The Concept of Geomechanics
Geomechanics concerns itself with the interplay between forces and materials in the earth. These forces range in scale from macroscopic gravitational and tectonic forces to the microscopic forces of attraction between grains, molecules and atoms.
Since we generally cannot observe these interactions first-hand in the earth, we rely on indirect methods from multiple geomechanics disciplines to try and make inferences about how forces and their resulting pressures and stresses mechanically interact with the fluids, grains, rocks and formations in the earth. Depending on the data available, these disciplines can include geophysics, geology, geostatistics, petrophysics, rock mechanics, soil mechanics and various sub-disciplines of petroleum engineering.
The Applications of Geomechanics
The interaction between the various forces and materials in the earth is pertinent to virtually all phases of hydrocarbon resource development.
In the exploratory phase, geomechanics is integrated into basin models that investigate how the basin may have evolved as sediments were deposited and subsequently deformed.
In the pre-drill phase, available information from various sources is integrated to model pore pressure, fracture gradient and risk of drilling problems such as hole collapse or differential sticking. The objective is to assist in the planning of casing, drilling fluid properties and drilling practices so as to minimize the overall cost of the well. A geomechanics study may sometimes also contribute to bit selection and choice of well trajectory.
While drilling, a pre-drill model is refined through observations of well behavior, petrophysical data, samples circulated from downhole and interactions between the bit and bottom-hole assembly with the formation. Modifications to the original mud weight plan or casing design may be required based on the information acquired while drilling.
After the drilling phase, geomechanics often plays a role in completion design and reservoir modeling. For example, the ability to fracture a rock and obtain enhanced production is highly dependent on downhole rock properties ascertained via a geomechanical investigation. Another application is the investigation of possible sanding problems during production. As the reservoir becomes pressure-depleted, the result is more stress on the grains, leading to loss of porosity and permeability and sometimes subsidence of the overlying formations—all of which are within the domain of geomechanics to model.
What Makes the Deepwater Environment Exceptional for Geomechanics?
The deepwater environment can pose some special challenges not seen in onshore wells. One of these challenges is simply logistical. For example, you cannot drive a truck out to a deepwater rig. Advanced planning for needs and contingencies is important to avoid lost time, which can be extremely expensive (tens of thousands of dollars per hour) on deepwater rigs. An accurate pre-drill geomechanics model, supplemented with real-time updates, is especially important for minimizing lost time due to problems such as kicks, stuck pipe, lost circulation and hole collapse.
In deepwater, the temperature at the seabed is often near that of the maximum density for water, which occurs at about 40 degrees Fahrenheit. When this temperature is coupled with the pressure of the sea water, an environment favorable for the creation and preservation of gas hydrates is created. These waxy mixtures of water and natural gas can create a drilling hazard when warm drilling fluid is circulated from further downhole and liberates the natural gas from the hydrate.
The deepwater geologic environment is not simply that of a typical onshore geologic environment placed underwater. In an onshore well, the vertical stress which is placed on a formation is simply due to the pull of gravity on the overlying formations, whereas in a deepwater well a significant part of this “overburden stress” is contributed by the pressure of the water column above the sea floor. In the deepwater environment, the vertical stress is almost always the stress with the highest magnitude. In areas where there has been mountain building, strike slip faults or thrust faults, the largest stress is typically closer to horizontal in orientation rather than vertical.
The sediments which accumulate on the seafloor in deep water have properties different from what is observed in other environments. With the support of the seawater, the ocean-derived and terrigenous (often clay-rich) materials at the seabed can have very high porosities (up to 80 percent) and correspondingly low densities. This obviously has an effect on the vertical stress which is generally not present in other environments.
In the deepwater environment, accumulations of sediments which have “fallen” from shallower water environments can create deeper accumulations called turbidites. Coarser-grained turbidites can become reservoirs after burial and subsequent migration of hydrocarbons into them.
Drilling In Deep Water Poses Additional Challenges
These challenges are due to the relationship between the overburden stress, the pore pressure and the fracture pressure. Typically, the fracture pressure (the mud pressure which will create fractures into which mud will be lost) is higher than the pore pressure and lower than the overburden stress. Near the seabed, the pore pressure and overburden stress are nearly equal because the bulk of the overlying material is just sea water. Consequently, the pressure created by pumping drilling mud often exceeds the fracture pressure. For this reason, a cheap, low density drilling fluid—sea water—is used for drilling the first one or two hole sections. Drilling without a riser to return the drilling fluid and cuttings to the surface is also a practical option for these near-seabed sediments in order to keep the borehole pressure in the “safe mud weight window” between the pore pressure and fracture pressure.
The stresses and rock mechanical properties that are ascertained from the pre-drill and drilling phases can then be applied to the design of completions and productions practices later in the life of the well.
In deepwater, geomechanics is invaluable. Because of the logistics associated with deepwater drilling locations, it is of vital economic importance to get it right the first time. If there’s a problem, driving a workover rig to the location is not an option.