Time, the fourth dimension, has become a prominent element in field characterization and management. Historically, different technical disciplines marched to their individual tunes, some barely acknowledging the existence of the others, much less sharing information and collaborating on the development and exploitation of the company’s assets. But things are changing with the appearance of new technologies and the integration of different disciplines to provide more complete understandings of the dynamics of a field.

Companies are learning how they can use time-lapse information and real-time monitoring to

Figure 1. 3-D Representation of the Mechanical Earth Model (MEM). (Images courtesy of Schlumberger)
understand what is happening in reservoirs as they are being produced — 4-D seismic and microseismic monitoring are just two examples of such technologies. But another interesting discipline is now allowing integration of available knowledge of a field from many different sources over time to predict performance and future risks. This discipline is geomechanics.

Historically used by individual disciplines to qualify sites for drilling or to help engineers design casing, completions or hydraulic stimulation programs, geomechanics was seen mostly as a tool for single-well applications. In fact, it provides a time map of the forces that have shaped the reservoir and will affect it in the future. In the hands of a skilled scientist or engineer, geomechanics can be used to determine the stresses and strains that have affected the reservoir throughout its life and will affect its performance in the years to come. Using such geomechanics information, engineers can plan wells to best access their reservoirs and reserves and can more effectively manage their field development and field management as well as mitigate risks associated with field operation. But, like any task with the potential to yield valuable results, the job is detailed and requires a meticulous approach.

Turning data into useful information
Schlumberger has been using geomechanics with great success in well planning, completion design and stimulation for several decades. Starting with wellbore stability modeling, geoscientists and engineers were able to make fast but effective studies of the formation structure surrounding the borehole, calculating safe and stable mud weights and evaluating potential drilling risk in terms of severity and likelihood of occurrence.

Another early application of oilfield geomechanics was hydraulic fracturing and stimulation. Engineers used log/rock properties correlations to design fracture treatments and estimate the extent of their propagation. Later, the techniques were expanded to address sand control and completions designs.

Years of meticulous laboratory testing of core samples in rock geomechanical laboratories
Figure 2. Fault representations through active reservoir layers.
provided benchmark data on fundamental rock properties. These in turn were correlated with logging measurements that complemented the static mechanical data from the labs with the dynamic elastic data from the log responses. The knowledge gained from these tests provided a vital link between petrophysical and geomechanical behavior that could light the way to simulation and predictability. But the solution was far more complex than it might initially appear. Instead of homogeneous test samples like those used in traditional mechanical properties analyses, the geoscientists had to deal with complex heterogeneous rocks that may or may not be fractured; may or may not be infused with reactive clay; or may be subject to chemical and mechanical changes as fluids, pressures and temperatures varied. Quantifying the effects of all these variables required considerable detective work.

Coupled geomechanics
The description of all the data relevant to geomechanics is the mechanical earth model. It is a consistent description of the earth stresses, pressures and rock mechanics properties through the reservoir and overburden, generated using all the information available from logs, geophysics, lab tests, drilling and field measurements. By coupling these models to reservoir and production simulations, geoscientists and engineers are able to not only analyze and design wells and completions but can also investigate the effects of production and injection and understand phenomena like subsidence or fault slippage that might affect both the reservoir and its surrounding rocks over the life of a field.

A real-world example
Recently, coupled reservoir modeling and geomechanics were used to understand and
Figure 3. Plastic shear zones at horizontal well perforations.
predict field behavior, compaction and potential well failures in the South Arne field, offshore Denmark. The operator, Amerada Hess, in collaboration with Schlumberger, constructed a 185,000-cell reservoir model using ECLIPSE reservoir simulation software to serve as a base line. The Schlumberger geomechanics team added further overburden and surrounding rock and, using a range of data from seismic, logs, lab and field measurements, populated the entire model with rock mechanics and strength properties and the 3-D geometries for multiple faults. Then the initial map of stresses throughout the reservoir and around the faults was computed using the VISAGE stress analysis simulator. At this point the model appropriately represented the reservoir’s initial petrophysical and geomechanical state.

Simulating the effects of known past production, the model was advanced in time to compute stress rotations, stress changes, rock deformations and changes in reservoir properties, yielding a greatly improved history match to observed production. The model was checked against all relevant field and well observations to ensure conformation with the laws of physics and reality, and no data were discounted — every observation from the field required a supportable explanation. Major changes in the reservoir behavior and well
performance were identified as resulting from compaction and depletion-induced rock deformations. In the simulation, one well emerged as being particularly significant and a good indicator of some of the geomechanics effects occurring in the field. As the model was advanced with time, a concentration of shear stress was predicted in the Tor Chalk formation along that particular well path. Other studies had indicated a range of possible reservoir compactions of between 4.8 ft and 11.5 ft (1.45 m and 3.5 m) at current day following six years of production. The coupled VISAGE simulation, which employed the Norwegian Geotechnical Institute’s chalk failure criterion, removed most of this uncertainty and predicted that compaction in the Tor Chalk could only be 4.8 ft. In addition, the modeling not only constrained the compaction solution uncertainties but at the same time predicted that at that degree of compaction a well failure would occur within a few months. In fact, a major proppant and chalk influx was observed in that same well two months after it had been predicted. A 4-D seismic inversion validated the technique and results by indicating, at 4.9 ft (1.5 m), the same reservoir compaction as predicted by the VISAGE coupling.

What does it mean?
With coupled geomechanics modeling and analysis, companies can game-play the effects of production or stimulation alternatives to see which will best stand the test of time and which field development plan and well configurations are likely to mitigate potential geomechanics problems. With this more accurate predictability, companies can plan and execute remediation steps before catastrophic failure occurs. With the ability to peer into the future, field development wells can be placed more advantageously to optimize reservoir production over time. With improved accuracy and resolution, the effects of changes from implementing enhanced recovery techniques can be modeled, affecting the operator’s ability to successfully manage the reservoir for maximum profitability.

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