Abstract
A variation of temperature produces a change in the microstructure of the rock due to the mineral thermal expansion and its residual strain. Depending on the temperature cycle and texture, microstresses may lead to the development of preexistent cracks or the creation of a new and irreversible cracking. The effect of temperature on reservoir rocks is an important topic since it conditions the permeability and the fluid flow. Two main questions arise from this: the first is if an irreversible cracking threshold is attained in the reservoir rocks at low temperature geothermal systems (around 100 °C); the second one is about the influence of thermal fatigue by the repetition of heating–cooling cycles on the different rock types. To answer these questions, four reservoir rocks (chalk, sandstone, fresh granite, and weathered granite) were submitted to two different thermal regimes. The first test was conceived to detect the irreversible cracking threshold, and for that, the rocks were submitted to progressive heating (90°, 100°, 110°, 120°, and 130 °C). The second test consisted of doing cycles of fast heating of the samples up to 200 °C. The microstructure variation was assessed by means of a scanning electron microscope, mercury porosimetry, and capillary water uptake combined with passive infrared thermography. Infrared thermography is an emerging tool in the field of rock study, used to detect water masses or determine thermal properties. The water transfer during the capillary tests of the rocks, before and after the tests, was monitored with this technique. In addition, the cooling rate index, a non-destructive parameter to detect cracking development, was calculated. The results made it possible to differentiate the behaviours in relation to the rock type, with a chalk and a weathered granite less susceptible to thermal stresses than a fresh granite and sandstone. In addition, infrared thermography resulted in being a very useful indirect technique to detect the changes on the surface, although they do not always correlate to the bulk microstructural changes.
Highlights
The influence of temperature on rock behaviour is studied in several geological domains, such as petroleum extraction, geothermal activity, or storage of radioactive waste, involving many rocks [1,2,3,4]
In the case of a fluid flow, chalk crystals are prone to dissolution or remobilisation, which creates a modification in chalk microstructure [12]
The main components of the rock are well-preserved coccolith fragments of different shapes, the majority are elliptical with no preferred orientation
Summary
The influence of temperature on rock behaviour is studied in several geological domains, such as petroleum extraction, geothermal activity, or storage of radioactive waste, involving many rocks [1,2,3,4]. The research studies show that high temperature changes the minerals and the void system of the rocks and the flow of the fluid in the rock may be enhanced or reduced [5,6,7,8,9]. The intensity of the modifications of the porous network is related to inherent stone characteristics such as mineralogy, texture, weathering degree [10], and thermal regimes involving extreme temperatures and repetition cycles. Sandstone, and granite are reservoir rocks, and their properties and behaviours facing different temperatures and pressure are often compared due to their different porous network, that is high microporosity for chalk, mesoporous distribution for sandstone, and low porosity of fissural type for granites. Despite the high and anisotropic thermal extension of calcite crystals, the high microporosity usually accommodates the expansion. In the case of a fluid flow, chalk crystals are prone to dissolution or remobilisation, which creates a modification in chalk microstructure [12]
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