Project title: ”Chalk Deformations – Physical and Chemical Processes”
The proposed work will be part of a 3 years research project “Water Weakening of Chalk – Physical and Chemical Processes”, funded by the Norwegian research council. The aim of this extensive project is to understand and model the impact of the chemical and the physical forces on the deformation of North Sea chalk.
Background: Since the detection of the Ekofisk subsidence some 20 years ago, there has been a considerable research activity concerning chalk behaviour in general and mechanical properties in particular. Due to the importance of water injection into chalk reservoirs to improve oil recovery, an extended experimental testing has been performed in order to further investigate the so-called water weakening effect. The mechanisms behind well instability, compaction, and subsidence experienced in the North Sea chalk reservoirs are, however, not completely understood.
Water weakening occurs to some extent in all rocks with initial low water saturation and has been believed to be a result of reduced capillary suction at grain contacts. The main objective of the work by UiS has thus been to gain further insight into the mechanisms behind water weakening of chalk, and to focus on the relationship between physico/mechanical and chemical effects. The results from our recent experimental research, dealing with impact of seawater injection on the mechanical behaviour of chalks, have shown that temperature as well as ion composition of the displacement water will affect the mechanical properties. So far it seems that chemical weakening can either be due to dissolution -and precipitation processes or chemical substitution reactions at the chalk surface (Hellman et al., 2002, Madland 2005, Heggheim et al. 2005, Korsnes et al. 2006a, 2006b, 2006c).
Several studies recently performed at the Rock Mechanics lab at UiS indicate that also other non-carbonate minerals and their distribution within the chalk structure play an important role when studying how mechanical properties are affected by the interaction between the rock and the various pore –and flooding fluids. Within this PhD work one will also thus by use of the scanning electron microscope (SEM) study in more detail the minerals before and after mechanical testing with –and without flooding of various brines. The deformation processes or weakening of the chalk, which take place while flooding with seawater, should also be studied by acoustic measurements.
Scope of experimental work (1 PhD and 1 Post doc.) : The objective of the experimental PhD and Post doc studies will be to further examine the behaviour of chalk flooded by seawater under different stress scenarios by performing mechanical tests, acoustic measurements as well as SEM studies. By use of standard acoustic velocity measurement equipment, both with compressional -and shear waves; one would be able to investigate how, for instance, the elastic parameters of the chalk are affected by the chemical composition of the actual flooding fluid. The results from the acoustic velocity measurements will, of course, be compared with the results obtained from standard triaxial tests. Studying chalks by acoustics and SEM, will most certainly give us an opportunity to also gain an improved insight into bonding properties and deformational processes.
Scope of modelling work (1 PhD or alternatively 1 Post doc.) : Modelling of the water weakening effect will be done on pore scale and core (Darcy) scale. The pore scale fluid flow modelling will be based on the Lattice Boltzmann algorithm, combined with thermodynamic modelling of chemical effects. The information from pore scale is to be incorporated in a one dimensional model on core scale in terms of appropriate partial differential equations. The ultimate goal will be to predict and explain the changes in the fabric of the porous media. Changes in the fabric is to be understood as a change in the pore geometry and the mineralogy, really how the sediment texture and pore geometry change as the result of stress and fluid flow. The PhD modelling work will be in collaboration with key personnel from UiS and IRIS.
Merete V. Madland, Ass. Prof.: Project leader
(UiS)
Aksel Hiorth, Senior research engineer
(IRIS)
Steinar Evje, Senior research engineer
(IRIS)
Reidar I. Korsnes, Post doc.
(UiS)
Runar Bøe, Ass. Prof.
(UiS)
Siv. ing Tron Golder Kristiansen:
(Global Geomechanics Advisor
& Geomechanics Network Leader, BP)