Core Ovality – A Stress Difference Indicator

Sigra have been measuring stress in rock for a long time. First by its two dimensional overcore system (IST2D) which commenced use in 1996. Sigra has also conducted hydrofracture and hydrojacking tests to measure stress. More recently it has developed a three dimensional stress measurement overcore system (IST3D) suitable for use at depth in a hole of any orientation.

All of these systems provide a stress measurement at a point within a borehole. The only method which has been available to deduce stress along a borehole is by examining acoustic televiewer images for borehole breakout. If, however, there is no breakout then there is no information to be gathered.

Stress can vary significantly through a rock mass due to the effects of lithification, diagenesis, cooling, erosion, tectonic movement and faulting. These changes may be quite marked between rock types or around faults.

It is quite unrealistic and uneconomic to exhaustively conduct multiple detailed stress measurements at close spacings. What is needed is some simple, near continuous system for detecting where the stress regime changes. This can then be used as an indicator of where to conduct detailed stress measurements.

To do this and to provide clients with some basis for determining where more precise stress measurements should be conducted Sigra has developed the core ovality measurement technique. This involves taking core and rotating it to precisely determine the major and minor diameters. This is repeated several times to arrive at an average difference value. The average diameter difference can then be combined with estimates of Young’s modulus and Poisson’s ratio to arrive at a stress difference between the major and minor stresses perpendicular to the core. The system takes about two minutes per measurement.

Figure 1 shows the core ovality test tool with an IST2D overcore sample on the bed. Figure 2 shows the sinusoidal core deformation curves with a mean value. To date quite good agreement has been achieved between the stress difference derived from overcores and those derived from core ovality measurement. Results have come from fine grained sandstones and from a meta-claystone.


Figure 1. The core ovality test instrument with an overcore sample on the rollers.


Figure 2. Core ovality test results from a meta claystone.

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