The permeability of fractured rocks in pressurised volcanic and geothermal systems

AbstractThe connectivity of rocks’ porous structure and the presence of fractures influence the transfer of fluids in the Earth’s crust. Here, we employed laboratory experiments to measure the influence of macro-fractures and effective pressure on the permeability of volcanic rocks with a wide range of initial porosities (1–41 vol. %) comprised of both vesicles and micro-cracks. We used a hand-held permeameter and hydrostatic cell to measure the permeability of intact rock cores at effective pressures up to 30 MPa; we then induced a macro-fracture to each sample using Brazilian tensile tests and measured the permeability of these macro-fractured rocks again. We show that intact rock permeability increases non-linearly with increasing porosity and decreases with increasing effective pressure due to compactional closure of micro-fractures. Imparting a macro-fracture both increases the permeability of rocks and their sensitivity to effective pressure. The magnitude of permeability increase induced by the macro-fracture is more significant for dense rocks. We finally provide a general equation to estimate the permeability of intact and fractured rocks, forming a basis to constrain fluid flow in volcanic and geothermal systems.</jats:p

Sintering of vesiculating pyroclasts

Hot volcanic pyroclasts can sinter, vesiculate, and outgas in concert – a combination of processes which remains poorly constrained. And yet this combination of processes can occur coincidently during deposition from pyroclastic density currents, in conduit-filling pyroclastic debris, and in tuffisites. In many of these settings, it is the sintering-driven evolution of permeability that is key to gas transport through the evolving deposit. Here, we experimentally and theoretically investigate the evolution of the permeable networks during sintering of hot fragmental volcanic systems, which are hydrous and oversaturated at the experimental conditions. Firstly, we find that vesiculation results in shutting of the inter-granular porous network as bubble growth drives expansion of the particles into one another, destroying interconnected pores. Secondly, we observe that degassing by diffusion out of the particle edge results in contraction of the vesicular particles, re-opening pore spaces between them. Therefore, we find that vesiculation, and diffusive outgassing compete to determine both the intra-fragment vesicularity and the permeability during sintering. The development of intra-fragment vesicularity directly impacts the inter-fragment pore space and its connectivity, which decreases during vesiculation and subsequently increases during diffusive outgassing, prompting complex, non-linear permeability evolution.The relative dominance of these processes is fragment size dependent; proportionally, fine fragments lose gas at a higher rate than coarser fragments during diffusive outgassing due to larger surface area to volume ratios. As the systems progress, larger fragments retain a higher proportion of gas and so attain greater vesicularities than finer ones – and therefore, the coarse fragmental pyroclasts experience a greater, yet transient, reduction in connected porosity and permeability. We suggest that where vesiculation is sufficient, it can lead to the complete loss of connected porosity and the sealing of permeable pathways much earlier than in a sintering-only system. Our results suggest that classical sintering models must be modified to account for these vesiculation and diffusive degassing processes, and that only a combined vesiculation, sintering, and diffusive outgassing model can resolve the evolution of permeability in hot clastic volcanic systems

Frictional Behaviour, Wear and Comminution of Synthetic Porous Geomaterials

During shearing in geological environments, frictional processes, including the wear of sliding rock surfaces, control the nature of the slip events. Multiple studies focusing on natural samples have investigated the frictional behaviour of a large suite of geological materials. However, due to the varied and heterogeneous nature of geomaterials, the individual controls of material properties on friction and wear remain unconstrained. Here, we use variably porous synthetic glass samples (8, 19 and 30% porosity) to explore the frictional behaviour and development of wear in geomaterials at low normal stresses ( 641\ua0MPa). We propose that porosity provides an inherent roughness to material which wear and abrasion cannot smooth, allowing material at the pore margins to interact with the slip surface. This results in an increase in measured friction coefficient from <0.4 for 8% porosity, to <0.55 for 19% porosity and 0.6\u20130.8 for 30% porosity for the slip rates evaluated. For a given porosity, wear rate reduces with slip rate due to less asperity interaction time. At higher slip rates, samples also exhibit slip weakening behaviour, either due to evolution of the slipping zone or by the activation of temperature-dependent microphysical processes. However, heating rate and peak temperature may be reduced by rapid wear rates as frictional heating and wear compete. The higher wear rates and reduced heating rates of porous rocks during slip may delay the onset of thermally triggered dynamic weakening mechanisms such as flash heating, frictional melting and thermal pressurisation. Hence porosity, and the resultant friction coefficient, work, heating rate and wear rate, of materials can influence the dynamics of slip during such events as shallow crustal faulting or mass movements

Characterization of acoustic infrasound signals at Volcán de Fuego, Guatemala: a baseline for volcano monitoring

Monitoring volcanic unrest and understanding seismic and acoustic signals associated with eruptive activity is key to mitigate its impacts on population and infrastructure. On June 3, 2018, Volcán de Fuego, Guatemala, produced a violent eruption with very little warning. The paroxysmal phase of this event generated pyroclastic density currents (PDC) that impacted nearby settlements resulting in 169 fatalities, 256 missing, and nearly 13,000 permanently displaced from their homes. Since then, Volcán de Fuego has been instrumented with an extensive network of seismic and infrasound sensors. Infrasound is a new monitoring tool in Guatemala. A key step toward its effective use in volcano monitoring at Volcán de Fuego is establishing a baseline for the interpretation of the recorded signals. Here, we present the first comprehensive characterization of acoustic signals at Volcán de Fuego for the whole range of surface activity observed at the volcano. We use data collected during temporary deployments in 2018 and from the permanent infrasound network. Infrasound at Fuego is dominated by the occurrence of short-duration acoustic transients linked to both ash-rich and gas-rich explosions, at times associated with the generation of shock waves. The rich acoustic record at Fuego includes broadband and harmonic tremor, and episodes of chugging. We explore the occurrence of these signals in relation to visual observations of surface activity, and we investigate their source mechanisms within the shallow conduit system. This study provides a reference for the interpretation of acoustic signals at Volcán de Fuego and a baseline for real-time monitoring of its eruptive activity using infrasound data. Our results suggest that changes in the style of activity and morphology of the summit crater are reflected in the acoustic signature of eruption; as such our study provides a reference for the interpretation of acoustic signals at Volcán de Fuego and a baseline for real-time monitoring of its eruptive activity using infrasound

Attitude toward contraception and abortion among Curaçao women. Ineffective contraception due to limited sexual education?

Background In Curaçao is a high incidence of unintended pregnancies and induced abortions. Most of the induced abortions in Curaçao are on request of the woman and performed by general practitioners. In Curaçao, induced abortion is strictly prohibited, but since 1999 there has been a policy of connivance. We present data on the relevance of economic and socio-cultural factors for the high abortion-rates and the ineffective use of contraception. Methods Structured interviews to investigate knowledge and attitudes toward sexuality, contraception and abortion and reasons for ineffective use of contraceptives among women, visiting general practitioners. Results Of 158 women, 146 (92%) participated and 82% reported that their education on sexuality and about contraception was of good quality. However 'knowledge of reliable contraceptive methods' appeared to be - in almost 50% of the cases - false information, misjudgements or erroneous views on the chance of getting pregnant using coitus interruptus and about the reliability and health effects of oral contraceptive pills. Almost half of the interviewed women had incorrect or no knowledge about reliability of condom use and IUD. 42% of the respondents risked by their behavior an unplanned pregnancy. Most respondents considered abortion as an emergency procedure, not as contraception. Almost two third experienced emotional, physical or social problems after the abortion. Conclusions Respondents had a negative attitude toward reliable contraceptives due to socio-cultural determined ideas about health consequences and limited sexual education. Main economic factors were costs of contraceptive methods, because most health insurances in Curaçao do not cover contraceptives. To improve the effective use of reliable contraceptives, more adequate information should be given, targeting the wrong beliefs and false information. The government should encourage health insurance companies to reimburse contraceptives. Furthermore, improvement of counseling during the abortion procedure is important

Integrated constraints on explosive eruption intensification at Santiaguito dome complex, Guatemala

Protracted volcanic eruptions may exhibit unanticipated intensifications in explosive behaviour and attendant hazards. Santiaguito dome complex, Guatemala, has been characterised by century-long effusion interspersed with frequent, small-to-moderate (<2 km high plumes) gas-and-ash explosions. During 2015–2016, explosions intensified generating hazardous ash-rich plumes (up to 7 km high) and pyroclastic flows. Here, we integrate petrological, geochemical and geophysical evidence to evaluate the causes of explosion intensification. Seismic and infrasound signals reveal progressively longer repose intervals between explosions and deeper fragmentation levels as the seismic energy of these events increased by up to four orders of magnitude. Evidence from geothermobarometry, bulk geochemistry and groundmass microlite textures reveal that the onset of large explosions was concordant with a relatively fast ascent of a deeper-sourced (∼17–24 km), higher temperature (∼960–1020◦C) and relatively volatile-rich magma compared to the previous erupted lavas, which stalled at ∼2 km depth and mingled with the left-over mush that resided beneath the pre-2015 lava dome. We interpret that purging driven by the injection of this deep-sourced magma disrupted the long-term activity, driving a transition from low energy shallow shear-driven fragmentation, to high energy deeper overpressure-driven fragmentation that excavated significant portions of the conduit and intensified local volcanic hazards. Our findings demonstrate the value of multi-parametric approaches for understanding volcanic processes and the triggers for enigmatic shifts in eruption style, with the detection of vicissitudes in both monitoring signals and petrological signatures of the eruptive products proving paramount

Rate-dependence of the compressive and tensile strength of granites

The strength and rupture of geomaterials are integral to subsurface engineering practices, such as those required to optimise geothermal energy extraction. Of particular importance is the time- and strain-rate-dependence of material strength, which dictates the energy released upon failure, and impacts the magnitude of induced seismicity, fracture architecture and thus hydraulic conductivity and system permeability. Here, we performed a series of uniaxial compression and Brazilian tensile strength measurements at a range of deformation rates in order to constrain the impact of strain rate on the strength of G603 granite. The dense, low permeability, medium-grained granites were mechanically tested at 4 strain rates (or diametric equivalent strain rates in the case of Brazilian tests) from 10−5 to 10−2 s−1, such that sample failure was achieved in anything from below 1s at the fastest rate in tension, to over 1000s at the slowest rate in compression. The applied rates encompassed those recommended by ISRM and ASTM material testing standards for compressive and Brazilian tensile testing. We found a significant rate strengthening effect, whereby compressive and tensile strength both increased by approximately 35 % across the 4 orders of magnitude of strain rate tested. We found that the static Young's modulus remained relatively constant across this range of deformation rates, however variability was reduced at faster rates, owing to the reduced time for equilibration of the system to imposed stresses. The lower strength at slower strain rates causes smaller stress drops, indicating that rocks driven to compressive and tensile failure at slower rates release less energy upon failure. Such constraints of the strain-rate-dependence of material strength, in contrast to the use of standardised material characteristics conventionally used in Engineering Geology applications, will prove useful as we develop increasingly sophisticated strategies such as cyclic soft stimulation to access resources using less energy, whilst reducing environmental risk and producing less waste.</p

Evolution of Mechanical Properties of Lava Dome Rocks Across the 1995–2010 Eruption of Soufrière Hills Volcano, Montserrat

Lava dome collapses pose a hazard to surrounding populations, but equally represent important processes for deciphering the eruptive history of a volcano. Models examining lava dome instability rely on accurate physical and mechanical properties of volcanic rocks. Here we focus on determining the physical and mechanical properties of a suite of temporally-constrained rocks from different phases of the 1995–2010 eruption at Soufrière Hills volcano in Montserrat. We determine the uniaxial compressive strength, tensile strength, density, porosity, permeability, and Young’s modulus using laboratory measurements, complemented by Schmidt hammer testing in the field. By viewing a snapshot of each phase, we find the highest tensile and compressive strengths in the samples attributed to Phase 4, corresponding to a lower permeability and an increasing proportion of isolated porosity. Samples from Phase 5 show lower compressive and tensile strengths, corresponding to the highest permeability and porosity of the tested materials. Overall, this demonstrates a reliance of mechanical properties primarily on porosity, however, a shift toward increasing prevalence of pore connectivity in weaker samples identified by microtextural analysis demonstrates that here pore connectivity also contributes to the strength and Young’s Modulus, as well as controlling permeability. The range in UCS strengths are supported using Schmidt hammer field testing. We determine a narrow range in mineralogy across the sample suite, but identify a correlation between increasing crystallinity and increasing strength. We correlate these changes to residency-time in the growing lava dome during the eruption, where stronger rocks have undergone more crystallization. In addition, subsequent recrystallization of silica polymorphs from the glass phase may further strengthen the material. We suggest the variation in physical and mechanical rock properties shown within the Soufrière Hills eruptive products be included in future structural stability models of the remaining over-steepened dome on Montserrat, and that consideration of rock heterogeneity and its temporal variation if possible, be made in other, similar systems