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Mechanical behaviour of partially and multiphase saturated chalks. Fluid-skeleton interaction : main factor of chalk oil reservoirs compaction and related subsidence


The compaction of chalky oil reservoirs during oil extraction induces the so-called sea floor subsidence which represents a huge problem in North Sea fields. This problem and other important ones, like the "casing collapse" or the "chalk production'' are related to the mechanical properties of chalk. Overall, the rock behaviour is modified during the waterflooding, i.e. the replacement of the initial saturating fluid (oil) by another (sea water for instance).
Although the subsidence is still occurring despite waterflooding, the subsidence mastering as well as (among others) the chalk production mastering is a major industrial challenge. It has consequences on the productivity and safety of oil fields. It requires to understand the actual causes of the mechanical behaviour of chalk.

In fact, a lot of studies on chalk have been performed since 1982. It is clear that many waterflooding tests have been run (since Newman, 1980). However, it is only recently that the capillary effect is suspected to be the main agent of this phenomenon. Further studies have confirmed this idea. Some recent numerical models describe the chalk behaviour during waterflooding. Among these models, the recent approach of J.M. Piau demonstrates clearly the relationships between the compaction occurring during the waterflooding and the constitutive laws of the chalk. But, the explanation of the phenomenon at microscopic scale and the modelling of the mechanism is still to be completed. Obviously, no satisfactory constitutive law can be written without this deep insight of the phenomenon.

The work done in the last fifteen years in unsaturated soils mechanics (triphasic medium: grains of soil, water, air) have pointed out the limits of the biphasic approach. This approach is unable to take into account the so called "suction" or "capillary pressure or the physical interactions between water and clay, silica, calcite or other minerals. It is especially the case when collapse or swelling phenomena associated with capillary pressure changes are considered.

The broadening of this multiphase approach to the case of reservoir rocks comes from the similarities between soils and chalks in terms of capillarity. In both cases, cohesion results from capillary effects of small water meniscus and is destroyed by full water saturation. As far as we know, it is the first time that such parallel has been clearly mentioned.
The scope and originality of this project is to apply the knowledge, the approach and the tools on unsaturated soils mechanics (which considers multiphase media in place of biphasic ones like in classical rock mechanics) to the understanding, description and modelling of the chalk behaviour during changes in saturation fluids, like waterflooding. It will be conducted in theoretical, numerical and experimental ways.

The achievement of the objective consist in

- a numerical tool, taking into account the physical mechanisms of fluid - fluid and fluids - skeleton interactions, for the modelling and forecasting of the behaviour of the reservoir rock.

- the experimental determination of the law parameters and their dependency with respect to the geological and physical conditions, using modified methods of unsaturated soils mechanics

- the implementation of the law into a computer finite elements code.
- models, calibrated on base of existing data, will be built in order to predict the chalk behaviour during oil extraction under several conditions and provide help for more efficient oil recovery and safe reservoir management.

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