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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 - part 2 (PASACHALK 2)


Objectives and problems to be solved:
In North Sea oilfields, the sea floor subsidence created by the compaction of chalk oil reservoirs during oil extraction is a serious problem. Together with the "casing collapse" and the "chalk production" problems, sea floor subsidence is related to some specific features of the mechanical behaviour of chalk. Chalk mechanical properties are modified during water flooding, when the initial saturating fluid (oil) is replaced by seawater.
Description of work:
The previous JOULE-THERMIE PASACHALK project was based on the application of unsaturated soils mechanics to chalk behaviour during water flooding. Unsaturated soils are three-phase porous media containing solid, water as a wetting fluid, and air as the non-wetting fluid. The aim of the present "PASACHALK 2" project is to enlarge the scope of the multiphase coupled analysis of the behaviour of chalk by including the additional effects related to the viscous behaviour and to the wettability effects in chalks. To our knowledge, this approach, in which the viscous behaviour and the wettability effects are included in a global multiphase approach, accounting for the effects of the oil-water "suction", has not been developed yet. Undoubtedly, fundamentals results obtained in this framework seem also essential for a better understanding of other problems related to chalk behaviour, such as chalk production in boreholes, and casing collapses. The PASACHALK 2 project is composed of an experimental, a theoretical and a numerical approach. It is aimed at providing an efficient numerical tool for a better management of the reservoir, through a better prediction of its behaviour during exploitation.
Expected results and exploitation plans:
The project deliverables will be some new concepts in rock mechanics:
an adaptation of geo-mechanical experimental apparatus;
the development or modification of a finite element code for the modelling of the coupling between the water flooding and the chalk compaction;
and a new model of chalk reservoir compaction and the associated subsidence and of "chalk production".
The exploitation policy will vary between the industrial partner and the university research centres. All new rock mechanics concepts, constitutive laws and experimental apparatus adaptations will be disseminated in the scientific community through papers in scientific and technical journals, and through presentations in international conferences. No patent policy or expected agreement will limit this diffusion. The results of the oil field modelling (stress and strain history, water and oil flow and retention, compaction and subsidence evolution, effect of different water injection scheme, etc.) will be the ELF property. These research results will only be diffused in a first step within the now TOTAL FINA ELF group, and also to partners associated in the exploitation of some chalk reservoirs.
The experimental part results consist mainly of the completion of a special laboratory equipment devoted to mechanical tests under controlled suction and/or following very particular stress path, including the waterflooding. This includes the testing procedures and the know-how. This kind of equipment and procedure can be used for other THM (thermo-hydro-mechanical) problems as pollutant transfers experiments.

The numerical modelling has been achieved through to a large experimental plan analysis and a literature review. So, basic mechanical and hydraulical phenomena involved during the variation of fluid content in chalk have been identified. They have been theoretically developed and implemented in a numerical model allowing to simulate various problems associated to the experimental plan and also to oil reservoir exploitation. The numerical model constitutes an exploitable result. It can be applied to reservoir engineering but also to other subsidence problems like these related to water catchments.

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