Hydraulic fracturing of poorly consolidated formations: Considerations on rock properties and failure mechanisms.

Abstract

This dissertation addresses the issue of hydraulic fracturing stimulation of poorly consolidated formations. First, a complete review about the mechanical properties of such formations was performed. Typical ranges of properties such as Young's modulus, Poisson's ratio, and uniaxial compression strength (UCS) were identified. In addition, the characteristic shape of the stress-strain curve was also recognized. Given their friable nature, weakly consolidated sands exhibit very low values of Young's modulus, and UCS. They could be located in the lower end of the sandstones trend in the Deere and Miller rock classification. Subsequently, a study on the reliability of the measurements of the rock mechanical properties for unconsolidated rocks was conducted. The effects of coring, freezing and testing were studied. It was concluded that coring-induced stress relaxation may cause permanent alterations of the rock mechanical behavior. However, the data about freezing-induced alteration of cores were deemed inconclusive and more research on this issue was recommended.

Finally, a discrete element model was built and calibrated in order to reproduce the mechanical and hydraulic responses of a selected unconsolidated sandstone. The hydraulic fracturing process was simulated and the relative importance of different failure mechanisms was evaluated. A remarkable finding by exercising such a model was that in the case of the Antler Sandstone (and possibly in more unconsolidated formations), shear failure seems to be more important than tensile failure during the hydraulic fracturing process. This conclusion is a clear contradiction to what has been traditionally accepted in the oil and gas industry.