formation microimager logs.

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