Arch. Min. Sci., Vol. 52 (2007), No 3, p

. Arch. Min. Sci., Vol. 52 (27), No 3, p. 281 296 281 JOANNA PINIŃSKA* THE NEED FOR HIGH PRESSURE ROCK TESTS FOR A GEOLOGICAL SURVEY AT GREAT DEPTHS POTRZEBA WYSOKOCIŚNIENIOWYCH BADAŃ SKAŁ DLA WGŁĘBNEGO ROZPOZNANIA GEOLOGICZNEGO This paper presents the results of high-pressure and high-temperature laboratory testing of rocks for modelling rock behaviour mechanisms at depths not directly accessible. The uniaxial and triaxial strength tests under controlled pressure and temperature were performed on various rocks: a Kośmin syenite, a Górażdże limestone and a Blue Cloud granite-gneiss from Canada. Samples confined in the thermo-compression chamber were tested using an MTS-815 machine at temperatures (T) 24 and 1 C under uniaxial conditions, and at confining pressures (p) of 3 and 6 MPa. Generally, all the rock types showed a significant increase in compressive strength with an increase of confining pressure while the thermal impact is variable. The strength of granite and granite-gneiss increased under uniaxial compression with an increase of temperature from 24 to 1 C, while the strength of the limestone decreased. Under triaxial compression conditions, an increase in temperature caused a decrease in the strength of granitic rocks and an increase in the strength of the limestone (Fig. 4). Combined pressure and temperature-induced deformation processes result in a variability of rock bulk density (ρ) changes (Fig. 9). Therefore, under the combined impact of pressure and temperature at greater depths, rock density depends on the balance between thermal and mechanical stresses. Thus the rock parameters determined under surface conditions without modelling the impact of great depth conditions cannot be used as a criterion for determining rock properties at greater depths. The simple extrapolation of rock properties from surface conditions to those at a greater depth will be inaccurate. Keywords: deformation, great depths, high pressure, high temperature, rock density, triaxial compression Dla weryfikacji i uszczegółowienia hipotez litologicznych i diagnostyki geologicznej na głębokości powyżej kilkunastu kilometrów opartych na badaniach geofizycznych niezbędne są badania znanych, rzeczywistych ośrodków skalnych w warunkach złożonego stanu naprężeń i temperatury. Badania te wykonywane w termicznych komorach wysokich ciśnień umożliwiają ustalenie rzeczywistych zmian gęstości, wytrzymałości i odkształcalności danej odmiany litologicznej skały na tle mechanizmów przebudowy jej struktury pod wpływem wzrostu ciśnienia i temperatury. * UNIVERSITY OF WARSAW, FACULTY OF GEOLOGY, DEPARTMENT OF GEOMECHANICS, AL. ŻWIRKI I WIGURY 93, 2-89 WARSZAWA, POLAND

282 W pracy przedstawiono wyniki badań laboratoryjnych nad reakcją skał na podwyższone ciśnienie i temperaturę na przykładzie sjenitu z Kośmina (skała magmowa), wapienia z Górażdży (skała osadowa) oraz granitognejsu Silver Cloud (skała metamorficzna). Do badań zastosowano wysokociśnieniową komorę termiczną, stanowiącą wyposażenie sztywnej prasy wytrzymałościowej MTS-815 (rys. 3). Badania prowadzono w warunkach ściskania próbek walcowych o średnicy 5 mm w temperaturze 24 C oraz 1 C, w warunkach jednoosiowego ściskania oraz stosując dwa stopnie ciśnienia okólnego (p) 3 MPa oraz 6 MPa. Rezultaty badań wskazują, że oddziaływanie temperatury, ciśnienia lub obu tych czynników równocześnie może skutkować bądź redukcją bądź wzrostem wytrzymałości skały w stosunku do warunków jednoosiowego ściskania w tradycyjnej temperaturze laboratoryjnej 24 C (rys. 4 i 5). Zależy to od struktury skały, architektury jej zespołu ziarnowego oraz właściwości termicznych. Podwyższona temperatura wywołuje wzrost wytrzymałości skał do chwili gdy rozszerzanie termiczne ziaren nie powoduje ich zniszczenia. Stąd w krystalicznych skałach magmowych i metamorficznych (sjenit, granitognejs) w podwyższonej do 1 C temperaturze w warunkach jednoosiowego ściskania, stwierdzono wyższą wytrzymałość niż w temperaturze 24 C, co jest związane z rozszerzeniem się mocnych ziaren i zwiększeniem ich powierzchni kontaktowych. Natomiast w warunkach trójosiowego stanu naprężeń działanie ciśnienia okólnego (3 MPa i 6 MPa) ograniczało możliwość rozszerzania ziaren, a zniszczenie nie mogących się swobodnie rozszerzać ziaren wywołało redukcję wytrzymałości skały. W osadowych skałach węglanowych (wapienie) o słabych, nieregularnych ziarnach naprężenia termiczne w temperaturze 1 C powodują, w warunkach jednoosiowego ściskania, obniżenie wytrzymałości w stosunku do tej właściwej temperaturze 24 C. Natomiast przy jednoczesnym działaniu ciśnienia (p = 3 MPa oraz 6 MPa) i temperatury do 1 C gdy ziarna od razu niszczone są wskutek naprężeń termicznych, działanie podwyższonego ciśnienia jest nadrzędne i manifestuje się ogólnym trendem wzrostu wytrzymałości skały. Pod działaniem naprężeń termicznych w warunkach trójosiowego stanu naprężeń dochodzi zatem do przemian strukturalnych, których efektem są zróżnicowane zmiany gęstości objętościowej ośrodka skalnego (rys. 9). Zmiany te nawet w zakresie liniowości odkształceń osiowych mają dla różnych poziomów ciśnienia obwodowego (p) i temperatury (t) charakter nieliniowy i początkowo w stanach przedzniszczeniowych prowadzą do wzrostu gęstości objętościowej, natomiast w stanach pozniszczeniowych na skutek kataklastycznego kruchego zniszczenia struktury, szczególnie w skałach krystalicznych wywołują znaczące rozluźnienie materiału. Zmiany strukturalne i wywodzące się z nich nieliniowe zmiany gęstości i wytrzymałości skał znajdują odzwierciedlenie w obrazie pola falowego. Stąd istotą termicznych badań wysokociśnieniowych jest ustalenie dla różnych odmian litologicznych skał, warunków utraty stateczności ich struktury w zmieniających się warunkach równowagi naprężeń termicznych i sił zewnętrznych. Słowa kluczowa: duże głębokości, gęstość skał, odkształcenie, trójosiowe ściskanie, wysoka temperatura, wysokie ciśnienie 1. Foreword Geological diagnostics for depths of more than a dozen kilometres which are not accessible for direct observation can only be based on remote geophysical methods. These methods use measurable parameters of the seismic field or of the density field (gravimetry). Based on a seismic or gravimetric record transformation into a geological image the, contour of the deep structures and their lithology is determined. As the rock material is not genetically homogeneous, this indirect information is ambiguous even under surface conditions and the inaccuracy of interpretation increases with the depth (Pinińska, 26a). Laboratory tests of rocks under high-pressure and high-temperature conditions used for modelling the rock behaviour mechanisms at great depth are the way to provide the more accurate seismic and gravimetric data needed for geological interpretation. The status of those tests is integrally related to the economical needs of the rock engineering and mining

Lublin region 283 industry development. Penetration to ever-increasing depths forces researchers, including those in Poland, to improve testing methodology and devices for more accurate simulation of great depth stress conditions in the laboratory (Frelkiewicz & Lis, 1975; Gustkiewicz et al., 1981; Gustkiewicz, 1985, 1989; Fabre & Gustkiewicz, 1997; Kwaśniewski, 1983; Nowakowski and Nurkowski, 1995; Sanetra and Szedel, 1997; Kłeczek et al., 1999). Such studies are expensive and time consuming (Buhler et al., 26) but another solution is conducting even more expensive in-situ tests in underground laboratories (Martin et al., 199; Martin & Simmons, 1992; Piguet, 21; Ślizowski, 25). Unfortunately, the laboratory studies in Poland have predominantly been aimed at sedimentary rocks only and do not address the issue of changes in the density of rocks under the simultaneous impact of pressure and temperature. However, integrated regional studies of magmatic rocks as well as sedimentary rocks and metamorphic rocks have been carried out at the Department of Geomechanics at the University of Warsaw for some time (Pinińska and Dziedzic, 1996, 24). These studies have proved that the data needed for transformation of remote geophysical data into the geological image of the deep structures, such as rock density, wave velocity and rock strength, are randomly distributed and data for different rock types overlap one another (Pinińska, 22). Consequently, these data do not universally define the lithological type of the rock and its parameters (Fig. 1 a, b). In addition to not being complemented with information on a) [g/cm ] 3 3.5 3. 2.5 2. granite travertine gabbro basal syenit diabase porphyry anortozite norite marble schist gneiss serpentinite tuff dolomite limestone 1.5 2 4 6 8 1 V p [m/s] limestone chest chalk marle sandstone sandstone

Lublin region 284 b) 45 4 granite travertine gabbro basal Uniaxial compressive strength c [MPa] 35 3 25 2 15 1 5 2 4 6 8 V p [m/s] syenit diabase porphyry anortozite norite marble schist gneiss serpentinite tuff dolomite limestone limestone chest chalk marle sandstone sandstone Fig.1. Variability of bulk density (a) and uniaxial compressive strength (b) of various Polish rocks versus longitudinal wave propagation velocity (V p ). Note: at a similar wave velocity (V p ), a particularly high regional density (ρ) variability is characteristic for carbonate rocks from the Lublin region (area marked with a dashed line) and a high strength (σ C ) variability is characteristic for magmatic and metamorphic rocks from the Sudety Mountains Rys. 1. Zróżnicowanie gęstości objętościowej (a) i wytrzymałości na jednoosiowe ściskanie (b) skał Polski w relacji do prędkości fali podłużnej (V p ) na tle litologii. Uwaga: przy podobnej prędkości fali podłużnej (V p ) szczególnie duża zmienność gęstości (ρ) cechuje skały węglanowe Lubelszczyzny (obszar zaznaczony linią przerywaną), a skały magmowe i metamorficzne Sudetów charakteryzuje duża zmienność wytrzymałości (σ C ) the influence of pressure and temperature, they are simply a poor source of information for in-depth interpretation. Based on the triaxial strength tests done in Poland to date, only few data on the variability of rock density under high pressure conditions have been obtained, e.g. for Strzelin granite (Gustkiewicz, 1985) and Mucharz sandstone (Łukaszewski, 24). These show that despite this assessment being determined for the linear part of the axial strain, the density itself increases in a non-linear manner with an increase of confining pressure (Pinińska, 26b). Taking into account that the intermediate principal stress

285 plays an important role in the dilatancy process (Kwaśniewski et al., 23), a linear approximation cannot be used for density variations versus depth under complex geological stress conditions and at great depths. Tests on samples of Kośmin syenite and Mucharz sandstone proved that the bulk density of these rocks increases at a varying rate with increasing confining pressure under both room temperature (24 C) and a high temperature (Fig. 2). a) b) 7 1 6 5 p [MPa] 4 3 Temp. 24 Temp. 1 p [MPa] 5 2 1 2.755 2.751 2.7515 2.752 2.7525 2.753 2.7535 2.754 3 Bulk density [g/cm ] 2.69 2.7 2.71 3 Bulk density [g/cm ] Fig 2. Bulk density (ρ) increase at increasing confining pressure (p) and temperature (T); a) Kośmin syenite, b) Mucharz flysch sandstone (volume density changes calculated based on the volumetric strain at the threshold of relative dilatancy) Rys. 2. Wzrost gęstości objętościowej (ρ) wraz ze wzrostem ciśnienia (p) i temperatury (T); a) sjenit z Kośmina, b) piaskowiec fliszowy z Mucharza (zmiany gęstości (ρ) ustalone na podstawie odkształceń objętościowych na progu dylatancji względnej) At the same time, the rock bulk density can decrease at higher temperatures when microdefects are created due to thermal expansion. Tests carried out on samples of Colombian basaltic rocks showed that the density of these rocks decreased from 2.82 g/cm 3 at 8 C to 2.7 g/cm 3 at 15 C (Carmichael, 199). For a majority of magmatic rocks, increasing the temperature above 1 C results in a decrease in their density by several per cent. In general case, changes in bulk density of more than 1% are a proof of transition into the flow state (Carmichael, 199) or of highly advanced damage to the rock structure. The above shows that determination of rock parameters for great depth conditions based on the surface data is of low accuracy, the more so as an increase in bulk density with an increase of stress and temperature has a non-linear character. The uncertainty of diagnoses is also enhanced by the incompatibility of test methodologies used in different research centres. Therefore, it is obvious that the surface data must be verified by laboratory tests before they are applied to the great depth conditions.

286 2. Verification tests in a high-pressure and high-temperature chamber Pilot studies on the simultaneous influence of pressure and temperature on rock strength, deformability and density were carried out at the Department of Geomechanics at the University of Warsaw. Three rock types of different origins were selected for the studies: magmatic rock Kośmin syenite, sedimentary rock Górażdże limestone and metamorphic rock Blue Cloud granite-gneiss from Canada (used in Poland as decorative material for the Metropolitan Building façade in Warsaw). Tests were conducted using a high pressure (14 MPa) and high temperature (2 C) chamber, combined with the MTS-815 testing machine (Fig. 3). a) b) c) Fig. 3. High pressure and high temperature chamber: a) sample with extensometers, b) chamber installed in the MTS-815 testing machine, c) temperature protective cover Rys. 3. Wysokociśnieniowa komora termiczna: a) próbka z ekstensometrami, b) komora umieszczona w maszynie wytrzymałościowej MTS-815, c) osłona termiczna Rock samples of the following dimensions: diameter (d) = 5 mm, height (h) = 1 mm, slenderness ratio (h/d) = 2. were used in uniaxial compression and conventional triaxial compression tests at temperatures of 24 C and 1 C and confining pressures of 3 MPa and 6 MPa. The axial strain (ε z ), circumferential strain (ε c ), and volumetric strain (ε V ) were recorded by electronic devices during each test. The tests objectives were determination of the following data under different pressures and temperatures: maximum axial stress or major principal stress at failure, stress-strain curves, deformation mode and bulk density changes.

287 3. Test results Table 1 shows the basic test results obtained for magmatic rock (syenite), sedimentary rock (limestone), and metamorphic rock (granite-gneiss). Based on the identification tests under uniaxial compression conditions, all three rocks are defined according to PN-EN ISO 14689-1 as rocks of high strength (1 MPa < σ C < 25 MPa). Laboratory test results Wyniki testów laboratoryjnych TABLE 1 TABLICA 1 Rock type Test type Bulk density ρ syenite limestone granitegneiss [g/cm 3 ] Major principal stress at failure σ 1 max [MPa] Static deformation modulus E st [GPa] Static Poisson s ratio ν st p = MPa, T = 24 C 2.755 155. 37.1.18 p = 3 MPa, T = 24 C 2.736 322.6 66.2.34 p = 6 MPa, T = 24 C 2.737 467.8 64..28 p = 3 MPa, T = 1 C 2.731 33.5 67.5.31 p = 6 MPa, T = 1 C 2.735 446.9 69.6.41 p = MPa, T = 24 C 2.628 147.2 43.3.41 p = 3 MPa, T = 24 C 2.629 186.5 57.2.33 p = 6 MPa, T = 24 C 2.576 266.8 49.2.33 p = MPa, T = 1 C 2.623 136. 59.8.25 p = 3 MPa, T = 1 C 2.551 219.5 6.8.39 p = 6 MPa, T = 1 C 2.632 291.4 45.7.29 p = MPa, T = 24 C 2.628 131.5 41.6.32 p = 3 MPa, T = 24 C 2.69 557.3 62.2.38 p = 6 MPa, T = 24 C 2.64 83.3 61.6.41 p = MPa, T = 1 C 2.622 156.7 53..63 p = 3 MPa, T = 1 C 2.597 539. 57.5.37 p = 6 MPa, T = 1 C 2.616 642.5 58..35 The general trends for the major principal stress at strength failure under different pressure and temperature conditions are shown in Figure 4. The tests proved that all rock types show a significant increase in compressive strength with an increase in confining pressure. Yet, different rock types react differently under increases in temperature. For syenite and granite-gneiss under uniaxial compression conditions, strength increases at a temperature of 1 C. Strength of the carbonate rock (limestone) decreases under the same conditions. Under triaxial compression conditions, when the temperature

288 and pressure were operating simultaneously, rocks behaved in the opposite manner. The temperature-induced strength increase for syenite and granite-gneiss was reduced due to the influence of pressure. At T = 1 C, under the simultaneous influence of pressure and temperature, the strength of both rocks was lower than at T = 24 C (Fig. 4). This indicates that under confined conditions starting from a confining pressure of 3 MPa, the structure of crystalline rocks becomes damaged under the impact of temperature. a) b) Max. axial stress 1max [MPa] 5 45 4 35 3 25 2 15 1 Temp. 24 Temp. 1 Syenite 5 1 2 3 4 5 6 7 Confining pressure p [MPa] c) Max. axial stress 1max [MPa] 9 8 7 6 5 4 3 2 1 Limestone 1 2 3 4 5 6 7 Confining pressure p [MPa] Fig. 4. Maximum axial stress σ 1 max (compressive strength) versus confining pressure (p); a) syenite, b) limestone, c) granite-gneiss Rys. 4. Maksymalne naprężenie osiowe σ 1 max (wytrzymałość na ściskanie) w relacji do ciśnienia okólnego (p); a) sjenit, b) wapień, c) granito-gnejs Max. axial stress 1max [MPa] 35 3 25 2 15 1 5 Granite-Gneiss Temp. 24 Temp. 1 Temp. 24 Temp. 1 1 2 3 4 5 6 7 Confining pressure p [MPa] In the case of limestone, when pressure and temperature acted simultaneously, strength was higher than at T = 24 C. This means that the effect of thermal stress which initially causes a decrease in strength is then reduced by the high pressure, and beginning from a confining pressure of 3 MPa the structure resistance of carbonate rocks increases under the influence of temperature. Therefore, the thermal stress effect is a complex issue as a result of the polymineral, inhomogeneous rock structure. For a rock built of strong grains featuring a high

289 thermal expansion, at the first stage of heating under uniaxial compression conditions, its strength increases due to the expansion of the rock grains and the widening of their contact surfaces. That is why in polymineral syenite and granite-gneiss that include strong quartz grains, strength under uniaxial compression (p = ) is higher at 1 C than at 24 C (Fig. 4a, c). Under triaxial compression, when the thermal expansion is impeded, the strength decreases. In a case where rock does not contain strong grains susceptible to thermal expansion (e.g. pelitic carbonate rock like Górażdże limestone), the grains will be damaged due to the thermal expansion. Therefore, the strength of limestone under uniaxial compression conditions decreases under the influence of temperature, whereas due to the balancing of thermal stresses at T = 1 C and p = 3 MPa and 6 MPa, limestone has a greater strength than at room temperature (Fig. 4b). Strength envelopes (Fig. 5) show different critical behaviours for granite-gneiss, syenite, and limestone under different pressures and temperatures. Generally, the higher the pressure, the greater the strength difference between heated and non-heated rocks. The thermal degradation with increasing pressure is particularly visible in the case of granite-gneiss. Temp 24 Granite-Gneiss Temp 1 [MPa] Temp 24 Temp 1 Syenite 8 Temp 1 Temp 24 Limestone 4-2 1 2 3 4 5 6 7 8 [MPa] n Fig 5. Strength envelopes for granite-gneiss, syenite and limestone under different pressure and temperature conditions Rys. 5. Obwiednie wytrzymałościowe granito-gnejsu, sjenitu i wapienia w różnych warunkach ciśnienia i temperatury

29 Stress-strain curves (σ 1 = f(ε z ), σ 1 = f(ε c ) and σ 1 = f(ε V )) indicate that under applied test conditions, the linearity threshold of strain increases for all rock types very clearly with an increase in pressure but the curve shape depends on the rock type lithology (Fig. 6). For instance, for crystalline granite-gneiss, the axial strain linearity threshold is a) b) Syenite 5 Limestone p=6 45 4 p=6 3 35 25 Axial stress [%] 1 p=3 p= p: 6 Temp 1 p: 3 Temp 1 p: 6 Temp 24 p: Temp 24 p: 3 Temp 24 3 25 2 15 1-1,5-1, -,5,,5 1, Strain [%] 5 c) Axial stress [%] 1 2 15 1 p: Temp 1 p: Temp 24 p: 3 Temp 1 5 p: 6 Temp 1 p: 3 Temp 24 p: 6 Temp 24-1, -,5,,5 1, Strain [%] Granite-Gneiss p=3 8 p=6 7 p=3 6 Axial stress [%] 1 5 4 3 2 p: Temp 1 p: Temp 24 p= p: 3 Temp 1 1 p: 6 Temp 1 p: 3 Temp 24 p: 6 Temp 24-2, -1,5-1, -,5,,5 1, 1,5 2, Strain [%] Fig. 6. Deformation curves under uniaxial and triaxial compression test conditions at different confining pressures (p) and temperatures (T); a) syenite, b) limestone, c) granite-gneiss Rys. 6. Krzywe deformacji w warunkach jednoosiowego i trójosiowego ściskania w różnych warunkach ciśnienia (p) i temperatury (T); a) sjenit, b) wapień, c) granito-gnejs

291 between 5 and 8 times higher under triaxial compression than under uniaxial compression conditions whereas, for limestone, it barely doubles. Axial stress-axial strain curves of both crystalline rocks (syenite and granite-gneiss) are very steep and indicate their susceptibility to brittle failure, unlike a carbonate rock (limestone) that features a high permanent deformation and a capacity for long-term residual strength. The influence of temperature is also clearly shown by the axial stress-volumetric strain curves (Fig. 7). The strong reaction of the granite-gneiss to a higher temperature under increased pressure is indicated by volume changes evidencing structural transformations (Fig. 7c) that are not observed in the limestone (Fig. 7b). The transformation character of syenite (Fig. 7a) is similar to that of granite-gneiss, however not as strong. Different reactions of crystalline and carbonate rocks to pressure and temperature are related to different microstructural bond failure mechanisms. Local, extensional fracturing processes cause the intragranular cracking of crystalline rocks, whereas for carbonate rocks intergranular cracking with shearing and dislocation along the slip surface are common (Pinińska, 25). Different failure mechanisms for crystalline and carbonate rocks, allow a relation to be drawn to the polygonal and trygonal structure model described by Napier and Pearce (1995). In the polygonal model of rock structure (crystalline rocks), the failure occurs by crushing each single grain and then the final failure of the rock structure occurs suddenly without any clear premonitory sign. In the trygonal structure of rocks (limestone), the failure related to the dislocations along the slip surface progresses slowly. Under temperature and pressure acting simultaneously, different paths of the intensification or reduction of both of these processes can be observed. This differentiates the behaviour of crystalline and carbonate rocks depending on the magnitude of thermal stresses generated inside the rock structure and its reduction by the mechanical stresses. The differences in the structural bond failure mechanisms for the polygonal and trygonal models are well represented by different characters of the axial strain (ε z ) vs volumetric strain (ε V ) relationship in Figure 8. For granite-gneiss and syenite (the polygonal model of structure), the volumetric strain (ε V ) increase is accompanied by a simultaneous increase in the axial strain (ε z ). The latter is caused by a permanent but local brittle cracking of grains, with no clear indications for the damage of the entire rock structure. The fluidal structure of granitegneiss (caused by metamorphism) seems to favour this phenomenon. To the contrary in limestone, the indicator of an advanced deformation can easily be observed as a significant volumetric strain (ε V ) accompanied by axial strain (ε z ) that is small yet stable. As a limestone structure undergoes damage by shearing processes (Pinińska, 25), and according to Buhler et al. (26) under triaxial compression such rocks at high temperatures (up to 25 C) react with a strong reduction of cohesion but only a small reduction of the friction angle; their friction-force-dependent compressive strength is less affected by the temperature.

292 a) Syenite b) 5 Limestone 35 Axial stress [MPa] 1 p=6 p=3 p= p: 6 Temp 1 p: 3 Temp 1 p: 6 Temp 24 p: Temp 24 p: 3 Temp 24 45 4 35 3 25 2 15 1 5-1,5-1,3-1,1 -,9 -,7 -,5 -,3 -,1,1,3,5-1, -,8 -,6 -,4 -,2,,2 Volumetric strain v [%] Axial stress [MPa] 1 p=6 p=3 p: 6 Temp 1 p: 3 Temp 1 p: 6 Temp 24 p: Temp 24 p: 3 Temp 24 p: Temp 1 p= 3 25 2 15 1 5 Volumetric strain v [%] c) Granite-Gneiss p=6 8 7 Axial stress [MPa] 1 p=3 6 5 4 3 2 p: 6 Temp 1 p: 3 Temp 1 p: 6 Temp 24 p: Temp 24 p= 1 p: 3 Temp 24 p: Temp 1-1, -,8 -,6 -,4 -,2,,2 Volumetric strain v [%] Fig. 7. Axial stress-volumetric strain curves for different confining pressures (p) and temperatures (T); a) syenite, a) limestone, c) granite-gneiss Rys. 7. Krzywe naprężenie osiowe-odkształcenie objętościowe dla różnych ciśnień okólnych (p) i temperatury (T); a) sjenit, b) wapień, c) granito-gnejs Complex pressure and temperature-induced deformation processes result in rock bulk density changes. Respective test data for granite-gneiss, syenite and limestone at 24 C and 1 C at various deformation stages were analysed versus the confining pressure. The

293 a) b) Syenite 1, Limestone 4, p: 6 Temp 24 p: 6 Temp 1 p: 3 Temp 1 p: 3 Temp 24 p: Temp 24,8,6,4,2, -3, -2,5-2, -1,5-1, -,5,,5 Volumetric strain [%] Axial strain [%] 3, p: 6 Temp 24 2, p: 6 Temp 1 p: 3 Temp 1 p: 3 Temp 24 1, p: Temp 24 p: Temp 1, -3, -2,5-2, -1,5-1, -,5,,5 Volumetric strain [%] Axial strain [%] c) Granite-Gneiss 1,5 p: 6 Temp 24 p: 6 Temp 1 p: 3 Temp 1 p: 3 Temp 24 p: Temp 24 p: Temp 1 1,,5 Axial strain [%], -3, -2,5-2, -1,5-1, -,5,,5 Volumetric strain [%] Fig. 8. Volumetric strain (ε V ) versus axial strain (ε z ) under different confining pressure (p) and temperature (T) conditions; a) syenite, a) limestone, c) granite-gneiss Rys. 8. Zależność odkształcenia objętościowego (ε V ) od odkształcenia osiowego (ε z ) w różnych warunkach ciśnienia (p) i temperatury (T); a) sjenit, b) wapień, c) granito-gnejs data indicate that the density increase may reach 2% and then advanced dilatancy causes a density reduction (Fig. 9). Therefore, depending on the state of stress and the forces of thermal expansion, compaction or cataclastic loosening occurs at a varying rate. The highest compactability and the greatest bulk density were found for granitegneiss at a confining pressure of 6 MPa and temperature of 1 C. The thermal stress is probably also the highest under these conditions. For limestone, the density increased mostly in the uniaxial compression test; in triaxial tests changes in density were very small. Therefore, under a complex state of stress and temperature, density depends on the balance between the thermal state of stress and the mechanical state of stress and the simple use of the surface density of the rock as a criterion for their lithological discrimination at a high depth is inappropriate.

294 Axial stress [MPa] 1 a) b) 5 45 4 35 3 25 2 15 1 p=6 p=3 p: 3 Temp. 1 p: 6 Temp. 1 p: 6 Temp. 24 Syenite 5 p: 3 Temp. 24 p: Temp. 24 2,68 2,7 2,72 2,74 2,76 2,78 9 35 3 25 2 15 1 5 Limestone 2,57 2,58 2,59 2,6 2,61 2,62 2,63 2,64 Bulk density [g/cm 3] Bulk density [g/cm 3] c) p= Axial stress [MPa] 1 Granite-Gneiss p=6 p=3 p: 3 Temp. 1 p: 6 Temp. 1 p: 6 Temp. 24 p: 3 Temp. 24 p: Temp. 24 p: Temp. 1 p= Axial stress [MPa] 1 8 7 6 5 4 3 2 1 p=3 p=6 p: 3 Temp. 1 p: 6 Temp. 1 p: 6 Temp. 24 p: 3 Temp. 24 p: Temp. 24 p: Temp. 1 p= 2,59 2,6 2,61 2,62 2,63 2,64 Bulk density [g/cm 3] Fig. 9. Bulk density changes under different confining pressure (p) and temperature (T) conditions; a) syenite, a) limestone, c) granite-gneiss Rys. 9. Zmiana gęstości objętościowej w różnych warunkach ciśnienia (p) i temperatury (T); a) sjenit, b) wapień, c) granito-gnejs 4. Summary Reaction of magmatic, metamorphic and sedimentary rocks to the simultaneous impact of pressure and temperature up to 6 MPa and 1 C, respectively, are different, depending on different structure, different inter-grain contacts and different strength properties of grains. Therefore, the result is either a reduction in rock strength or its increase. Under uniaxial compression conditions at a temperature of 1 C, magmatic and metamorphic (crystalline) rocks exhibit an increase in strength, which results from the expansion of strong grains and the extension of contact surfaces. Under triaxial

295 compression conditions with pressure inhibiting the crystalline grain expansion, the rock strength decreases with an increase in temperature. A temperature of 1 C alone results in a strength reduction of sedimentary carbonate rocks or in a strength increase under the simultaneous impact of pressure and temperature. This means that depending on the inter-grain contacts and their strength, thermal stresses and pressure either add to each other or subtract from each other. The greatest effects of both stress and temperature were found in the case of strong metamorphic rocks (granite-gneiss), which were most susceptible to brittle cataclastic failure. The variability of the rock structure demonstrated above results in changeability in the bulk density of rocks. Even in the linear range of deformation the density increases nonlinearly in a manner specific to the given lithological variety. It is difficult, therefore, to extrapolate it to great depth conditions without detailed study. It should be added, that in the conditions of complex geological stresses, the extrapolation becomes even more difficult, particularly when in the post-failure state a local cataclastic loosening of rock material occurs. The factors discussed above are responsible for the heterogeneity of the elastic wave field in rock masses which results in unequivocal geological identifications based on the varying wave velocity (Bakun-Czubarow, 26). It is obvious, therefore, that hypotheses related to a detailed survey of geological structures at great depths require tests on known rocks under high-pressure and high-temperature conditions for verification of remote geophysical measurements. The MTS system with its high-pressure and high-temperature chamber facilitates such tests because it is open to the installation of additional test software and sensors. The data acquisition and processing system is compatible with well-known and widely applied spreadsheet programs such as Surfer or Microsoft Excel. Yet, the system has a number of significant drawbacks, for example: no sample deformation viewing option when the sample is enclosed inside the pressure chamber during the triaxial test; a difficult and lengthy tuning procedure from the uniaxial test to the triaxial test; large thermal inertia of the chamber (lengthy warming-up and cooling down). That is why each test with the application of high pressure and temperature is extremely expensive and time-consuming. The most significant drawbacks, however, are the very high costs of service and spare parts. Thus, the fear of damaging the apparatus and/or sensors and extensometers discourages more complex experiments. REFERENCES Bakun-Czubarow N., 26. Wybrane parametry minerałów i skał w funkcji ciśnienia i temperatury oraz ich przydatność do interpretacji danych geofizycznych. Prace PIG. t. 188. Wyd. PIG, Warszawa. 9-24. Buhler M., Mutschler Th., Natau O., 26. Investigation of the stability of deep wellbore in sedimentary rocks including time and temperature effects. Universität Karlsruhe, Institut für Bodenmechanik und Felsmechanik (Poster). Carmichael R.S. (ed.), 1989. CRC Practical Handbook of Physical Properties of Rocks and Minerals. CRC Press, Boca Raton. 741 p.

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