Stresses in the Earth
Hydroshearing can be done without hydrofracturing, but not the other way around, because hydrofracturing is always accompanied by a component of hydroshearing. We can understand this only by understanding something about stresses and pore pressures. In the earth, the natural stress state is highly compressive, and the three principal stresses are different from one another, so natural shear stresses exist in the rock mass. We usually call these stresses in the ground
Natural Fractures in Deep Crystalline Rock Masses
It is possible to increase the permeability of an igneous rock mass by injecting a fluid under high pressure into the deep rock mass (10 in the next figure), called hydroshearing or hydrofracturing.
Suppose we slowly increase the pore pressure in the rock mass around one of the deep wells by gradually increasing the injection rate. The principal stresses shown in the first figure do not change substantially, and at some point, because the stresses are different in different directions, favorably oriented joints will slip. This is because the increase in pressure counteracts the compressive stress holding the joint tightly together. When slip happens, the rock surfaces across the joints are displaced, usually by a millimeter or less. Because the joint surface is rough, this slip also opens some additional space which stays open after injection is stopped because the slip is not reversible: it is like pushing a brick along a table; when you stop pushing, the brick stays where it is. The process is shown in the figure below, and the opening of additional space, labeled remnant conductivity, is called “shear dilation”, which is the dominant mechanism in hydroshearing. By maintaining the injection pressure at a suitable level, or perhaps by increasing it slowly, a zone of hydroshearing will slowly grow around the injection point, increasing the rock mass permeability around the well. However, hydroshearing only opens some of the joints, and the increase in fluid conductivity may be limited in terms of the size of the region affected around the wellbore, and the amount of dilation the joints may experience.
The injection pressure that is maintained during hydroshearing by continuous pumping is close to the minimum stress in the ground (usually σhmin), perhaps 95 to 99% of its value – so pinj » σhmin. During hydroshearing, the flow rate and pressure are carefully monitored to sustain the shearing process and allow it to propagate slowly outward. Each shear displacement event shown above is not a gradual process; it takes place as a “stick-slip” event accompanied by the emission of small bursts of seismic energy, called microseisms, or microseismic activity. These are like earthquakes, but because they generally are never felt at the surface, they are called “microseismic events”. The propagation of the hydrosheared zone can be tracked as it moves outward by recording and mapping these microseismic events, so it is possible to learn a great deal about how the deep rock mass reacts. This information, along with pressure and flow rates over time, aids in the design of future activities and decisions as to whether hydroshearing or hydrofracturing is the better option in particular cases.
We differentiate clearly between natural and “induced” fractures, although in competent but jointed rock the latter are simply joints that have been opened by the high injection pressure. This process is widely used in the petroleum industry to increase the productivity of oil and gas wells. To enhance and maintain the beneficial effects, it is common to also place a granular agent such as quartz sand or artificial ceramic beads to hold the fracture open, a process called “propping”, and the granular material is called a “proppant” because it “props” open the fractures after the injection ceases. This is also similar to what Civil Engineers may do in a fractured rock mass under a hydroelectric dam: they inject a fine-grained cement grout or a polymer at hydraulic fracture pressures, greater than σhmin, so the grout can be forced far into the rock mass and help seal the joints.
Of course, for geothermal development, it is necessary to improve the permeability, so the propping material that is added to the hydrofracturing fluid during high pressure injection is coarse-grained. Then, the propped fractures retain a high fluid conductivity, and the overall permeability of the rock mass is enhanced. It is possible, with proper fluid design and aggressive injection, to open joints and prop them to distances of about 100-150 m from the injection point. Much more than this requires such large volumes of fluid and proppant that it may not be economical.
During hydrofracturing, high pressures are not confined to the region where the joints are propped open. Pressures propagate far in advance of the propped zone, so in advance of the zone being propped, hydroshearing is taking place. This is well known because large hydrofracturing processes in the oil and gas industry have been carefully monitored for microseismicity, and it is clear that the microseismicity is far in advance, perhaps several hundred meters, of the zone that is wedged and propped. Also, because the rock mass is composed of rock blocks that are quite rigid (granite and gneiss), when a fracture is wedged open, it generates movements in the surrounding rock far from the proppant. This effect is shown in the figure below, a simple model of a blocky rock mass.
Different strata will respond differently because of differences in the extent and openness of the natural fractures, the difference and directions in the natural stresses, differences in the mechanical properties of the rock, and differences in the rate and properties of the injected fluid. These must be understood to achieve good results in geothermal stimulation of low permeability igneous rock masses, and in linking adjacent wellbores. Also, how can one increase the extent of the hydrosheared zone? That is a story for another day. But remember, success in geothermal development depends on economic and effective drilling and accessing the energy, whether it is hot fluids in permeable rock or the heat in hot dry rocks. With ingenuity, we will access the great amount of heat at depth in an economic manner, sustainable, and highly useful in our cold climate.