Modern Hydraulic Fracturing: Rock Mechanics
Rock mechanics is a critical decision-making tool in the development of petroleum reservoirs. Engineers use the concept in applying the concepts of solid mechanics to address a range of issues related to the drilling of oil and gas wells at different phases. “Rock mechanics applies the principles of continuum and solid mechanics and geology to quantify the response of a rock that is subjected to environmental forces caused by human-induced factors which alter the original ambient conditions” (Aadnoy and Looyeh 65). Therefore, engineers ought to fully understand the basic mechanics of rock removal and the principles of rock mechanics to drill oil and gas wells. The concept of rock mechanics focuses on the application of Newton’s ideas of mechanics in the exploration of underground rocks. Notably, the idea entails understanding how underground rocks behave when disturbed by the impact of excavation, stress changes, fluid flow, changes in temperature, and many other factors (Aadnoy and Looyeh 69). Therefore, having knowledge in rock mechanics is vital in understanding the metrics of oil and gas production. The present study explores the various aspects of rock mechanics that play a critical role in the construction of oil and gas related wells.
Underground Stressors
The concept of rock mechanics also entails understanding underground stresses and their impact on well-drilling process. Typically, underground formations ought to carry the weight of the overlying creations. The underground stress state contains three interrelated major stresses as well as pressures resulting from the pore (Fjer et al. 104). In most cases, many engineers in the petroleum industry tend to assume that vertical stress is a significant problem. However, in significantly sloping surfaces, vertical stress may be less from being considered a principle stress (Fjer et al. 104). However, this form of stress must be taken into consideration because it interacts with horizontal stress to cause major problems in well-drilling.
In theory, the state of underground stress coincides with both vertical and horizontal-based stresses. As early observed, some engineers are tempted to believe that vertical stress is the only major problem when drilling oil and gas production sells. Nonetheless, in some instances, mostly in tectonic environments, horizontal stress may be significant than the vertical stress (Aadnoy and Looyeh 116). While engineers have proposed many indirect methods for the evaluation of the magnitude of the state of underground stress, the inversion technique has proved to be the most effective. Table 1 provides a highlight of the existing methods of determining the state of in-situ stress. As observed, the inversion method (σv) by Aadnoy has the ability to estimate both optimum and minimum horizontal stress magnitudes and orientations (Aadnoy and Looyeh 116). Overall, the technique is critical in enabling engineers to analyze fractures in oil and gas well drilling, thus preventing the collapse of the project.
Table 1
Common techniques of examining underground rock stress and the efficacy.
Examining Method
σv
σH
σh
Individual LOT Technique
√
Empirical LOT technique
√
Extended LOT Technique
√
Inversion of LOT Technique
√
√
√
Breakout Analysis
√
Image Logs
√
The inversion technique is effective in examining horizontal stress because of its comprehensive approach. The technique utilizes data from a multiple leak-off examinations performed in wells of different inclinations and azimuths (Fjer et al. 306). The technique is founded on the principles of linear elastic theory, which emphasizes using back calculation to evaluate horizontal stress. In spite of its strength, the method also suffers from some weaknesses. Notably, the technique is associated with some uncertainties in regards to the leak-off pressure relative to fracture breakdown as proscribed by the elastic model (Fjer et al. 306). Additionally, the method suffers uncertainties because of spatial variation in the horizontal stresses. According to Fjer et al., examinations may entail compartments of substantially different stress regimes when using data from multiple wells (306). Therefore, these weaknesses underline the need for a careful application of the technique when evaluating horizontal stresses during oil and gas well-drilling.
Pore Pressure
The aspect of pore pressure is a crucial parameter in the exploration of rock mechanics, especially porous, fluid-filled rock mechanisms. In principle, porous, saturated and permeable rocks behave consistent with effective stress principle (Fjer et al. 114). For this reason, it is significantly crucial to study borehole stability during the drilling of oil and gas production wells. Naturally, saturated formation tends to experience pore pressure when sediments are being buried (Fjer et al.114). In essence, the process underlines the potential adverse impact on the stability of wells. A normal pore pressure is usually achieved when the pore fluid escapes and migrates to the surface at about the same rate relative to compaction (Fjer et al. 114). Nonetheless, pore pressure within a zone may have a different value from the anticipated one in some instances. Such a scenario takes place when the zone is abnormally pressured, making a reservoir more prolific (Fjer et al. 114). The presence of abnormal pressures in pores presents hazardous effects on the drilling process. Notably, the process has three impacts: tectonic loading that results in undrained stress, pore expansion through thermal or chemical process, and a higher rates of sedimentation and compaction than those of fluid expulsion and migration (Fjer et al. 115). Therefore, it is important for engineers to understand the status of a zone’s pore pressure to determine the best approaches to maintain normal pressure when drilling oil and gas production wells.
Sedimentological Aspects
The concept of rock mechanics also entail the aspects associated with sediments. Rock sedimentation is determined by many factors, including erosion of rock fragments, deposition, transportation, and lithification (Fjer et al. 117). In essence, the multiplicity of these factors makes it necessary for engineers to study the mechanical features of a rock. Theoretically, the production of oil and gas involves drilling of wells deep past several types of rocks into the underground. Therefore, it may be significantly difficult for the process to be smooth without proper knowledge of the underlying rocks.
Grains
Grains are critical components in the examining the concept of rock mechanics. Grains are characterized by their size, shape, and sorting technique. The phi-scale, which is defined by phi=-log2 (grain diameter in mm) is utilized to determine the grain size of rock sediments. As highlighted in table 2, grain size is used to classify sediments into different categories.
Table 2:
Classification of sediments using their respective grain size.
Grain Diameter range (mm)
Phi-scale
Term
>256
<-8
Boulder
64-256
-6- -8
Cobble
4-64
-2- -6
Pebble
2-4
-1- -2
Granule
1/16-1/16
8-4
Silt
<1/256
>8
Clay
Grain size, shape, and sorting are essential aspects in determining the packing of grains in the rock. The packing of grains determines the extent to which a rock is permeable and porous (Fjer et al. 118). In this regard, these characteristics are dependent on the size, shape, and sorting of grains. Comparatively, round grains cause less friction than their angular counterparts when evaluated at the same porosity while a poorly sorted sand is associated with a higher friction angle compared to the properly assorted one (Fjer et al. 118). From a broader perspective, grains properties determine the resulting friction when drilling a borehole.
Minerals
Additionally, the concept of rock mechanics involves an understanding of the various minerals that make up rocks. Having a knowledge of these minerals is critical to guide the metrics of drilling boreholes during the extraction of oil and gas. The silicate group is the most dominant mineral group in the composition of rocks, making up of about 90% of the earth’s crest (Fjer et al. 118). The tetrahedral units of these silicates combine with oxygen to form complex compounds, which play a critical role in the formation of rocks. For example, potassium feldspars react with oxygen to form potassium silicate oxide (Fjer et al. 118). Nonetheless, clay is the most essential mineral in the formation of shaly rocks. Such minerals contain silica and octahedral sheets, which combine to form a rock (Fjer et al. 118). Rocks also comprise other minerals. The main minerals in the composition of rocks include oxides, phosphates, carbonates, and sulphides (Fjer et al. 118). Overall, the knowledge of the minerals that make up rocks is critical in guiding a smooth drilling of oil and gas wells.
Mechanical Properties of Sedimentary Rocks
The study of the mechanical properties of rocks is also a central aspect in rock mechanics. Sedimentary rocks are highly crucial for the exploration and extraction of hydrocarbon components, especially oil and gas. The permeability and porosity of this type of rocks make them the most preferable commercial reservoirs in the world (Sochon 4). The rocks are created by a series of physical, biological, and chemical processes. Sedimentary rock formation begins by the process of disaggregating the magnetic, sedimentary and metamorphic sources, whereby they are weathered to resistant residual, secondary minerals, and water soluble ions of silica and sodium, among many others (Sochon 4). The formation process moves to the next state in which the weathered components are transported to newer destinations. Notably, the materials are transported by air, water, or wind and deposited in other sites (Sochon 4). The final process entails the combination of various minerals. The settled sedimentary materials undergoes a process of lithification, whereby they are compacted to create diagenetic minerals (Sochon 4). Therefore, knowledge about the formation of rocks play a critical role for oil and gas companies to determine where to undertake their explorations.
Conclusion
The aspect of rock mechanics provides oil and gas production companies with crucial information about the extraction process and procedures. In addition to determining the appropriate sites to situate oil and gas reservoirs, engineers need to understand the features of the underlying rocks to understand the best approaches to drill the required wells. The comprehension of underground pressures is one of the important functions of rock mechanics. Through this element, engineers can have insights into the impact of both horizontal and vertical stressors on wells, thus enabling them to employ the appropriate techniques during the drilling process. Failure to apply this principle may lead to the collapse of wells.
At the same time, rock mechanics involves the aspect of pore pressure. Abnormal pressures can cause disastrous effects on the stability of wells. For this reason, engineers must understand the true status of a zone’s pore pressure to ascertain the right approaches to well-drilling. Additionally, the concept of rock mechanics entails the attributes of both minerals and grains that comprise specific rocks. Similarly, having these insights is crucial to informing the appropriate approach to well-drilling. Furthermore, the concept of rock mechanics involves the understanding of the physical features of sedimentary rocks. Information about this aspect is crucial to ascertaining oil and gas-rich sites.
Work Cited
Aadnoy, Bernt and Reza Looyeh. Petroleum Rock Mechanics: Drilling Operations and Well Design. Gulf Professional Publishing, 2011.
Sochon, Jurgen. Physical Properties Of Rocks: A Workbook. Elsevier, 2011.
Fjær, Erling, Rune M. Holtm Per Horsrud Arne M. Raaen, and Rasmus Risnes. Petroleum Related Rock Mechanics (2nd Edition). Elsevier, 2008.