Tectonically Stressed Shales

Stressed shales occur in areas where diastrophic movement has occurred. (This is the process by which the earth's crust is reshaped, producing continents, oceans, mountains, etc.) The shales may incline considerably from the horizontal, having steeply dipping bedding planes. Forces may be acting upon the formation which, when relieved, cause the shale to fall into the hole (see Figure 2).

Figure 2 Tectonically Stressed Shales

Figure 2 Tectonically Stressed Shales

The problem may be further aggravated if the bedding planes become wet with water or oil. The Atoka and Springer shales of the American mid-continent are classic examples of shales of this type.

It is agreed that formation stresses induced by diastrophic movement make these shales vulnerable to sloughing. It is also recognized that the natural material cementing these shales is relatively weak. It may be amorphous silica, an aluminum or calcium silicate, or an organic material that is sensitive to oil. There is evidence that chemical inhibition is helpful in minimizing the problem, but it is not the entire answer. There is also evidence that slightly higher mud densities can be helpful, but here again it does not seem to be the entire answer.

To more effectively control these shales, a way has to be found to seal the formation against fluid invasion. This is typically accomplished by carefully controlling the high pressure, high temperature filtration properties of muds. The improvement can be significant, but still does not completely solve the problem.

Blended organic compounds, containing an emulsifier and a sulfonated blown asphalt or modified gilsonites are materials used in plugging the micro-fractures in shale. This minimizes fluid contact along the fractures, and when combined with other remedies will generally reduce the severity of the problem.

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  • Angelica
    What is tectonically stressed shales?
    9 years ago

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