1. Introduction

The engineering applications of Energy Geomechanics take place in the Earth's shallow lithosphere (depth $<10$ km). Surface weathering processes and the movement of tectonic plates have changed the shallow lithosphere over millions of years. Hence, the current state of the subsurface is the result of the evolution of the Earth's lithosphere over geological time. Geomechanics is involved in all stages of hydrocarbon recovery including:

• Exploration (e.g., structural geology and geophysical methods for oil and gas prospection)
• Drilling and Completion (e.g., wellbore stability and hydraulic fracturing)
• Production (e.g., sand production, reservoir compaction, surface subsidence, EOR)
• Waste disposal (e.g., fault stability, surface heave, fracturing)

Recent engineering actions, such as reservoir depletion for hydrocarbon or water extraction, can also alter the state of stress.

The following sections review quickly the disciplines involved in Energy Geomechanics and the most important applications. Figure 1.1 shows a summary of applications of geomechanics in the petroleum industry. Similar applications are also relevant to other energy fields, such as deep geothermal energy.

Figure 1.1: Summary of applications of geomechanics in the energy industry.

1.1 Structural Geology

Structural Geology is the branch of geology that studies the three-dimensional deformation of geological formations as they deposit, metamorphose, fold, and fracture. The Earth's crust has changed considerably over geological time going through periods of burial, uplifting, contraction and extension (see animated Earth evolution here https://dinosaurpictures.org/ancient-earth#0).

Detailed description and models of the subsurface require in-depth structural geology characterization. Past knowledge of a given formation is essential to predict how it will evolve in the future under imposed engineering changes.

The Earth is composed of minerals (mostly plagioclase, feldspar, quartz, pyroxene, mica and clay minerals) that solidify on the surface and form a brittle crust underlain by magma. Magma currents underneath the lithosphere are not balanced and force the lithosphere to collide into each other and crack.

Faulting and folding are the result of these tectonic displacements that originate from the movement of tectonic plates. For example, convergent plate boundaries cause lateral compression and structures that result in tall mountain ranges (e.g., Himalayas). Divergent plate boundaries cause lateral extension (e.g., Mid-Atlantic ridge that forms Island). Tectonic plates moving parallel with respect to each other in opposite direction form transform boundaries and create a shear damage region around the plane of discontinuity (e.g., San Andreas Fault in North America).

Tectonic plate movement changes the stresses in the lithosphere (Figure 1.2). For example, a region near convergent tectonic plates is expected to have higher lateral compressive stresses than a region near a divergent plate boundary. The determination of local stresses are critically important to Energy Geomechanics because they affect wellbore stability, fault reactivation, and propagation of hydraulic fractures.

Figure 1.2: Plate tectonics influence directly the distribution of stresses in the Earth's lithosphere (Image credit: Zoback [2010]).

Gravity results in sedimentary layers that are mostly horizontal and parallel to each other at the time of deposition. Tectonic movements result in changes of lateral stresses (tectonic stresses) that deform sedimentary layers (Figure 1.3 and 1.4). Ductile rock deformation leads to folding, e.g., anticline, with a a gradual change in deformation without an apparent discontinuity. Brittle deformation leads to fractures and faulting, i.e., a clear discontinuity in sedimentary layers. Hydrocarbon accumulations take advantage of folds and low permeability faults. The deformational behavior of rocks is crucial in defining the variation of rock permeability with changes of stresses.

Figure 1.3: Tectonic stresses result in folding and faulting of sedimentary layers. The image shows an example from a seismic survey cross-section.

Figure 1.4: Give enough time, stress, and temperature and rocks can deform like bread dough. This example is from UK Jurassic Coast taken by the author.

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.

1.3 Problems

1. Completion through horizontal wellbores and multistage hydraulic fracturing has dramatically impacted the production of oil and gas in the US. Answer the following questions:
   1. What was the percentage of tight/shale gas production with respect to total gas production in the US for 2010 and 2020 according to EIA “US dry natural gas production by type” (https://www.eia.gov/energyexplained/natural-gas/where-our-natural-gas-comes-from.php)?
   2. Out of shale gas for 2022, what percentage comes from TX/NM and what percentage comes from Marcellus Shale according to EIA “Monthly dry shale gas production” (https://www.eia.gov/energyexplained/natural-gas/where-our-natural-gas-comes-from.php)?
   3. What was the percentage of tight/shale oil production with respect to total oil production in the US for 2004 and 2019 according to EIA “Monthly US crude oil production (2004-2018)” (https://www.eia.gov/todayinenergy/detail.php?id=38372#)?
   4. Out of tight oil for 2023, what percentage comes from TX/NM according to EIA “US tight oil production” (https://www.eia.gov/energyexplained/oil-and-petroleum-products/where-our-oil-comes-from-in-depth.php)?
   5. How many times did tight oil production grow from 2023 to 2007 according to EIA “US tight oil production” (https://www.eia.gov/energyexplained/oil-and-petroleum-products/where-our-oil-comes-from-in-depth.php)?
2. What is the concept of “decarbonizing the fossil fuel energy industry”. What are the different types of CO$_2$ geological storage according to the target storage formation?
3. What is the heat flow rate of the Earth's molten core (provide answer in TeraWatts)? What is an enhanced geothermal system?

Note: you may use this (https://apps.automeris.io/wpd/) web app to digitize plots.

1.4 Further Reading and References

1. Decarbonizing the economy: https://equinor.ft.com/articles/there-will-be-no-net-zero-without-carbon-capture-and-hydrogen
2. Carbon capture and storage: https://www.netl.doe.gov/coal/carbon-storage/faqs/carbon-storage-faqs
3. Future of geothermal energy: https://www.vox.com/energy-and-environment/2020/10/21/21515461/renewable-energy-geothermal-egs-ags-supercritical
4. Geologic hydrogen: https://www.usgs.gov/news/featured-story/potential-geologic-hydrogen-next-generation-energy