Fluid-assisted fracturing in geological materials
Gwaba, Devon Chikonga
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We have developed and advanced novel experimental techniques to study, in the laboratory and in-situ, fluid-assisted fracturing in geological materials. Fractures and fluids influence numerous mechanical processes in the earth's crust, but many aspects of these processes remain poorly understood; in large part, because of a scarcity of controlled field experiments at appropriate scales. Faulting processes are a good example. In the laboratory, faults are typically simulated at the centimeter to decimeter (cm-to-dm) scale using load cells. Laboratory results are then routinely up-scaled by several orders of magnitude to understand faulting and earthquakes in a wide range of conditions. We show, however, that a scale of at least a few meters is required to adequately simulate earthquake nucleation processes. For this reason, we developed an experimental approach that aims to induce new faults or reactivate existing faults in-situ at scales of 10 to 100 meters. The approach uses thermal techniques and fluid injection to modify in situ stresses and the fault strength to the point where the rock fails. Mines where the modified in-situ stresses are sufficient to drive faulting, present an opportunity to conduct such experiments. Another example is hydraulic fracturing in unconsolidated sediments and soils, which is important for many applications ranging from compensation grouting to sand control in petroleum reservoirs, to sand diking in shallow formations. In this work, we advance experimental methods of hydraulic fracturing of cohesionless particulate media. The hydraulic fracturing behavior in natural sands, obtained from a production well, is compared and contrasted with that in synthetic sands. Conventional wisdom suggests that it is not possible to monitor hydraulic fracturing in cohesionless materials using passive acoustic emission and active ultrasonic measurements. Yet, we show that not only is this possible but it is an effective monitoring technique allowing for important insights into the fracturing process and fracture geometry mapping. Also, this technique gives a reference for interpreting microseismic data recorded in the field.