Geomechanics (from the Greek prefix geo- meaning "earth"; and "mechanics") involves the geologic study of the behavior of soil and rock. By definition, rock mechanics is the theoretical and applied science of the mechanical behavior of rocks in the force fields of their physical environment. In practice, so-called “rock engineering” is concerned with the application of principles of engineering mechanics to the design and construction of structures of any type either on or in the rock, such as tunnels, mine shafts, underground excavations, open-pit mines, road cuts, dams, skyscrapers, waste repositories, and oil or gas wells.
Though initially developed for mining and civil engineering purposes, geomechanics found its way into the oil and gas industry in the ’80s in order to improve hydraulic fracturing and drilling operations. In the contemporary petroleum industry, geomechanics is defined as the discipline that integrates rock mechanics, geophysics, petrophysics, and geology to quantify the response of the Earth to any changes in the state of stress, pore pressure, and formation temperature.
Geomechanics: The Oil and Gas Industry’s Missing Link
Although the systematic application of rock mechanics in the oil and gas industry is relatively new, it was recognized and appreciated by many oil companies in a short period of time and has become a fast-growing field due to its applicability and effectiveness in reducing nonproductive time (NPT).
As the virgin state of stress is disturbed by different oil and gas activities, the rock’s mechanical state changes, too, and consequently influences drilling, completions, and production performance. These changes can result in serious and unexpected cost and time overruns if not properly predicted and managed. Dodson et al. (Offshore, Vol. 64, No. 1, 2004) conducted a survey of Gulf of Mexico wells and reported wellbore stability issues were the cause of almost 40% of drilling-related NPT, resulting in an annual cost of around USD 8 billion.
As a result of experiencing significant improvements in drilling and production operations by utilizing geomechanics, it has hence become an important and integral part of each and every field development plan, from the early stages of exploration to even after field abandonment. With the recent boom in the development of unconventional oil and gas resources, the use of geomechanics principles has become even more imperative due to the sensitivity and complexity of these reservoirs. Geomechanics is playing a critical role in successfully maximizing shale gas production by helping optimize the use of hydraulic fracturing technology.
Geomechanical applications in the oil and gas industry include pore pressure prediction; helping ensure cap-rock integrity; field problem diagnosis; formation properties evaluation; in-situ stresses estimation; drilling performance evaluation; wellbore stability; borehole trajectory optimization; sand production prediction and control; underbalanced drilling feasibility; fractured reservoir characterization; and production maximization affected by natural fractures, hydraulic fracturing, fluid and steam injection, reservoir compaction, surface subsidence, and casing shear and collapse. It’s a long list! Clear knowledge of how to apply geomechanics appropriately will increase exploration and development efficiency in both conventional and unconventional resources.
Geomechanical Modeling: Turning Impossibilities Into Possibilities
To conduct any of the aforementioned studies using rock
mechanics, the first step is to construct a geomechanical Earth model (GEM). A
GEM consists of six core components that need to be either calculated or
estimated using field data:
Vertical stress, δv (often referred to as
the overburden stress)
Maximum horizontal stress, δHmax
Minimum horizontal stress, δHmin
Stress orientation, Azi δHmax
Pore pressure, Pp
Rock mechanical properties
Modeling techniques in geomechanics encompass analytical, experimental, and numerical methods, each having its pros and cons. Generally, numerical models have higher accuracy over analytical ones but require additional input data and more time. Analytical techniques are in return quicker with less complexity. Experimental models are based on physical and mechanical laboratory tests on rock core samples. It is usually costly and time-consuming to perform such tests, though they do provide valuable information about rock properties.
As a generic workflow, constructing a 1D geomechanical model starts with rock mechanical property estimation using petrophysical logs in conjunction with core test results. There are different empirical models to make a strong profile; however, laboratory data are required to calibrate these models.
The second step is building a continuous overburden profile
using density logs.
Pore-pressure prediction using logs and available good test
data (or seismic data if available) is the next step. Minimum horizontal stress
can be calculated using either empirical equations or fracturing data (LOT
[leak-off tests]/X [extended] LOT or minifracturing tests) or ideally, a combination
of both. Drilling incidents such as ballooning and mud losses can help to
constrain the minimum horizontal stress and fracture gradient.
The last steps are determining the azimuth and magnitude of the
maximum horizontal stress. This is the most complicated part of geomechanical
modeling, as no direct way of measuring δHmax is available.
Analyzing wellbore failures such as breakouts and drilling-induced tensile
fractures from image logs is one of the existing techniques to determine a
reasonable range for δHmax and find its orientation. Using
caliper logs, sonic logs, and laboratory measurement of elastic strain recovery
are alternative techniques.
Many field examples have proved that geomechanical analyses
can open opportunities for drilling into harsh and challenging environments
which previously looked impossible. In an example in southeast Asia, where
drilling a vertical well was identified as impossible due to lack of a safe
operating mud weight window, the well was made possible by geomechanical analysis
that led to changing the good trajectory to the safest orientation in a
specific formation and thereby widening the window. Geomechanics can also
improve casing design and provide a wider mud weight window for drillers. There
are examples in Northwest Shelf Australia where geomechanical modeling reduced
the number of casings, resulting in significant cost savings for the operators.
In the context of production from naturally fractured
reservoirs, a GEM can make a real difference in maximizing production by
identifying critically stressed fractures which are, in fact, productive
fractures. Identifying the orientation of these fractures enables optimization
of drilling orientation to intersect the maximum number of them. Field examples
in the Middle East and Southeast Asia have shown notable increases in
production using these types of studies.
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