Nicola Tisato

A new Seismic Wave Attenuation Module to experimentally measure low-frequency attenuation in extensional mode

A Seismic Wave Attenuation Module is developed to experimentally measure the attenuation in extensional-mode QE−1 and the Youngs modulus of copper jacketed, 60 mm long and 25.4 mm in diameter samples in a gas medium (Paterson) rig. The new module is suitable for natural rock samples and was tested under confining pressure up to 50 MPa and room temperature. The module is designed to operate at a strain < 10−6, for which rocks behave linearly. To calculate attenuation, both the applied force and the bulk shortening of the sample are measured employing linear variable differential transformers. This technique allows measuring samples with a high degree of heterogeneity. Attenuation at low-seismic frequencies (10−2–102 Hz) is obtained for rocks at dry and various saturation conditions. We present a series of measurements on Berea sandstone in a room-dry condition and saturated with different fluids: water and glycerine solutions with viscosities of 10 cP and 22 cP, respectively, at a confining pressure of 10 MPa and with a pore pressure of 1 MPa. The accuracy of the attenuation data expressed as a phase shift is 0.0019 rad.

An overview of laboratory apparatuses to measure seismic attenuation in reservoir rocks

Intrinsic wave attenuation at seismic frequencies is strongly dependent on rock permeability, fluid properties, and saturation. However, in order to use attenuation as an attribute to extract information on rock/fluid properties from seismic data, experimental studies on attenuation are necessary for a better understanding of physical mechanisms that are dominant at those frequencies. An appropriate laboratory methodology to measure attenuation at seismic frequencies is the forced oscillation method, but technical challenges kept this technique from being widely used. There is a need for the standardization of devices employing this method, and a comparison of existing setups is a step towards it. Here we summarize the apparatuses based on the forced oscillation method that were built in the last 30 years and were used to measure frequency-dependent attenuation in fluid-saturated and/or dry reservoir rocks under small strains (10 −8 ‐10 −5 ). We list and discuss important technical aspects to be taken into account when working with these devices or in the course of designing a new one. We also present a summary of the attenuation measurements in reservoir rock samples performed with these apparatuses so far.

Synchrotron-based X-ray tomographic microscopy for rock physics investigations

ABSTRACTSynchrotron radiation X-ray tomographic microscopy is a nondestructive method providing ultra-high-resolution 3D digital images of rock microstructures. We describe this method and, to demonstrate its wide applicability, we present 3D images of very different rock types: Berea sandstone, Fontainebleau sandstone, dolomite, calcitic dolomite, and three-phase magmatic glasses. For some samples, full and partial saturation scenarios are considered using oil, water, and air. The rock images precisely reveal the 3D rock microstructure, the pore space morphology, and the interfaces between fluids saturating the same pore. We provide the raw image data sets as online supplementary material, along with laboratory data describing the rock properties. By making these data sets available to other research groups, we aim to stimulate work based on digital rock images of high quality and high resolution. We also discuss and suggest possible applications and research directions that can be pursued on the basis o...

Bubbles attenuate elastic waves at seismic frequencies: First experimental evidence

The migration of gases from deep to shallow reservoirs can cause damageable events. For instance, some gases can pollute the biosphere or trigger explosions and eruptions. Seismic tomography may be employed to map the accumulation of subsurface bubble-bearing fluids to help mitigating such hazards. Nevertheless, how gas bubbles modify seismic waves is still unclear. We show that saturated rocks strongly attenuate seismic waves when gas bubbles occupy part of the pore space. Laboratory measurements of elastic wave attenuation at frequencies <100 Hz are modeled with a dynamic gas dissolution theory demonstrating that the observed frequency-dependent attenuation is caused by wave-induced-gas-exsolution-dissolution (WIGED). This result is incorporated into a numerical model simulating the propagation of seismic waves in a subsurface domain containing CO2-gas bubbles. This simulation shows that WIGED can significantly modify the wavefield and illustrates how accounting for this physical mechanism can potentially improve the monitoring and surveying of gas bubble-bearing fluids in the subsurface.

Microbial mediation of complex subterranean mineral structures

Helictites—an enigmatic type of mineral structure occurring in some caves—differ from classical speleothems as they develop with orientations that defy gravity. While theories for helictite formation have been forwarded, their genesis remains equivocal. Here, we show that a remarkable suite of helictites occurring in Asperge Cave (France) are formed by biologically-mediated processes, rather than abiotic processes as had hitherto been proposed. Morphological and petro-physical properties are inconsistent with mineral precipitation under purely physico-chemical control. Instead, microanalysis and molecular-biological investigation reveals the presence of a prokaryotic biofilm intimately associated with the mineral structures. We propose that microbially-influenced mineralization proceeds within a gliding biofilm which serves as a nucleation site for CaCO3, and where chemotaxis influences the trajectory of mineral growth, determining the macroscopic morphology of the speleothems. The influence of biofilms may explain the occurrence of similar speleothems in other caves worldwide, and sheds light on novel biomineralization processes.

Integrated numerical and laboratory rock physics applied to seismic characterization of reservoir rocks

A good understanding of the effect of rock and pore-fluid properties on seismic waves is necessary for the characterization of a subsurface hydrocarbon reservoir from a seismic data set. Information about the rock and fluids in the reservoir can be obtained, for example, through well logging and laboratory tests with samples cored from the wellbore. Together with seismic data, this information can be extrapolated for the entire dimension of the reservoir to provide valuable quantitative estimates for production. Additionally, this information can be extrapolated in time for monitoring the spatial redistribution of fluids during production. Making such space and time extrapolations more accurate using seismic data is the main goal of rock physics. For that, identifying and understanding the physical processes taking place in a reservoir rock at different scales is an important step and the subject of our article.

Seismic attenuation in partially saturated Berea sandstone submitted to a range of confining pressures

Using the forced oscillation method, we measure the extensional-mode attenuation and Youngs modulus of a Berea sandstone sample at seismic frequencies (0.5–50 Hz) for varying levels of water saturation (~0–100%) and confining pressures (2–25 MPa). Attenuation is negligible for dry conditions and saturation levels <80%. For saturation levels between ~91% and ~100%, attenuation is significant and frequency dependent in the form of distinct bell-shaped curves having their maxima between 1 and 20 Hz. Increasing saturation causes an increase of the overall attenuation magnitude and a shift of its peak to lower frequencies. On the other hand, increasing the confining pressure causes a reduction in the attenuation magnitude and a shift of its peak to higher frequencies. For saturation levels above ~98%, the fluid pressure increases with increasing confining pressure. When the fluid pressure is high enough to ensure full water saturation of the sample, attenuation becomes negligible. A second series of comparable experiments reproduces these results satisfactorily. Based on a qualitative analysis of the data, the frequency-dependent attenuation meets the theoretical predictions of mesoscopic wave-induced fluid flow (WIFF) in response to a heterogeneous water distribution in the pore space, so-called patchy saturation. These results show that mesoscopic WIFF can be an important source of seismic attenuation at reservoir conditions.

Mushroom Speleothems: Stromatolites That Formed in the Absence of Phototrophs

Unusual speleothems resembling giant mushrooms occur in Santa Catalina Cave, Cuba. Although these mineral buildups are considered a natural heritage, their composition and formation mechanism remain poorly understood. Here we characterize their morphology and mineralogy and present a model for their genesis. We propose that the mushrooms, which are mainly comprised of calcite and aragonite, formed during four different phases within an evolving cave environment. The stipe of the mushroom is an assemblage of three well-known speleothems: a stalagmite surrounded by calcite rafts that were subsequently encrusted by cave clouds (mammilaries). More peculiar is the cap of the mushroom, which is morphologically similar to cerebroid stromatolites and thrombolites of microbial origin occurring in marine environments. Scanning electron microscopy investigations of this last unit revealed the presence of fossilized extracellular polymeric substances (EPS) – the constituents of biofilms and microbial mats. These organic microstructures are mineralized with Ca-carbonate, suggesting that the mushroom cap formed through a microbially-influenced mineralization process. The existence of cerebroid Ca-carbonate buildups forming in dark caves (i.e., in the absence of phototrophs) has interesting implications for the study of fossil microbialites preserved in ancient rocks, which are today considered as one of the earliest evidence for life on Earth.

Low frequency measurements of seismic wave attenuation in Berea sandstone

We have designed and set up a pressure vessel for 250 mm long and 76 mm in diameter cylindrical samples to measure seismic wave attenuation in rocks at frequencies between 0.01 and 100 Hz and to verify the occurrence of fluid-flow induced by stress field changes. A dynamic stress is applied at the top of the rock cylinder by a piezoelectric motor generating either a stress step of several kPa in few milliseconds or a mono-frequency force. A load cell measures force and a strain sensor the bulk axial shortening across the sample. Five pressure sensors are buried at different heights of the cylinder to measure pore pressure changes related to stress field changes. The sample is sealed in a pressure vessel that can reach confining pressures of 25 MPa. We present datasets collected at room pressure and temperature. Three attenuation data curves measured on reference samples demonstrate the accuracy of the apparatus. A test of the influence of the static stress applied on the sample on the attenuation measurements and measurements conducted for frequencies between 0.1 and 50 Hz with strain < 5e-6 on partially saturated Berea sandstone are presented. Timeevolution pore-pressure curves due to stress field changes are also given.

Source and dynamics of a volcanic caldera unrest: Campi Flegrei, 1983–84

Despite their importance for eruption forecasting the causes of seismic rupture processes during caldera unrest are still poorly reconstructed from seismic images. Seismic source locations and waveform attenuation analyses of earthquakes in the Campi Flegrei area (Southern Italy) during the 1983–1984 unrest have revealed a 4–4.5 km deep NW-SE striking aseismic zone of high attenuation offshore Pozzuoli. The lateral features and the principal axis of the attenuation anomaly correspond to the main source of ground uplift during the unrest. Seismic swarms correlate in space and time with fluid injections from a deep hot source, inferred to represent geochemical and temperature variations at Solfatara. These swarms struck a high-attenuation 3–4 km deep reservoir of supercritical fluids under Pozzuoli and migrated towards a shallower aseismic deformation source under Solfatara. The reservoir became aseismic for two months just after the main seismic swarm (April 1, 1984) due to a SE-to-NW directed input from the high-attenuation domain, possibly a dyke emplacement. The unrest ended after fluids migrated from Pozzuoli to the location of the last caldera eruption (Mt. Nuovo, 1538 AD). The results show that the high attenuation domain controls the largest monitored seismic, deformation, and geochemical unrest at the caldera.