Effects of temperature cycles on mechanical parameters of chalk

PhD Project: Thermal properties of chalk. The role of pore fluids, minerals and diagenesis

Tijana Livada 2, Anders Nermoen 1,2, Reidar Inger Korsnes 2, Ida Lykke Fabricius 3

1 The National IOR Center of Norway 2 University of Stavanger, 3 Technical University of Denmark,

References: A. Luque, B. Leiss, P. Alvarez-Lloret, G. Cultrone, S. Seigesmund, E. Sebastian, and C. Cardell. Potential thermal expansion of calcitic and dolomitic marble from Andalusia (Spain). Journal of Applied Crystallography, 44:122{1237, 2011. J. L. Rosenholtz and D. T. Smith. Linear thermal expansion of calcite, var. Iceland Spar, and Yule marble. American Mineralogist, 35:1049-1054, 1950.

Method

Results/Discussion

Brazilian test for tensile strength. The results are shown in the probability distribution in Figure 5a and the

main observations are:

• The average tensile strength does not depend on the number of temperature cycles (Figure 1). As seen

in Figure 5b where the results are sorted from low to high value, the highest tensile strength remains

the same for all series. However, since the lowest tensile strength is significantly lower for the 50 cycles

tests this is reflected in the std. dev. that was 0.8 MPa compared to 0.4-0.6 MPa in the other tests.

• Tensile strength shows no dependency on porosity (see Figure 5c).

Hydrostatic tests for bulk modulus dynamics

The stress-strain during stress cycle with constant temperature and with temperature cycles included are

shown in the left and right column of Figure 6, respectively.

• The loading and un-loading curves display hysteresis effects.

• More strain is accumulated when the temperature has been cycled. This may indicate changes in

The stiffness parameters increase (work hardening) less in the temperature cycle test than the

constant temperature tests (see Table 1).

The plastic component of the total volume strain after unloading is increased

Introduction

Future work To present, the hydrostatic test has only been performed on chalk saturated with calcitic brine. Future

work includes the same method but with chalk saturated with Isopar H oil, as well as the repetition of the

same method on different lithologies (sandstone and shale).

Figure 2. Some sample of the chalk used for testing

Figure 3. Brazilian test setup and core preparation

Hydrostatic tests for elastic modulus

Two core samples were saturated by calcitic water and tested in

triaxial cell (Figure 4). Test 1: Confining stress cycles at constant

temperature (30℃). The confining stress was cycled between 1.2

and 5.2 MPa and the stress-strain behavior was analyzed to

quantify the bulk modulus evolution over 10 cycles (one cycle each

day). Test 2: To quantify the impact of temperature, the temperature

was cycled between 30 and 130℃ (high temperature for 8 hours

and low for 16 hours) for each stress cycle. The stress-strain

evolution is compared to the test performed at constant temperature

such that the impact of temperature cycles could be observed.

Figure 4. Experimental set up for the hydrostatic test and core preparation

Figure 5. Results from the Brazilian compressive test: (a) probability of the sample to fail within tensile strength range.

In (b) the variation of chalk failure for different cycles; (c) Tensile strength vs porosity.

Figure 1. Left: marble displaying flakes and granular

disintegration in Granada, Spain; right: thermal expansion

coefficient and residual strain r for marble samples WM, TM,

AR and FH from Granada, Spain (Luque, 2011).

Acknowledgement: The authors acknowledge the Research Council of Norway and the industry partners; ConocoPhillips

Skandinavia AS, BP Norge AS, Det Norske Oljeselskap AS, Eni Norge AS, Maersk Oil Norway AS, DONG Energy

A/S, Denmark, Statoil Petroleum AS, ENGIE E&P NORGE AS, Lundin Norway AS, Halliburton AS, Schlumberger

Norge AS, Wintershall Norge AS of The National IOR Centre of Norway for support.

Figure 6. Results from hydrostatic

test for constant temperature (left

column a, c and e) and with

temperature cycling (right column b,

d and f). In a and b the stress

versus volume strain are shown

without and with temperature cycle

in between, respectively. In c and d

the slope in stress-strain curves

during loading – unloading cycle are

shown (1.2 MPa → 5.2 MPa → 1.2

MPa). In e and f, the irreversible

component from each stress cycle

are plotted.

Conclusion Two different test methods were used to determine if temperature cycling would effect chalk mechanical

strength. The Brazilian test on dry samples reveals that there is no significant weakening observed with

temperature variation, however the standard deviation of the tensile strength is doubled. Hydrostatic tests

shows that cycling the temperature for each stress cycle lead to additional volumetric strain. The results in

Figure 6 display a larger irreversible component than if the temperature was kept constant. In addition, the irreversible component increases with the number of cycles as opposed to becoming constant.

Material

Chalk from Kansas was used in this study. It is a firmly

indurated chalk with wackestone texture. The samples have

average porosity 0.34 and permeability 0.9 mD (Figure 2).

Brazilian tests for tensile strength

In the Brazilian test, a sample is loaded by two opposing normal

strip loads and the tensile strength, T0, is calculated by:

𝑇0 = 2𝐹

𝜋𝐷𝐿

Where F is the applied force and D and L are diameter and length of

the sample (Figure 3),

Sixty disk shaped dry samples tested after heating/cooling cycles:

(135ºC for 8 hours, allowed to cool down for 16 hours).

Deformation induced by thermal cycles cause marble

cladding on marble monuments. Temperature cyclicity

occurs at repeated seasonal change (Figure 1). As marble,

chalk is mostly composed of calcite. The thermal expansion

of the calcite mineral is temperature dependent and

anisotropic. If temperature is increased, the grain expands

parallel to the c-axis, while it contracts in the perpendicular

direction (Rosenholtz and Smith, 1950). The spalling of

chips observed in marble arise due to the combined effect of

the expansion of single calcite crystals, thermal expansion

coefficient differences, and the crystalline angle between

neighboring crystals.

Can thermal expansion differences at grain level lead to

degradation of inter-granular cementation in chalks? We

hypothesize that the difference in thermal expansion

coefficient cause weakening of chalk if cementation is

present.

Which forces dictate chalk strength? If cementation is the major contributor to binding chalk

grains together, then repeated cooling/heating cycles would

induce weakening. However, if electrostatic forces (e.g. van

der Waal) are bind chalk grains, then temperature cycling would not impact the mechanical properties.

# Const. temperature Temperature cycles

Kbulk

loading

Kbulk

unloading

Kbulk

loading

Kbulk

unloading

2 2437 2767 2260 2951

4 2478 2705 2666 3397

6 2466 2644 2742 3369

8 2504 2664

10 2520 2545

Table 1. Bulk modulus evolution

during loading and unloading.