Expert Viewpoint - The Importance of Geomechanics

Schlumberger acquired the company formerly known as V.I.P.S. Ltd in May 2007 and Marsden is excited about the opportunities that it is providing. The company developed the world's first coupled geomechanics stress-dependent reservoir simulation capability, which integrates the disciplines of geomechanics and rock mechanics with those of reservoir engineering and simulation.

Geomechanics is becoming an essential component of reservoir simulation. Reservoir models traditionally consider porosity, permeability, fluid movement and pressure changes, with production history used to update the model and for forward modelling. Geomechanics has often been ignored, but rock deformation and stress caused by drilling and/or production is very likely to affect permeability and porosity. The removal of hydrocarbons from a reservoir or the injection of fluids changes the rock stresses and geomechanics environment, potentially leading to compaction and subsidence and impacting well and completion integrity, cap-rock and fault-seal integrity and fracture behavior. We can now calculate dynamic changes in the reservoir resulting from production and injection. This additional information, which has been considered for many years in civil engineering and mining applications, is becoming increasingly recognised as highly valuable in oilfield management. Geomechanics should also be considered when designing 4D seismic projects and systems for thermal recovery and CO2 disposal.

A baseline model starts with a mechanical model of the reservoir incorporating rock type and faults. Stresses are carried by stiffer rocks. Geomechanics considers rock stiffness and the overburden before and after a well is drilled.

After drilling a well, these stresses will transfer to the wellbore. We can model the complex 3D stresses around the well and consider various “what-if” scenarios to be modelled for well construction planning. Accurate modelling enables the trajectory, mud weight and casing strength to be designed so as to minimize the risk of collapse during drilling and production. Marsden considers the task to be analogous to designing a structure or airframe, where stress and deformation are integral parts of the model.

For designing a new “green field” production system, adding geomechanics will help to optimize production, reduce the risk of well and/or completions failure and investigate and help mitigate against the risk of environmental impact such as surface subsidence, of which Ekofisk is a well documented example. There are many other examples of this problem throughout the world, some resulting in significant changes such as modified river flow patterns.

Geomechanics can be even more critical for brown-field sites. There are several examples where infill wells, constructed 20 years after the original wells, have failed, despite the use of newer and better equipment. This is because geomechanical challenges and the risk of reservoir failure increase with time. “We have a lot to learn from civil and mining engineering”, says Marsden. “Without geomechanics we are often like firemen – dealing with problems after the event of failure. It is much better to prepare and mitigate. Geomechanics provides many opportunities to do this”. 

Heavy Oil is often associated with soft, unconsolidated near-surface reservoirs in which wellbore stability can be a key issue both while drilling and while producing. Consideration of the geomechanical properties of these low-strength rocks and the fluids they contain is particularly important. 

Thermal Recovery will change stress patterns due to the heat itself and through heat-induced changes in fluid pressure. Thermally induced stresses, such as from in-situ combustion or injection of hot steam and/or cold water, can also change porosity and permeability. It is a closed loop in which several factors interact. For example, while steam injection will reduce viscosity, it may lead to stress induced formation failure leading to well breakage and/or collapse, perhaps extending to subsidence at the surface. Conversely, expansion due to heating a reservoir or injection of large volumes of fluids may lead to heave or uplift at the surface, with associated HSE implications.

The VISAGE* stress analysis simulator is widely acknowledged as the most advanced and comprehensive Finite Element Method system available for geomechanical modeling. By solving complex stress equations and relating the rock stresses to reservoir properties, the technology is key to the development of 3D and 4D mechanical earth models that predict the geomechanical behavior of the reservoir during production and injection. In addition to hydrocarbon production, VISAGE has also been widely used for applications such as the design of bridges and mines.

Schlumberger is developing links between VISAGE and ECLIPSE* reservoir simulation software and other reservoir workflows such as the Petrel* seismic-to-simulation technology. This will enable seamless mechanical earth modeling from seismic, logs and laboratory tests through to coupled geomechanics and reservoir analyses and engineering designs.

ECLIPSE focuses on dynamic fluid flow, modeling properties such as porosity, permeability, temperature, pressure, saturation and viscosity. ECLIPSE is considered to be the worlds most advanced reservoir simulator. VISAGE, the worlds most advanced geomechanical simulator, models properties related to solids, such as stress and strain, that can impact the rock and fluids. For example, it is not unusual for reservoirs to compact and lose up to 70% of their original permeability. VISAGE provides a more complete picture of the reservoir and the overburden and surrounding environment, including rock stress, strain and displacement. VISAGE offers unique functionality in that computed properties are fed back to ECLIPSE through a coupling process. This enables variables in the dynamic reservoir model to be updated as a result of stress and changes in stress.  

VISAGE-Viewer is an interface tool used to enhance an ECLIPSE reservoir model into a larger model, including the over- and underburdon, to which geomechanical properties such as stiffness and strength are added. VISAGE-Viewer also provides tools to view the model. Geomechanics adds more realistic scenarios and can look ahead to predict models after several years of production

Geomechanical modelling follows a stringent workflow and process – understanding physics and how rocks react to stress. “We take a holistic approach”, says Marsden. “All observations and events, both past and present, are used as input. It is a truly dynamic model, changing whenever new data become available.”

The VISAGE workflow can be applied to model thermal effects related to heavy oil production - stresses and strains caused by pressure and temperature changes. A typical SAGD field will have many wells and rapidly changing stresses. VISAGE can work at both a large scale, for example to understand reservoir processes or design infill wells, and at a small scale, for example to predict geomechanical effects on cement, designing completions equipment and the hardware required for the production system.

Modeling can be coupled with industry thermal simulators such as ECLIPSE Thermal, which considers the thermal effects on fluids. VISAGE adds the thermal effect on solids. VISAGE cans also model rock property changes and solids production in CHOPS operations.

Schlumberger has built the largest oilfield geomechanics team in the world and is integrating geomechanics into a wide range of applications. For example, geomechanics impacts 4D seismic, whereby changes in the seismic response of a reservoir over time are used to estimate changes in fluid content and saturation.  Compaction and changes in stress will also change the seismic response, so should be added to the model.

The addition of Koutsabeloulis and his team provides many exciting opportunities to integrate geomechanics into even more Schlumberger offerings. In many cases, it is helping to explain the divergencies between predicted and observed reservoir behavior.

The team also brings experience of microseismic monitoring of oilfields – predicting the occurrence and location of fractures resulting from production and injection. Geomechanics expertise can help to optimize recording arrays and provide more data into the resulting models.

Rock strength models are based on laboratory data, such as tests on cores by TerraTek, which became part of Schlumberger in 2006. Models also use data from tools such as Sonic Scanner*, which provides radial and axial measurements of the stress-dependent properties of the rocks near the wellbore.

Models are built from 3D structural models plus all available 1D models from down hole measurements interpolated and extrapolated using Petrel. Far-field boundary conditions are then imposed and matched with the observed effects of stress in wells, such as fracturing and breakout from FMI data. Other inputs to geomechanics studies include seismic data, pressure and temperature measurements as provided by Sensa optical fiber systems, and GPS terrestrial surveys, locating changes in elevation.

Schlumberger is working on geomechanics projects from around the world, including new “green field” development projects, field optimization, integrity planning and EOR schemes, including several aimed at enhancing heavy oil recovery programs.