Sigra Overcoring

Agapito Associates, Inc. (AAI) provides services for measuring in-situ horizontal stresses using an innovative down-hole measurement technique, developed by Sigra Pty, Ltd. of Australia, that represents the most advanced biaxial overcore stress measurement system in the world.

Measurements are conducted from the surface, in boreholes up to 5,000 feet (1,500 meters) deep, in conjunction with normal HQ wireline coring operations using Boart Longyear HQ wireline drill string equipment. The technique is applicable for any rock type, provided that reasonably intact core can be retrieved. A specially designed downhole in-situ stress tool (IST) measures stress-relief borehole deformations during overcoring and stores the data in an internal battery-powered data logger. The borehole orientation is also determined by onboard magnetometers and inclinometers. Rapid on-site analysis of the data, supplemented by laboratory testing of the core, provides the magnitudes and directions of the principal stresses in the plane perpendicular to the borehole.

The IST technique provides more information than the competing hydraulic fracturing method and can be conducted at significantly lower cost.  Measurements can be made rapidly, with delays typically on the order of 30 to 90 minutes per test, depending on the depth of the hole.  The system components are rugged and reliable and sufficiently compact and light (400 pounds, 180 kilograms) to permit deployment in remote sites. In addition to the IST, the system utilizes three other custom wireline tools: a counterbore tool; a pilot hole tool; and the IST setting tool.  Once coring has reached the desired measurement depth, the counterbore tool is lowered to remove any remaining core stump and drill a conical centering socket.  The counterbore is withdrawn, and the pilot hole tool is lowered.  Using water pressure for thrust and rod rotation, the tool drills a 1-inch (25-millimeter) diameter hole, 20 inches (500 millimeter) deep, centered in the bottom of the HQ hole.  The IST is inserted into the setting head and lowered into the hole where it is forced into the pilot hole and wedged in place by a setting weight.  The rods are raised about 20 feet (6 meters) so that the magnetometers in the IST can record the tool orientation free from magnetic interference.  The core barrel is then lowered and normal coring commences. During the overcoring operations, a continuous record of pilot hole diameter, measured in six radial directions, is obtained and stored electronically.  Once coring has been completed past the pilot hole, the core containing the IST is retrieved, and the data are downloaded and reviewed to evaluate the quality of the overcore test.  Preliminary results are available almost immediately; however, the core is wrapped and sent to a laboratory to determine the elastic modulus and Poisson’s Ratio, which are necessary for calculating the stress magnitudes.  Although stresses can be calculated using diameter measurements in only three directions, the six diameters measured by the IST provide redundancy and permit evaluating measurement errors and selecting the best possible solution.

Click the following link for a video of how the process works: In-situ Stress Testing by Overcoring

IST Tool
IST Tool
IST in-Situ Stress Measurement Tool
Sigra IST In-Situ Stress Measurement Tool

Have questions? Contact our technical staff for answers.

Project Summaries & Publications: Stress Measurements

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