EMRP1.15

Carbon oxygen ratio (C/O) logging is an important method for the accurate determination of hydrocarbon saturation in the reservoir region. One of the advantages of the measurement, that it is independent of Cl content. Furthermore, the insensitivity of high energy neutrons to the casing, makes it possible to use it in cased boreholes too. In addition to the application in the oil industry, monitoring of CO₂ reservoirs is also possible. In our study, the modeling of the time-dependent coupled neutron-gamma field produced by the tool was carried out, using MCNP, a general-purpose Monte Carlo radiation transport code. The energy spectrum of gammas reaching the scintillation detector crystal were simulated in different detector positions, and different tool environments: reservoir rock, reservoir porosity, hydrocarbon saturation, and well status (cased or open). The effect of the parameters above are illustrated by vertical cross sections of the particle fluxes around the tool, and shown by the changes of the interpretation charts. By the introduction of a “goodness” factor, derived from the interpretation chart, the potential, and the limitations of C/O logging are investigated.

How to cite: Szucs, J. G. and Balazs, L.: Sensitivity study of C/O logging measurements by the Monte Carlo method, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-228, https://doi.org/10.5194/egusphere-egu22-228, 2022.

At the field scale, petro-elastic models linking seismic velocities with porosity have been widely used to estimate properties of reservoirs and subsurface domains in general. At the laboratory scale, frame elastic properties and porosity are not enough to predict the full ultrasonic wave propagation, and other factors like texture, pore space topology and fluid interactions play a significant role. In dry volcanic rocks characterized by larges vesicles, the heterogeneities triggering the perturbations of the ultrasonic wavefield mainly correspond to the pore space topology. However, the sensitivity of S-waveforms to the pore space has not been examined in volcanic rocks.

To assess the role of the pore space on ultrasonic wave propagation, we performed computational simulations on 2D synthetic samples analogous to volcanic rocks, resembling the acquisition of S-waveforms in laboratory experiments. The computational framework applied is the spectral-element method. The porosity and aspect ratio on the study samples was kept constant along the simulations to focus on the effect that the pore space parameters have on the wave arrival, amplitude, and shape of the waveforms.

This study shows that the pore space topology controls the waveform of ultrasonic waves in dry volcanic rocks, and parameters like amount, size, and even the location of the pores impact the elastic wave propagation independently of the porosity value. This finding has important implications for forward modelling seismic signals of heterogeneous volcanic media at the field scale.

How to cite: Di Martino, M. D. P., De Siena, L., and Tisato, N.: The role of pore space topology on ultrasonic wave propagation in volcanic rocks., EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-660, https://doi.org/10.5194/egusphere-egu22-660, 2022.

Dolostones represent one of the major hydrocarbon reservoirs in the world. Understanding the elastic behavior of these units is crucial for hydrocarbon exploration and/or development. In this study, 100 samples from five formations within the Arabian Platform, were used to examine the main controlling factors on the sonic velocity of dolostones. A combination of field and laboratory analyses were conducted on the collected samples including; thin-section petrography, SEM, XRD, digital image analysis, porosity and permeability measurements, velocity measurements, and rock physics modeling. The studied samples have a wide range of porosity (1- 45%, averaging 18.5 %), and permeability (0.01 – 2000 mD, averaging 196 mD). Compressional V_P and shear wave V_S velocity ranges from 3.0 - 6.7 km/s, and 1.6 – 3.7 km/s, respectively. In general, porosity-velocity trajectory is showing a negative relationship with a coefficient of determination R² of 0.82. However, some samples are deviated from this trendline due to their inherited and diagenetic parameters. These parameters include texture, mineralogy, pore type, and crystal size. Fabric-preserving dolostones have, relatively, higher velocities than non-fabric preserving dolostones. Although 95% of the studied samples are dominated by dolomite, samples with higher content of calcite and quartz, have lower velocities. Moldic and vuggy pore-dominated samples have, relatively, higher velocities than samples dominated by intercrystalline pores and microporosity. For non-fabric preserving dolostones, samples with larger crystals show higher velocities than samples with smaller crystals. Using equivalent pore aspect ratio (EPAR), a clear distinction between permeable (>10 mD) and tight (< 10 mD) samples can observed, where most of the permeable samples have high EPAR values, while the tight samples have low EPAR values. The result of this study might significantly help in the interpretation and understanding of the sonic logs and seismic data from dolostone strata.

How to cite: Salih, M., El-Husseiny, A., Reijmer, J. J. G., Eltom, H., Abdelkarim, A., and Kaminski, M. A.: Controls on Sonic Velocity in Dolostones, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-945, https://doi.org/10.5194/egusphere-egu22-945, 2022.

Stress changes have a large impact on the hydraulic and mechanical properties of fractures and can be caused by varying the fluid pressure in a subsurface reservoir or by tectonic movements. Innovative tools to assess the stress conditions are still of major importance for most subsurface applications. We derived an experimental procedure to reveal stress signals preserved in fractures in the laboratory.

In a set of complex experiments various fractured low-permeability rocks, two sandstones and two crystalline rocks, were cyclically loaded in a MTS tri-axial compression cell. The preconditioned cylindrical samples were split into two halves to generate an artificial tensile fracture and a rigid shear displacement was applied before installing the sample into the apparatus. Two different loading scenarios were applied: “continuous cyclic loading” (CCL) and “progressive cyclic loading” (PCL). During continuous cyclic loading samples were loaded from 2 to 60 MPa in several repeated cycles. In the progressive cyclic loading experiments the hydrostatic confining pressure was increased using a step-wise function (15, 30, 45 and 60 MPa) and was unloaded after every sub-cycle, while the pore pressure was kept constant at a low level. The mechanical fracture closure was monitored continuously during the experiments using axial and circumferential extensometers and the specific fracture stiffness could be calculated at a very high resolution. The fracture permeability was measured continuously using four high-pressure fluid pumps. A 3D surface scanner was used to analyze the fracture surface geometry before and after the experiments to reveal possible changes to the surface topography as well as to quantify changes in aperture and contact-area ratio.

The specific fracture stiffness was shown to be irreversible when a fracture was hydrostatically loaded once. Further, a “stress-memory” effect of fracture stiffness could be shown during progressive loading. It is characterized by a change from non-linear to linear stiffness evolution when a previous stress-level is exceeded. This phenomena can be used to identify previous stress states preserved within fractures. Additionally, this data is important for elasto-plastic contact theories of rough fractures. The impact of progressive loading on fracture permeability evolution showed varying results based on the heterogeneity and mineral composition of each rock type and the resulting fracture geometry.

How to cite: Kluge, C., Muhl, L., Schramm, D., and Blöcher, G.: How to discover ancient stress-levels preserved within fractures using the stress-memory effect of specific stiffness, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-3523, https://doi.org/10.5194/egusphere-egu22-3523, 2022.

The poroelastic behaviour of cracked rocks is expected to depend on the geometry and properties of the crack network. Any preferred orientation of microcracks produces anisotropy in physical rock properties, including poroelastic parameters. Under conventional triaxial loading there is an alignment of cracks parallel to the vertical direction of compression, leading to vertical transverse isotropy in the cracked rock.

Here, we repeatedly measured transversely isotropic poroelastic parameters during increasing amplitude cyclic loading in a sample of Westerly granite saturated with water. Independent step changes in confining pressure and differential stress were repeated at selected levels of differential stress to measure the change and reversibility in the transversely isotropic parameters throughout the loading and unloading cycles.

We used miniature differential pressure transducers which were located directly around the sample surface, allowing for direct measurement of the pore pressure in the sample. The direct measurements of pore pressure allow us to estimate undrained properties, including Skempton’s coefficients. Axial and radial strain gauges allow for the calculation of elastic moduli from the step changes in axial and radial stress. We determine the undrained moduli from the initial short-term response, and the drained moduli following pore pressure equilibration for each step change in stress.

Results show that the radial Skempton’s coefficient increases with increased differential stress, and the axial coefficient decreases and even becomes negative (where increases in axial stress cause a decrease in pore pressure) at high stress (i.e., about 80% of failure stress). During unloading, the measured Skempton coefficients are observed to be recovered, without hysteresis.

How to cite: Elsigood, B., Brantut, N., Meredith, P., Mitchell, T., and Healy, D.: The poroelastic response of cracked Westerly granite to cyclical changes in load, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-5194, https://doi.org/10.5194/egusphere-egu22-5194, 2022.

A method for the numerical determination of pressure-dependent permeability in sandstones is developed. The proposed method is restricted to a hydrostatic pressure load that is below the pore collapse pressure.

Our modelling approach is generally based on the idea of digital rock physics. Starting from a µCt-scan, the pore space of a given rock sample is detected and transferred into a numerical model. Subsequently, the stationary Stokes equations are solved, and the permeability is determined from the simulated pressure and velocity fields.

To model the pressure dependence, it is assumed that the deformation of the rock's micro-structure due to pore throat closing has a significant influence on the change in permeability. In our workflow, the respective pore throats between the individual grains of the original CT image are reconstructed using the watershed algorithm and combined in a separate phase of the numerical model. During several simulations, a steadily increasing artificial flow resistance is assigned to the pore throat phase and the respective permeability of the whole sample is determined. Finally, the pressure-dependent permeability curve can be reconstructed via a correlation between flow resistance and pressure load.

The proposed workflow is validated with externally published data of a Bentheim sandstone sample. It is observed that the model is generally able to reproduce the characteristics of a experimentally determined pressure-dependent permeability curve.

How to cite: Siegert, M., Gurris, M., and Saenger, E. H.: Numerical modelling of pressure-dependent permeability in Bentheim sandstone, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-8360, https://doi.org/10.5194/egusphere-egu22-8360, 2022.

We have conducted a flow-through experiment using a Flechtingen sandstone sample containing a single macroscopic fracture. Based on this experiment, we obtained range of various intrinsic rock parameters, such as permeability and specific stiffness of the combined matrix-fracture system under hydrostatic loading. In order to quantify the processes behind the laboratory observations, we carried out coupled hydro-mechanical simulations of the matrix-fracture system. Navier-Stokes flow was solved in the 3-dimensional open rough fracture domain, and back-coupled to Darcy flow and mechanical deformation of the rock matrix.

To capture the volumetric shape of the fracture, the two fracture surfaces were scanned using a 3D-profilometer (Keyence VR-3200) before and after the experiment. The resulting fracture surfaces were aligned using a grid-search algorithm and subsequently offset to mimic the shear displacement as applied during the laboratory experiment. Based on the obtained 3D representation of the fracture volume embedded in a porous media, the stress path of the laboratory experiment was simulated numerically. By means of the simulation results, values of fracture closure, increase of contact area, fracture permeability and fracture stiffness due to normal load on the fracture surface were obtained.

The results demonstrate that the numerical simulation could capture the elastic and inelastic behaviour as well as the related permeability alteration of the fracture domain. Both, the laboratory experiments as well as the numerical simulation indicate an inelastic deformation of the single fracture even at low normal stress. The inelastic deformation is expressed by an increase of the fracture contact area and therefore fracture stiffness with increasing stress. The increase in the contact area is due to a reduction in mean aperture and is therefore accompanied by a reduction in the fracture permeability. The development of the contact area is irreversible and thus indicates the maximum stress that the sample previously experienced. We call this behaviour "stress-memory effect".

We present the workflow to obtain the numerical results and a comparison with the laboratory experiment to show that the dominant processes were captured by the simulation.

How to cite: Blöcher, G., Kluge, C., Cacace, M., Deng, Q., and Schmittbuhl, J.: Numerical investigation of hydro-mechanical responses of a single fracture embedded in a porous matrix, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-8775, https://doi.org/10.5194/egusphere-egu22-8775, 2022.

Magnetotelluric inversions spanning the Pacific-Australian Plate boundary in New Zealand’s South Island indicate there is a localized zone of low electrical resistivity that is spatially co-incident with the mid-crustal part of the Alpine Fault Zone (AFZ), that currently accommodates shear strain by temperature-sensitive creep. We explored the source of this anomaly by measuring the electrical properties of samples collected from surface outcrops approaching the AFZ that have accommodated a gradient of systematic strain and deformation conditions. We investigated the effects of tectonite fabric, fluid saturated pore/fracture networks and grain surface conduction on the bulk electrical response and the anisotropy of resistivity of these samples measured under increasing confining pressures up to 200 MPa. We find that for fault rock protoliths, Haast and Alpine Schist, resistivity and change in anisotropy of resistivity with confining pressure (δ(ρ_‖/ρ_⊥)/ δ(p_eff)) increases while porosity decreases approaching the AFZ. This indicates the electrical response is controlled by pore-fluid conductivity and modified during progressive metamorphism. AFZ mylonites exhibit low electrical resistivities at low porosities, and lower δ(ρ_‖/ρ_⊥)/ δ(p_eff) than the schists. These reflect changes in both the porosity distribution and electrical charge transport processes in rocks that have experienced progressive grain size reduction and mixing of phases during development of mylonitic fabrics due to creep shear strain within the AFZ.

How to cite: Toy, V., Kluge, E.-K., and Lockner, D.: Electrical properties and anisotropy of schists and fault rocks from New Zealand’s Southern Alps under confining pressure, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-9616, https://doi.org/10.5194/egusphere-egu22-9616, 2022.

Continuous seismic noise recordings has demonstrated a remarkable ablitiy to monitor changes of the investigated media at various scales. In this study, we focused on the link between seismic velocity variations (dv/v) derived from seismic noise cross-correlations between pairs of stations and hydrological variations observed both at the field and lab scales. The field-scale experiment was performed at the water supply pumping site of Crépieux-Charmy (Lyon, France). 99 3-C velocimeters were deployed during 20 days around an infiltration basin operated for managed aquifer recharge and designed to generate an hydraulic barrier to prevent a potential contamination from the nearby river. This dense seismic network set-up allowed to dynamically image the seismic velocity variations during two filling/drainage cycles of the basin. Punctual values extracted from computed high resolution tomographies of the velocity variations were compared to local measurements of the water table level using piezometers. A remarkable agreement was found between the 2 observables in particular during the establishment of a 3D dome in the water table. During drainage phases, systematic response delays were observed which are most probably due to variations of water content in the unsaturated zone between the basin and the water table.
To better understand these effects occurring in the critical zone, we tried to reproduce a similar monitoring experiment at the laboratory scale. A tank filled with sand was designed in order to characterize controled hydrological variations (water table depth, water saturation). We used continuous seismic sources deployed on the edges of the tank. The seismic noise was recorded using 10 3-C accelerometers . The combination of these two approaches at different scales provides a better understanding of the links between seismic velocity and hydrological (water table level and water content in the vadose zone) variations.

How to cite: Gaubert-Bastide, T., Garambois, S., Bordes, C., Voisin, C., Brito, D., Roux, P., and Oxarango, L.: Seismic velocity variations generated by controlled hydrological changes : field and laboratory studies based on seismic noise crosscorrelation, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-10087, https://doi.org/10.5194/egusphere-egu22-10087, 2022.

Zeta potential is an important petrophysical property that controls electrostatic interactions between mineral, water, and non-aqueous phase fluids. These interactions play an important role in defining the wetting state of reservoir rocks. The zeta potential can be interpreted from the streaming potential measurements, which are shown to be an efficient means for a broad range of applications including monitoring of single- and multi-phase flows in subsurface settings, characterization of fracture networks, efficiency of CO₂ sequestration, hydrogen underground storage and enhanced oil recovery. It is widely agreed that the zeta potential in carbonate rocks is controlled by the concentration of potential determining ions (PDI), but the understanding of this is still poor and there are very limited experimental data on quantitative characterization of the dependence of the zeta potential on concentration of negative potential determining ions (PDI) such as SO₄^2-, CO₃^2-, HCO₃^-, especially when their concentration is high and exceeds that of the positive PDIs.

In this study, the streaming potential method is used to investigate the zeta potential of natural carbonate rock samples in contact with natural aqueous solutions of low-to-high ionic strength and with varying concentration of sulphate (SO₄^2-) and carbon (C₄) related (HCO₃^-, CO₃^2-) ions. In each set of experiments the total ionic strength was kept constant to eliminate the impact of concentration on the zeta potential. The study probed the concentration of negative PDIs that has never been reported before, with their respective lowest concentration consistent with previously reported values, and the highest concentration equal to the maximum achievable by stripping the tested solutions of Cl^-.

Our results demonstrate that zeta potentials strongly depend on concentration of the negative PDIs, thus providing explicit empiric relationship between the zeta potential and a broad range of PDI concentration. Our findings improve the current understanding of the complex physicochemical processes that take place at calcite-water interface and provide important experimental data for surface complexation modelling of carbonate-brine systems.

How to cite: Atiwurcha, N., Derksen, J., Vega-Maza, D., and Vinogradov, J.: Zeta Potential of Intact Carbonate Core Samples Saturated with Natural Aqueous Solutions with Varying Concentration of Negative Potential Determining Ions, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-13060, https://doi.org/10.5194/egusphere-egu22-13060, 2022.

Anomalously high seismic P- to S-wave velocity ratios (Vp/Vs) have been observed in subduction zones, in locations where varieties of earthquakes and slips are expected to occur, interpreted as highly pressurized heavily fractured zones. Assuming the rocks isotropic, Vp/Vs can be directly linked to rocks Poisson’s ratio in the elastic regimes relevant to both the field and laboratory measurements. From dedicated measurements across the frequency range it was shown that such insights hold, in agreement with a micromechanical model for isotropic micro-cracked rocks: Anomalously high Vp/Vs exist for any low porosity isotropic rocks of any mineral content, if heavily micro-fractured and at near-lithostatic fluid pressures, i.e. at very low Terzaghi effective pressure.

Extending that understanding, one could question if such anomalous Vp/Vs could also be observed and similarly explained in isotropic porous reservoir rocks. From the typical micromechanical inclusion models for predictions at the sample’s scale, such is unlikely as Poisson’s ratio should largely decrease with an increasing content of spherical pores. Yet, that is not what is measured in the relevant undrained elastic regime in well-cemented porous sandstones. For these, most rocks Poisson’s ratio remain anomalously high and comparable to that retrieved in low porosity rocks. Moreover, while models would then predict a dependence of Poison’s ratio to the liquid’s bulk modulus that is again not consistent with the measurements.

From comparing literature datasets reporting drained and undrained Poisson’s ratio and bulk modulus for sandstones of varying porosity, the aim of this work is to investigate and discuss (i) how the measured properties compare, (ii) if one property or the two deviate from existing models and why.

How to cite: Pimienta, L.: Do anomalously high Vp/Vs exist in porous reservoir rocks?, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-13560, https://doi.org/10.5194/egusphere-egu22-13560, 2022.