1.2 Geomechanics in the Energy Industry

Geomechanics has had an increasing importance in Energy Resources Engineering, particularly in the Oil and Gas Industry. Other applications within the Energy umbrella include: deep geothermal energy, deep nuclear waste disposal, and CO$_2$ geological storage. The applications of energy geomechanics can be broadly grouped into:

• Drilling and wellbore stability,
• Wellbore completion, and
• Reservoir geomechanics.

1.2.1 Drilling and Wellbore Stability

Subsurface access requires wellbores. However, a cavity in the subsurface intensifies stresses around it and requires support to be open. Imagine making a hole in beach sand. You might be able to make a hole near the water level, but not in dry sand, nor in sand below the water level. Early wellbores were exclusively vertical and a few hundred meters deep (100 m is equivalent to 325 ft) (Figure 1.5). Today's wellbores are kilometers-long and may deviate horizontally several kilometers more (1 km is about 0.62 miles). Optimal wellbore design and drilling requires knowing various properties of the rocks including the strength of rocks being drilled.

Figure 1.5: From the beginning of drilling to today's ultra-deepwater drilling.

1.2.2 Wellbore Completion

A single wellbore offers little contact area with the reservoir. Hydraulic fractures seek to increase such contact area, particularly in low-permeability reservoirs. Hence, even a small flow velocity can result in a large flow rate [bbl/min] if the contact area is large. Hydraulic fracture propagation depends on local stresses, bounding layers, and reservoir rock heterogeneity. Hydraulic fracturing has been used in the petroleum industry since $\sim$1950. Improved completion methods take advantage of horizontal wellbores, proppants, acid solutions, and a variety of chemicals to enhance reservoir drainage.

Figure 1.6: Pictorial representation of multistage hydraulic fracturing in a horizontal well.

1.2.3 Reservoir Geomechanics

Hydrocarbon recovery changes reservoir pore pressure. Reduction of pore pressure (needed for pumping and production) leads to stress transfer from the pore fluid to the rock solid skeleton. High increases of stress on the rock skeleton lead to reservoir compaction, reduction of reservoir permeability, and disturb the mechanical equilibrium of overlaying strata and faults. Increases of the magnitude of stresses and stress anisotropy may also cause rock failure near the wellbore and produce fine solid particles that may plug and damage pumps and surface facilities.

Figure 1.7: Reservoir depletion and pore pressure change.

1.2.4 Carbon and Hydrogen Geological Storage

Burning fossil fuels produces carbon dioxide (CO$_2$) among other subproducts at power plants. CO$_2$ can be separated from flue gas and captured. After capturing, CO$_2$ can be injected into deep geological formations in order to mitigate CO$_2$ emissions into the atmosphere. The entire process is known as Carbon Capture and Storage (CCS).

Carbon dioxide injection into reservoirs has been done for decades in the oil and gas industry for Enhanced Oil Recovery (EOR). However, a prerequisite for carbon geological storage is permanent storage. The assurance of permanent storage depends on petrophysical and geomechanical processes. Geomechanics is essential for CO$_2$ geological storage to ensure: integrity of injection wells and stability of adjacent faults. CO$_2$ injection at rates and volumes higher than threshold limits can trigger injector fracturing or fault reactivation.

Figure 1.8: Selected types of carbon geological storage.

An alternative solution for high energy and low weight fuels or electric batteries is hydrogen. However, moving from a fossil-fuel dominated market for vehicles and on-demand energy supply to hydrogen would require large hydrogen storage places. One option is hydrogen geological storage in salt caverns and clastic reservoirs (such as saline aquifers and depleted hydrocarbon reservoirs). Hydrogen geological storage implies similar technical challenges as carbon geological storage with the added problem of cyclic changes due to seasonal demand.

1.2.5 Geothermal Energy

Deep geothermal systems are potential sources of energy with zero carbon emission during production. Some of the limitations of the current geothermal industry include: drilling costs, drilling challenges in high temperature environments, low surface area of geothermal reservoirs, and prediction of stress changes with heat mining. New geomechanics breakthroughs are helping the geothermal energy industry by

1. enabling faster and reliable drilling in high-temperature formations (T $>200^{\circ}$C),
2. achieving high surface area in geothermal reservoirs by drilling horizontal wells and creating fracture networks, and
3. building reliable models of the subsurface to capture thermo-poro-elastic processes.

Figure 1.9: Selected types of deep geothermal systems for heat mining.