Where holes have already been drilled, Sigra can use its hydrofracturing system to determine the rock stress regime. This provides values of major and minor principal stresses even in cases where tensile stresses exist at the wall of the test hole. The system can be regarded as providing a lesser level of accuracy than the overcoring via the Sigra IST system and at a greater cost. However in fractured ground it is the only way to gain an indication of stress across joints.

Practically, hydrofracture is a biaxial technique. The solution of the stress field perpendicular to a borehole also requires material that has not failed and behaves in a linear elastic manner. This measurement also requires that the borehole is uncased and assumes that a failure will occur parallel to the hole axis in the direction of minimum stress. In fact to achieve a pressure seal in an open hole, packers must be at a higher pressure than the fluid being injected and therefore they will initiate failure unless some pre-existing rock fracture exists in the test zone.

If the rock stresses are sufficiently anisotropic it is possible that a tensile zone and associated fracture exist parallel to the borehole wall prior to the fluid pressure being raised. This axial fracture will compromise borehole sealing and makes analysis more complex. In the event that the minimum stress is perpendicular to the borehole most packer systems will not permit loading of the rock in the axis of the hole because the packers are mechanically linked and the initial fracture will be parallel to the hole. After initiation the fracture will roll over into a plane which is perpendicular to the minimum stress. This also complicates analysis as the closure pressure analysed is not the intermediate stress perpendicular to the hole. In the instance where a pre-existing fracture exists transecting the hole, then fluid pressure may act on its faces and jack the fracture open.

Where borehole wall failure occurs prior to fracturing it compromises any seal and causes a deviation from the theoretical equations of stress at a borehole wall. In this case it is still possible to get a closure pressure following hydrofracturing which represents the minimum stress. This is the process generally used in the oilfield to establish a (minimum) stress.

Another problem with hydrofracture stress measurement in an open hole is that the pressure that the packers exert on the borehole wall must exceed the fluid pressure. The force they exert will therefore tend to initiate failure separate to the hydrofracture fluid.

In summary, the use of hydrofracture as a stress measurement technique is fraught with practical and analytical problems. It can still be a useful second choice to measure stress, especially in hard rock where the deformations due to overcoring are very small, or if a minimum stress is all that is sought. In the latter case this can be achieved through a cemented casing.


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