A Novel Experiment Method of Evaluating the Brittleness of Rock

The selection of favorable well /layer and the hydraulic fracturing strategy are all based on the rock brittleness in shale gas. Two main methods of evaluation the brittleness of rock have been widely used in field. One is based on the Young’s modulus and Poisson’s ratio, and the other is based on the brittle mineral component content. But these 2 methods are not good enough in some shale, because the parameter of brittleness is related to the rock failure behavior while the 2 methods can’t reveal the failure property. Using the feature points on the complete stress-strain curve is a good option to evaluate the brittleness. But complete stress-strain curve is often hard to be gotten if the stiffness of load frame is not high enough, especially for the shale.

A novel experimental method of rock brittleness evaluation is developed from the nature of rock failure, which is based on the AE (acoustic emission) technology. Because the AE events can reflect the rock failure process, it can characterize the rock brittleness well. More brittle the rock is, more concentrated the AE events occur. That is to say, the concentrated AE events feature shows the rock is brittle, while dispersed AE events feature means the rock is ductile.

Several experiments have been carried with this method on shale and sandstone. The results show that the shale is much more brittle than the sandstone. Meanwhile, the evaluation results are in good agreement with the results from complete stressstrain curve methods and have effectively guided the hydraulic fracturing in field.

This method is easy, accurate, universal and low demand on the load frame. The results can be used to guide the well/layer selection and fracturing design, and to establish the empirical relationship between the rock brittleness and logging data in new region.