Deformation of Chalk by Pressure Solution

Roland Hellmann Crustal Fluids Group, LGIT-IRIGM BP53X, 38041 Grenoble Cedex, France

J. P. Gratier Crustal Fluids Group, LGIT-IRIGM BP53X, 38041 Grenoble Cedex, France

P. Renders Crustal Fluids Group, LGIT-IRIGM BP53X, 38041 Grenoble Cedex, France

Experimental Conditions

The deformation behaviour of chalk from the Paris basin was investigated using an open triaxial compaction cell. The confining stress (s3) was maintained constant at 4.0 MPa, while the vertical (axial) stress (s1) varied from 5.0 to
8.8 MPa. The experiments were run at temperatures ranging from 25 to 90ƒC. The compaction cell was configured to permit the flow of fluids through the sample under stress. The fluid pressure could be set independently of the applied stress with a back pressure regulator. The maximum experimental fluid pressure was 2.0 MPa, and flow rates ranged from 0.01 to 0.1 ml/min. The fluids that were reacted with the chalk were either degassed, deionized water, or a saline solution (0.05 m MgSO4 + 0.5 m NaCl). All samples were initially dried at 60-100ƒC for a few days. The length of individual experiments was variable, ranging from several weeks to 3 months.

Preliminary Experimental Results

The time evolution of the axial deformation of chalk samples was measured as a function of several experimental parameters; presence or absence of fluids, fluid pressure (Pf), fluid flow rate, applied differential stress (s1 - s3), temperature, and fluid composition.

In order to exclude the possibility that the recorded deformation was attributable to a purely mechanical mechanism(s), several samples were deformed in the dry state
(s1 - s3 = 2 to 4 MPa). After an initial period of rapid deformation (ª0.5%), the samples ceased to deform (i.e. strain rate (b) =0). Only the permeation of circulating fluids caused deformation to be reinitiated. Deformation in the presence of fluids was always characterized by rapid strain rates over several days, followed by a gradual approach to constant, steady-state strain rates over periods of weeks to months. This result indicates the critical importance of fluids in maintaining long term steady-state rates of deformation. Typical amounts of total strain recorded were in the range of 0.5-2%, and strain rates were on the order of 10-10 - 10-9 s-1.

Changes in fluid pressure (0.5-2.0 MPa) had an immediate effect on deformation, although steady-state deformation rates were relatively insensitive to these pressure changes. On the other hand, fluid flow rates did influence steady-state rates of deformation. In some runs, fluid circulation was stopped, thereby causing the strain rate to decrease by up to a factor of 3. This effect can be related to a decrease in the chemical affinity of the dissolution reaction at grain-to-grain contacts. This is indirect evidence for a pressure solution mechanism of deformation.

The effect of increased differential stress was to increase the steady-state strain rate. A log-log plot of strain rate vs. applied differential stress is linear, with a slope ª 1. The response of deformation rates to increasing temperature was also positive, but not very pronounced. The lack of a strong temperature dependence (i.e. low energy of activation) suggests that the rate of dissolution is not surface reaction controlled, but rather is controlled by diffusion in the fluid film at grain contacts.

The last parameter that was examined was the chemical composition of the fluid. Preliminary results in water and in saline solutions show that rates of deformation decrease in the latter. This is as a surprising result since chalk (calcite) is far more soluble in saline solutions than in pure water. This effect needs to be substantiated, however. Additional experiments are planned that will use non-polar solvents, such as alcohol. Deformation in fluids in which the solid is sparingly soluble is predicted to be much slower. If this can be substantiated, then the importance of a chemical mechanism inherent to pressure solution reactions during deformation can be confirmed.