The promise of geomechanic modeling

Feb. 1, 2006
The technically challenging nature of today’s reservoirs and the cost risk of unpleasant surprises are changing workflow processes.

The technically challenging nature of today’s reservoirs and the cost risk of unpleasant surprises are changing workflow processes. An integral component of these new workflows is the science of geomechanics.

Geomechanics modeling is rapidly becoming a core discipline for drilling and field development in reservoirs at risk for geomechanical events. Consider the following:
• Wellbore instability costs the drilling industry $6 billion per year, through kicks, losses, stuck pipe, sidetracks, extra casing strings, etc.
• More than 20,000 wells are hydraulically fractured annually, and for a variety of reasons, many of these treatments fall short of expectations.
• The effects of reservoir compaction go far beyond the reservoir and its production; they also may impact production facilities as overburden stresses change.

Additionally, many of our biggest new reservoirs are offshore in deepwater. Largely, they consist of high-porosity, high-permeability sediments prone to sand production, compaction, permeability reduction, and casing deformation. Understanding of the geomechanical regime is key to successful operations in these areas.

Geomechanics is the discipline dealing with the prediction and management of rock deformation. The most critical geomechanical issues are generally in weak rock systems, as in deepwater reservoirs, and in tight, fractured formations. Focusing on the entire sedimentary column, from 0 to ~8,000 km (~27,000 ft) depth, geomechanics helps design drilling, perforating, and stimulation applications, and predict wellbore instability, sand production, subsidence, and other undesirable occurrences.

One of the biggest obstacles to well and reservoir optimization is neither economic nor technical, but simply the slow evolution of our business processes as we respond to our changing environment. Adding a new step or dimension meets resistance, especially if people believe it is complex and time-consuming. The good news is we have the ability - and the appropriate measurements - right now to build useful models, and their construction is neither as complicated or lengthy a process as many suspect. Rather than beginning with a full-field, fully coupled geomechanics reservoir model, we can start at the single-well level with a near-wellbore model and proceed outward as new development wells are drilled. Alternatively, we can include geomechanics in reservoir optimization by simply modifying an existing reservoir model. We have all the pieces needed for this:
• 3D geologic framework models from logs, borehole and/or passive seismic
• 1D mechanical earth models from logs, cores and drilling data
• Interpretation and simulation software
• Startup pore-pressure models delivered in only a few days
• Calibration of the geomechanics reservoir model with real-time data from day one and continuing feedback through the workflow• Hydraulic fracture mapping.

With this kind of an earth model, for example, it is possible to detect the onset of reservoir compaction that can lead to structural and casing deformation, even sheared casing in some instances.

An example of this concerns an incident in the GoM where a field was shut in prior to a recent hurricane. When brought back online after the storm, one of the field’s best producers had filled with sand. The field was equipped with permanent pressure gauges, but the data had not been monitored.

Back analysis indicated that a geomechanics reservoir model could have predicted the sanding problem, as the model would have shown reservoir damage from pre-storm pressure cycling. However, without such a model, there was no baseline or failure criterion with which to interpret the pressure data. Even a simple near-wellbore geomechanics model, had it been monitored, would have highlighted the detrimental impact of pressure cycling. With this knowledge, the operator could have chosen different shut-in and start up methods to minimize reservoir damage during early production and minimize the risk of and sand production upon startup.

All this and more is possible with just a simple, near-wellbore geomechanics model. In time, by incorporating passive seismic and permanent pressure monitoring, this can be expanded into a 4D near-wellbore model. It is technically possible to develop a full 4D reservoir model with complete geomechanics, and this is being done in some high-risk deepwater fields.

Elsewhere, it seems quite likely that geomechanics modeling for reservoirs will get a foothold in the near-wellbore region, where all the measurements and feedback signals are readily available and the value of this technology is apparent. Single near-wellbore geomechanics models are a start.

Jeff Spath
President, Schlumberger Data & Consulting Services