Hydraulic Fracturing of Soft and Hard Rocks. Part 2: Acoustic Emissions, Source Mechanisms and Energy

The hydraulic fracturing (HF) has been used to create new flow paths or enhance existing flow paths, thus, enabling the permeability of rocks by increasing the connected porosity of rocks. During HF, the release of energy in the form of seismic waves causes tremors and minor earthquakes. Experimental results on the performance of rocks during HF tests are shown in the form of two parts, by varying between a high and low injection rate of 20 and 2 ml/min, respectively, into the matrix of two different rock types (soft gypsum and hard granite). Part 2 attempted to simulate real-life HF events in the field. To acquire the micro-seismic events, acoustic emission sensors were attached to each lateral surface of the specimen and later processed into four source mechanisms (tensile, compressive, shear and mixed-mode). The micro-seismic activity was larger for granite subjected to higher injection rate. The source mechanisms were found to be mainly material-dependent because three distinct time intervals (stages) were identified for each material type. The location of AE events was interpreted to be material- and injection-rate-dependent. The AE events recorded in gypsum tests were spread across their surface with no apparent correlation between new cracks and fluid fronts, while for granite, they were close to the new propagated cracks and/or within the fluid fronts (compressive mechanisms were less predominant in the middle of the specimen), while there were distinct behaviors for each material subjected to different injection rates.