Frictional instabilities of basalts and calcite-built marbles in the presence of pressurized H2O- and CO2-rich fluids

Fluid-rock interactions control earthquake nucleation and the evolution of natural and man-induced seismic sequences. Experimental studies of fault frictional properties in the presence of pressurized fluids can provide unique insights into these interaction. We performed 14 friction experiments on cohesive silicate-built (basalts) and calcite-built rocks (Carrara marbles) in the presence of pressurized pure H2O, pure CO2 and H2O+CO2 fluids to investigate the triggering of frictional instabilities associated to CO2 storage in basalts (e.g. injection H2O+CO2 mixtures to fix CO2 to newly formed carbonate minerals) and, to a less extent, the processes driving to earthquake triggering in carbonate-built rocks. Experiments were performed at room temperature on 50/30 mm external/internal diameter hollow-shaped rock cylinders with the rotary shear apparatus SHIVA (INGV, Rome). Sample were inserted in a pressure vessel and the experiments performed under drained conditions. After imposing an initial normal stress of 15 MPa, an initial shear stress of 5 MPa and an initial pore fluid pressure of 2.5 MPa, the pore fluid pressure was increased in steps of 0.1 MPa every 100 s till the main frictional instability was triggered. The main instability was defined as the instant at which the sample accelerated to a slip rate of >0.3 m/s (seismic slip rate). Un-deformed and deformed samples, the slip surfaces, the slipping zones and the wall rocks were investigated with optical microscope, XRD, XRF and micro-Raman spectroscopy; H2O+CO2 and H2O fluids were recovered after the experiments to determine the enrichment of the chemical species (Ca++, Mg++, etc.). Carrara marble was more prone to slip in the presence of pressurized H2O+CO2 mixtures than in pure CO2 and H2O fluids; instead, in basalts, the injection of pressurized H2O+CO2 delayed the main frictional instability with respect to the experiments performed in pure H2O and anticipated with respect to pure CO2 fluids. Main instabilities were preceded by creep and slip burst events ("precursory events"): the number and frequency of slip burst events was larger in the experiments performed on basalts. Moreover, in basalts enriched in clay minerals (1) fault reactivation occurred at lower pore fluid pressures at a given normal stress and (2) the frequency of precursory events decreased, making the fault more “silent” and unstable than the fault made of less altered basalts. In the experiments, fluids may play both a chemical and mechanical role. Pure CO2 mainly contributes to pressurize the experimental faults in both basalts and Carrara marbles, with minimal chemical interaction with the host rock. Instead, H2O+CO2 mixtures resulted in formation of H+ ions which caused dissolution of the two rock types, as suggested by the enrichment in Ca2+ and Mg2+ cations measured in their respective aqueous solutions. Noteworthy, in the case of basalts, the high concentration of the Ca2+ and Mg2+ cations in solution and dissolved from glass, pyroxene and feldspars resulted in precipitation of calcite and dolomite (= mineral carbonation) in the experimental slipping zone. The rapid carbonation processes observed in our experiments, which last only 30-40 minutes, demonstrates the great effectiveness of the large scale CO2 storage projects in basaltic rocks as the CarbFix in Iceland.ope

Frictional, transport properties, and microstructures of simulated basalt faults

I terremoti indotti dalle attività antropiche costituiscono una discriminante per il successo delle attività industriali quali l’iniezione di acque reflue, l’estrazione di petrolio e gas naturale, lo sfruttamento di energia geotermica e lo stoccaggio geologico di anidride carbonica. Negli ultimi anni, i basalti hanno catturato l’attenzione dell’industria del settore energetico e della comunità scientifica, a causa della loro vasta diffusione nella litosfera oceanica e della loro capacità di mineralizzare la CO2 “trasformandola in roccia” (New York Times 9/2/2015). Tale proprietà, consente di fatto di sottrarre a lungo termine l’anidride carbonica presente in atmosfera, contribuendo pertanto alla riduzione locale delle emissioni di CO2 di origine antropogenica. Comprendere le proprietà di attrito, meccaniche e idrologiche di faglie e fratture in basalto ha assunto pertanto un'importanza fondamentale, per le dirette implicazioni riguardanti l'enucleazione dei terremoti, la loro propagazione e l'arresto in ambienti geologici dominati dai basalti. Per meglio comprendere le proprietà meccaniche di faglie e fratture in basalto, e in particolare la fase di enucleazione dei terremoti, esperimenti di attrito sono stati realizzati mediante l’apparato biassiale BRAVA e l’apparato di tipo rotativo SHIVA, entrambi installati presso l’Istituto Nazionale di Geofisica e Vulcanologia (INGV, Roma). Invece, per caratterizzare le proprietà di trasporto delle carote di basalto e delle faglie sperimentali, la trasmissività idraulica è stata misurata mediante il permeametro, prima e dopo gli esperimenti di attrito su SHIVA. Sono stati trattati tre principali argomenti seguendo un approccio sperimentale per caratterizzare: 1) le proprietà di resistenza di attrito, stabilità e di healing delle faglie sperimentali in basalto (ovvero, faglie polverizzate e superfici di faglia) in condizioni di umidità atmosferica e in condizioni bagnate, integrando i dati meccanici con quelli microstrutturali (Capitolo 2); 2) le instabilità dell’attrito ed i processi di carbonatazione delle superfici di faglia sperimentali aventi diversi gradi di alterazione, cagionati dall’iniezione di fluidi ricchi in H2O, CO2, misture H2O-CO2, e Argon (Capitolo 3); 3) le variazioni delle proprietà idromeccaniche delle superfici di faglia sperimentali e la loro influenza sul loro comportamento durante l’iniezione di acqua in pressione (Capitolo 4). Per quanto concerne i cilindri cavi descritti nel capitolo 4, l’analisi accurata dello stato di sforzo negli esperimenti di tipo rotativo, ha richiesto lo sviluppo di un modello basato sui dati sperimentali che tenesse conto della geometria cilindrica dei campioni montati su SHIVA, la quale modifica il modo in cui la pressione di fluido influisce sullo sforzo normale efficace agente sulla faglia (Appendice 1). Tutti i test sono stati realizzati a temperatura ambiente, che può emulare le condizioni di temperatura di un sito energetico a bassa entalpia. In questa tesi, complessivamente si osservano valori del coefficiente di attrito statico intorno a μ ~ 0.6 – 0.8, a diverse condizioni che spaziano dall’umidità atmosferica a quelle sovra-idrostatiche, indipendentemente dello stato di alterazione dei basalti e della composizione chimica del fluido iniettato durante gli esperimenti a breve termine (< 60 min). Pertanto, le faglie in basalto sono considerate “forti”, e gli elevati tassi di healing testimoniano la loro abilità di riguadagnare la resistenza al taglio durante il periodo intersismico. Secondariamente, metto in evidenza come la struttura delle faglie controlli le proprietà di rate and state e la stabilità delle stesse: mentre le polveri sono più propense ad enucleare terremoti (ovvero possiedono un comportamento di indebolimento con l’aumento di velocità: velocity weakening) quando, a seguito di processi cataclastici con riduzione della granulometria, la deformazione diventa localizzata lungo zone di deformazione ben sviluppate, al contrario, le superfici di faglia passano a un comportamento di incremento dell’attrito con l’aumentare della velocità (velocity strengthening), a seguito di processi di dilatanza che accompagnano la produzione di detrito durante lo scivolamento. Infine, si è osservato che i cambiamenti nelle proprietà idromeccaniche durante la pressurizzazione di fluido possono diventare dominanti rispetto agli effetti prodotti dai cambiamenti di attrito di secondo ordine predetti dalle leggi di rate. A tale riguardo, ho rilevato un più pronunciato indebolimento idromeccanico, laddove la trasmissività idraulica della faglia è minore. Questa osservazione fornisce un efficace meccanismo per l’indebolimento delle faglie e in ultima istanza, portare all’enucleazione di terremoti anche nelle porzioni faglie in basalto caratterizzate da un comportamento “velocity strengthening”.Earthquakes induced by anthropic activities are a major concern for the success of the industrial operations associated with in-situ underground wastewater injection, oil and gas withdrawals, geothermal energy exploitation, and geological carbon sequestration. Over the last few decades, basalt rocks have drawn heightened attention from the geo-energy industry and the scientific community because of their widespread occurrence in the oceanic lithosphere and their efficiency to act as carbon sinks, thus contributing to locally reduce the CO2 anthropogenic emissions. Given the direct implications for earthquake nucleation, propagation, and arrest in basaltic-dominated environments, understanding the frictional, mechanical, and transport properties of basalts-bearing faults and fractures has become of paramount importance. To gain better insights on the mechanical behavior of basalt-hosted faults, notably the earthquake nucleation phase, friction experiments were performed using the biaxial deformation machine BRAVA and the rotary-shear apparatus SHIVA, both installed at the National Institute of Geophysics and Volcanology (INGV, Rome), Italy. Whereas, to characterize the transport properties of basalt cores and simulated faults, hydraulic transmissivity was measured on the permeameter and before and after friction tests on SHIVA. Three main scientific topics were addressed using an experimental approach: 1) the frictional strength, stability, and healing properties of basalt-built experimental faults (i.e., simulated gouge and bare rock surfaces) under room-dry and wet conditions, by integrating the mechanical data with fault microstructures (Chapter 2); 2) the frictional instabilities and carbonation processes of simulated initially bare rock surfaces with different degree of alteration, triggered by injection of pressurized H2O, pure CO2 , CO2 - rich water, and Argon (Chapter 3); 3) the hydromechanical properties changes of simulated initially bare rock surfaces and their influence on the fault slip behavior during water pressurization (Chapter 4). The accurate stress paths analysis from rotary-shear tests involving hollow bare rock surfaces in Ch.4 required the development of an experimentally derived model accounting for the cylindrical geometry of SHIVA samples, that modifies the fluid pressure contribution on the effective normal stress acting on the laboratory fault, (Appendix 1). All the tests were performed at ambient temperature, which may mimic the temperature conditions in low enthalpy geo-energy sites in basalts. In this dissertation, overall, I demonstrate that the static friction coefficient of basalts is in the range of μ ~ 0.6 – 0.8, at conditions ranging from room-dry to supra-hydrostatic, regardless of the alteration state of basalts and the fluid chemistry during short-term laboratory experiments (< 60 min). Therefore, basalts are inherently frictionally strong and the high healing rates testify their ability to regain shear strength during the interseismic period. Secondly, I show that fault microstructure controls their frictional stability: while simulated gouge are more prone to host earthquake nucleation (i.e., velocity weakening behavior) when deformation becomes localized along well-developed shear zones formed in response to cataclasis and grain size reduction, bare rock surfaces show the opposite behavior, transitioning to velocity strengthening behavior promoted by dilatancy processes coupled with gouge production during shearing. Finally, I illustrate that changes in coupled hydromechanical properties during fluid pressurization can dominate over the effects of second-order frictional changes predicted by the rate-and state-friction laws. In this regard, I observed that hydromechanical weakening effects become more pronounced the lower the fault transmissivity. This evidence provides an effective mechanism for inducing fault weakening and ultimately, to bring about earthquake slip also in velocity-strengthening basalt fault patches

Frictional Instabilities and Carbonation of Basalts Triggered by Injection of Pressurized H2O- and CO2- Rich Fluids

The safe application of geological carbon storage depends also on the seismic hazard associated with fluid injection. In this regard, we performed friction experiments using a rotary shear apparatus on precut basalts with variable degree of hydrothermal alteration by injecting distilled H2O, pure CO2, and H2O + CO2fluid mixtures under temperature, fluid pressure, and stress conditions relevant for large-scale subsurface CO2storage reservoirs. In all experiments, seismic slip was preceded by short-lived slip bursts. Seismic slip occurred at equivalent fluid pressures and normal stresses regardless of the fluid injected and degree of alteration of basalts. Injection of fluids caused also carbonation reactions and crystallization of new dolomite grains in the basalt-hosted faults sheared in H2O + CO2fluid mixtures. Fast mineral carbonation in the experiments might be explained by shear heating during seismic slip, evidencing the high chemical reactivity of basalts to H2O + CO2mixtures

Frictional Instabilities and Carbonation of Basalts Triggered by Injection of Pressurized H2O- and CO2- Rich Fluids

The safe application of geological carbon storage depends also on the seismic hazard associated with fluid injection. In this regard, we performed friction experiments using a rotary shear apparatus on precut basalts with variable degree of hydrothermal alteration by injecting distilled H2O, pure CO2, and H2O + CO2 fluid mixtures under temperature, fluid pressure, and stress conditions relevant for large-scale subsurface CO2 storage reservoirs. In all experiments, seismic slip was preceded by short-lived slip bursts. Seismic slip occurred at equivalent fluid pressures and normal stresses regardless of the fluid injected and degree of alteration of basalts. Injection of fluids caused also carbonation reactions and crystallization of new dolomite grains in the basalt-hosted faults sheared in H2O + CO2 fluid mixtures. Fast mineral carbonation in the experiments might be explained by shear heating during seismic slip, evidencing the high chemical reactivity of basalts to H2O + CO2 mixtures

Frictional instabilities of basalts and calcite-built marbles in the presence of pressurized H2O- and CO2-rich fluids

Fluid-rock interactions control earthquake nucleation and the evolution of natural and man-induced seismic sequences. Experimental studies of fault frictional properties in the presence of pressurized fluids can provide unique insights into these interaction. We performed 14 friction experiments on cohesive silicate-built (basalts) and calcite-built rocks (Carrara marbles) in the presence of pressurized pure H2O, pure CO2 and H2O+CO2 fluids to investigate the triggering of frictional instabilities associated to CO2 storage in basalts (e.g. injection H2O+CO2 mixtures to fix CO2 to newly formed carbonate minerals) and, to a less extent, the processes driving to earthquake triggering in carbonate-built rocks. Experiments were performed at room temperature on 50/30 mm external/internal diameter hollow-shaped rock cylinders with the rotary shear apparatus SHIVA (INGV, Rome). Sample were inserted in a pressure vessel and the experiments performed under drained conditions. After imposing an initial normal stress of 15 MPa, an initial shear stress of 5 MPa and an initial pore fluid pressure of 2.5 MPa, the pore fluid pressure was increased in steps of 0.1 MPa every 100 s till the main frictional instability was triggered. The main instability was defined as the instant at which the sample accelerated to a slip rate of >0.3 m/s (seismic slip rate). Un-deformed and deformed samples, the slip surfaces, the slipping zones and the wall rocks were investigated with optical microscope, XRD, XRF and micro-Raman spectroscopy; H2O+CO2 and H2O fluids were recovered after the experiments to determine the enrichment of the chemical species (Ca++, Mg++, etc.). Carrara marble was more prone to slip in the presence of pressurized H2O+CO2 mixtures than in pure CO2 and H2O fluids; instead, in basalts, the injection of pressurized H2O+CO2 delayed the main frictional instability with respect to the experiments performed in pure H2O and anticipated with respect to pure CO2 fluids. Main instabilities were preceded by creep and slip burst events ("precursory events"): the number and frequency of slip burst events was larger in the experiments performed on basalts. Moreover, in basalts enriched in clay minerals (1) fault reactivation occurred at lower pore fluid pressures at a given normal stress and (2) the frequency of precursory events decreased, making the fault more “silent” and unstable than the fault made of less altered basalts. In the experiments, fluids may play both a chemical and mechanical role. Pure CO2 mainly contributes to pressurize the experimental faults in both basalts and Carrara marbles, with minimal chemical interaction with the host rock. Instead, H2O+CO2 mixtures resulted in formation of H+ ions which caused dissolution of the two rock types, as suggested by the enrichment in Ca2+ and Mg2+ cations measured in their respective aqueous solutions. Noteworthy, in the case of basalts, the high concentration of the Ca2+ and Mg2+ cations in solution and dissolved from glass, pyroxene and feldspars resulted in precipitation of calcite and dolomite (= mineral carbonation) in the experimental slipping zone. The rapid carbonation processes observed in our experiments, which last only 30-40 minutes, demonstrates the great effectiveness of the large scale CO2 storage projects in basaltic rocks as the CarbFix in Iceland