Getting to grips with geomechanics

Rockfield Software is near the end of a three-year joint industry project focused on prediction of the initiation and evolution of fractures and faults within hydrocarbon reservoirs.

The Swansea-based company, which specialises in advanced numerical analysis, is specifically known for its “Finite Element-based” simulation systems and, since setting up in 1985, it has applied this expertise to a wide variety of engineering applications, including the modelling of wellbore stability; expandable completions under field conditions; explosive fracturing of rock for the mining industry, and structural integrity analyses such as the collapse of tin cans under pressure.

The JIP currently rounding off is part of a programme launched in 2005 by the Industry Technology Facilitator (ITF) to tackle issues surrounding geomechanics.

This discipline – which, until recently, had a relatively low profile – has become highly important as it is concerned with the behaviour of soil and rocks in response to tectonic movement, deposition of sediment and erosion.

As the industry has moved to recover hydrocarbons from deeper, higher-temperature and pressure environments, the impact of the resultant stresses and strains within the rock matrix on hydrocarbon flow has become increasingly recognised as an important factor with respect to geomechanics, as this aspect also needs to be fully considered in maturing reservoirs.

Since the fluids within reservoir pore spaces exert pressure, it stands to reason that drilling a hole in a reservoir and extracting the fluid will cause changes in the forces at work in the reservoir as a whole. The stresses and strains within the rock fabric will continue to change as the reservoir is depleted.

Furthermore, it is not only the geomechanics of the reservoir rocks that are important, but also that of the rocks surrounding the oil bearing strata.

“Geomechanics is important for producing reservoirs and becomes more important as they mature because the structural stresses and strains increase,” said ITF’s subsurface technology manager, Duncan Anderson.

“The industry relies on reservoir models to help guide optimum hydrocarbon recovery. If reservoir models don’t fully account for the influence of geomechanics then, ultimately, the calculations that they produce will be wrong.

“There is an appetite within the industry to act quickly to ensure that geomechanical factors are adequately incorporated into reservoir modelling, and this project, along with the others in our geomechanics programme, should go some way to achieving that.”

The project aims to provide greatly improved models for the rock formation subsequent to fracture or fault formation – that is, issues ranging from the prediction of the sand face behaviour through to the incorporation of geomechanics into geological (or, more specifically, “palinspastic”) reconstructions and the prediction of the structural form of sediments near intrusive salt bodies.

John Cain, business development manager at Rockfield, said: “Changes in the stresses and strains within or near a reservoir often result in the development of localised deformation zones that are manifested as fractures at the wellbore scale, or faults at the reservoir scale.

“These faults and fractures have a pivotal role in the behaviour of the reservoir and, therefore, the reservoir management strategy.

“Changes in stresses resulting from drilling a well can ultimately result in the loss of that well; the drill bit may get jammed or things may start to move after the well has been drilled.

“Fractures can either block or enhance the flow of fluids to a producing well, so this work is underpinning science that will be applied to both the successful depletion of the reservoir and a reduction in the loss of wells at the wellbore scale.

“While prediction of the initiation and subsequent evolution of fractures/faults is essential for accurate geomechanical modelling, rigorous formulations that address this issue are not available in other commercial codes. It is this fundamental limitation that has been addressed in this project.”

A second JIP was kicked off in September, 2007, with, currently, six of the original seven partners participating.

The focus of this project is to drive the application of these advanced modelling techniques to structural geology and basin-modelling environments.

For more information, contact Duncan Anderson at ITF (d.anderson@oil-itf.com) or John Cain at Rockfield (j.cain@rockfield. co.uk)

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