An advanced computational intelligent framework to predict shear sonic velocity with application to mechanical rock classification

Shear sonic wave velocity (Vs) has a wide variety of implications, from reservoir management and development to geomechanical and geophysical studies. In the current study, two approaches were adopted to predict shear sonic wave velocities (Vs) from several petrophysical well logs, including gamma ray (GR), density (RHOB), neutron (NPHI), and compressional sonic wave velocity (Vp). For this purpose, five intelligent models of random forest (RF), extra tree (ET), Gaussian process regression (GPR), and the integration of adaptive neuro fuzzy inference system (ANFIS) with differential evolution (DE) and imperialist competitive algorithm (ICA) optimizers were implemented. In the first approach, the target was estimated based only on Vp, and the second scenario predicted Vs from the integration of Vp, GR, RHOB, and NPHI inputs. In each scenario, 8061 data points belonging to an oilfield located in the southwest of Iran were investigated. The ET model showed a lower average absolute percent relative error (AAPRE) compared to other models for both approaches. Considering the first approach in which the Vp was the only input, the obtained AAPRE values for RF, ET, GPR, ANFIS + DE, and ANFIS + ICA models are 1.54%, 1.34%, 1.54%, 1.56%, and 1.57%, respectively. In the second scenario, the achieved AAPRE values for RF, ET, GPR, ANFIS + DE, and ANFIS + ICA models are 1.25%, 1.03%, 1.16%, 1.63%, and 1.49%, respectively. The Williams plot proved the validity of both one-input and four-inputs ET model. Regarding the ET model constructed based on only one variable,Williams plot interestingly showed that all 8061 data points are valid data. Also, the outcome of the Leverage approach for the ET model designed with four inputs highlighted that there are only 240 "out of leverage" data sets. In addition, only 169 data are suspected. Also, the sensitivity analysis results typified that the Vp has a higher effect on the target parameter (Vs) than other implemented inputs. Overall, the second scenario demonstrated more satisfactory Vs predictions due to the lower obtained errors of its developed models. Finally, the two ET models with the linear regression model, which is of high interest to the industry, were applied to diagnose candidate layers along the formation for hydraulic fracturing. While the linear regression model fails to accurately trace variations of rock properties, the intelligent models successfully detect brittle intervals consistent with field measurements

CO2 Long-term Periodic Injection Experiment at Mont Terri (CO2LPIE).

Peer reviewe

Effect of Caprock Relative Permeability on Co2 Flow through it

Geologic carbon storage is needed to meet the climate goal of limiting global warming to 1.5 ºC. Injecting in deep sedimentary formations brings CO2 to a supercritical state, yet less dense than the resident brine making it buoyant. Therefore, the assessment of the sealing capacity of the caprock lying above the storage reservoir is of paramount importance for the widespread deployment of geologic carbon storage. We perform laboratory-scale supercritical CO2 injection into a representative caprock sample and employ numerical simulations to provide an in-depth understanding of CO2 leakage mechanisms. We explore the effect of relative permeability curves on the potential CO2 leakage through the caprock. We show that capillary breakthrough is unlikely to take place across a non-fractured caprock with low intrinsic permeability and high entry pressure. Rather, CO2 leakage is dominated by the intrinsically slow molecular diffusion, favoring safe storage of CO2 over geological time scales.publishedVersio

¿Es seguro almacenar millones de toneladas de CO₂ bajo tierra?

Nos encontramos en un estado de emergencia climática provocado por las actividades humanas. La emisión a la atmósfera de grandes cantidades de dióxido de carbono (CO₂) procedentes de la quema de combustibles fósiles (carbón, petróleo y gas) han producido un calentamiento de la superficie de la Tierra de unos 1,2 ℃ de media por encima del nivel preindustrial. Los impactos del calentamiento global ya se han manifestado en fenómenos meteorológicos extremos más intensos y frecuentes. Ante este panorama, las tecnologías de eliminación del carbono desempeñarán un papel indispensable en el camino a la descarbonización. La capacidad instalada actual de captura de CO₂ es de unas 40 megatoneladas (1 millón de toneladas) al año. Pero aún estamos peligrosamente lejos de cumplir los objetivos climáticos. La capacidad de captura y almacenamiento de CO₂ debe multiplicarse aproximadamente por 100 de aquí a 2050. Aunque las realidades económicas y políticas son determinantes, el temor a las fugas de CO₂ a la superficie ha provocado retrasos en la implementación generalizada de esta tecnología. Un miedo que, según nuestros estudios, no tiene por qué hacerse realidad.Iman Rahimzadeh Kivi recibe fondos del Ministerio de Ciencia e Innovación (Ref. PCI2021-122077-2B). Víctor Vilarrasa Riaño recibe fondos del Consejo de Investigación Europeo (ERC por sus siglas en inglés) (Ref. 801809) y del Ministerio de Ciencia e Innovación (Ref. PCI2021-122077-2B y Ref. CEX2021-001198).Peer reviewe

An experimental investigation on the poroelastic response of a water-saturated limestone to hydrostatic compression

This contribution addresses an experimental study on the poroelastic behavior of a water-saturated limestone. Samples come from Sarvak limestone, a major hydrocarbon-producing reservoir in Iran. Three back-saturated intact specimens were subjected to hydrostatic compression with different boundary pressure conditions. A coherent workflow from testing to data interpretation was established to efficiently deal with possible sources of uncertainty and provide accurate results. Obtained results revealed that drained and undrained bulk moduli, as well as the Biot and Skempton coefficients, strongly depend on the applied Terzaghi effective stress. Conversely, calculated values for the unjacketed solid modulus K′s were found to be constant in the range of applied stresses. Variations of the poroelastic moduli with the effective stress were modeled and used to extract depth-dependent poroelastic properties of the studied lithotype along a well. Besides, evaluated poroelastic moduli were utilized to verify the linear theory of poroelasticity. The indirectly calculated unjacketed pore modulus K′′s appeared to be highly variable with changes in the effective stress and possesses even negative values when the Skempton coefficient is lower than a critical value. A comparison between unjacketed bulk moduli and the bulk deformation modulus of the main mineral constituents Ks also disclosed that they are basically uncorrelated, and the assumption of ideal porous rock, K′′s=K′s=Ks, does not hold in general for the examined rock samples. Nevertheless, consideration of the ideality assumption in the theory of poroelasticity may result in reliable estimates for the Skempton coefficient in certain circumstances, particularly at high effective stresses.Peer reviewe

[Dataset] Numerical models for simulating supercritical CO2 intrusion into shaly caprock specimens

This dataset is composed of the input files of numerical models for simulating laboratory-scale CO2 intrusion into shaly caprock specimens. Six numerical models are included in six distinct folders whose names represent a short description of the corresponding model. The names of the folders and the model descriptions are as follows: - “RS_Referencecase.gid”: simulation of CO2 intrusion into the Remolded Shale with a reference set of parameters including sample length l=1 cm, molecular diffusion coefficient D=1·10-9 m2/s and capillary entry pressure of porous disks p0=0.01 MPa - “OPA_Referencecase.gid”: simulation of CO2 intrusion into Oplinus Clay with the same set of reference parameters mentioned above - “RS_l=10cm.gid”: simulation of CO2 intrusion into a 10-cm-long RS specimen - “RS_D=1e-10m2s-1.gid”: simulation of CO2 intrusion into the RS specimen by considering a lower diffusion coefficient D=1·10-10 m2/s - “RS_p0=0.1MPa.gid” and “RS_p0=0.2MPa.gid”: simulations of CO2 intrusion into the RS specimen with increased p0=0.1 MPa and p0= 0.2 MPa, respectively. In each folder, there is a file with the name of the folder ended as “_gen.dat” which contains the input data of the model, including material properties, initial and boundary conditions and the time intervals. There is also a file ended as “_gri.dat” that includes the information on the mesh. The file “root.dat” includes the name of the model. To run the simulation, just execute the Code_Bright executable “Cb_v9_3.exe” in a folder that contains the three input files and the executable.Peer reviewe

Laboratory and numerical assessment of potential CO2 leakage through the caprock

The emission of huge amounts of anthropogenic CO2 into the atmosphere have given rise to global warming and therefore climate change. Currently, it is widely accepted that geologic storage of CO2 in deep underground formations has to be part of the solution to mitigate these harmful consequences (IPCC, 2018). The injected CO2 is a non-wetting fluid in the reservoir rock and it is also lighter than the in-situ brine, leading to upward buoyant transport of CO2 across the storage reservoir. The migration of CO2 out of the reservoir is supposed to be principally restricted by a low-permeability and high-entry pressure caprock lying immediately above it. Therefore, the successful and safe retainment of CO2 in place over geologic time scales is strongly controlled by the sealing capacity of the caprock. As CO2 gets in contact with the caprock, the concentration gradient of the dissolved CO2 into brine drives molecular diffusion of the non-wetting fluid out of the storage repository through the fluid-filled interconnected pore network of the caprock (Busch et al., 2008). On the contrary, the bulk penetration of CO2 as a free phase into the caprock encounters capillary resistance imposed by fluid-fluid and fluid-rock interfacial forces operating at tight pore throats. The injection-induced excess pressure and buoyancy forces may increase the differential pressure between CO2 and brine at the reservoir-caprock interface. If the overpressure of the non-wetting fluid exceeds the entry capillary pressure P0 acting at the largest pore throats, it starts invading the caprock. Further increase in the differential pressure overcoming another capillary threshold, termed breakthrough pressure Pbrth initiates a continuous filament of CO2 across the pore system. From this point on, the two-phase flow dominates the volumetric displacement of CO2. It is generally believed that the pressure-driven bulk flow of the non-wetting fluid mainly governs the potential leakage through the intact caprock and that molecular diffusive loss toward the surface is negligible (Song and Zhang, 2013). However, to arrive at a fundamental understanding of the CO2 transport behavior and distinguish between the prevailing leakage mechanisms, further experimental investigations under representative reservoir conditions are required. These experimental studies should be accompanied by appropriate interpretation techniques to accurately deal with the CO2-brine-rock system complexities. The main objective of this study is to combine methods from two complementary disciplines of experimental observations and numerical simulations to get a better insight into the dominant leakage mechanisms of CO2 through the caprock. Our focus is on the potential leakage through the rock matrix. Laboratory tests on an analogous caprock sample are first carried out under the in-situ conditions with the primary goal of evaluating CO2 penetration and flow properties across the caprock. Experimental results are then used to parameterize a two-phase flow model for the numerical simulation and interpretation of the core-scale CO2 breakthrough and flow behavior.I.R.K. and V.V. acknowledge funding from the European Research Council (ERC) under the European Union’s Horizon 2020 Research and Innovation Program through the Starting Grant GEoREST (www.georest.eu) (Grant agreement No. 801809). R.M. is thankful for the support from US DOE through CarbonSAFE Macon County Project DE-FE0029381.Peer reviewe

Supercritical CO2 intrusion into caprocks: insights from numerical simulation of lab-scale CO2 injection

Carbon capture and storage in deep geological formations suggests a promising large-scale CO2 mitigation option. Currently available pathways to meet the Paris climate goal of limiting global warming to well below 2 ºC involve significant contributions of geologic carbon storage. Nonetheless, this mitigation technology is not exempt from challenges. In particular, the potential CO2 leakage through the caprock is a major concern. CO2 is less dense than the in-situ brine in deep saline formations and tends to float. The upward migration of the buoyant CO2, if causing leakage through the caprock, can put at stake the large-scale implementation of geologic carbon storage. Therefore, the assessment of the caprock sealing capacity is of paramount importance. This study provides an improved understanding of CO2 flow mechanisms across the caprock based on insights gained from numerical simulations of core-scale CO2 injections. We inject supercritical CO2 into a caprock sample under representative subsurface conditions. We parameterize a two-phase flow model using laboratory data and reproduce the CO2 injection experiments. Overall, we conclude that advective CO2 flow is unlikely to take place through a caprock with sufficiently high capillary entry pressure and low intrinsic permeability. The ubiquitous molecular diffusion principally dominates CO2 leakage. These findings favor long-term storage of CO2 underground. Nevertheless, diffusive leakage over geological time scales has yet to be assessed through field-scale numerical simulations.Peer reviewe

Supercritical CO2 intrusion into caprocks: experimental observations and numerical simulations

Geologic storage of carbon dioxide (CO2), mainly in deep saline aquifers, has emerged as a key solution to reach the Paris Agreement goal of limiting the global temperature increase below 2ºC and effectively mitigate climate change. The injected CO2 is less dense at the storage condictions than the resident brine and tends to flow upward by buoyancy. Therefore, to achieve the primary objective of permanently storing CO2 underground during geological time scales, the host reservoir should be overlain by a low-permeability and high-entry pressure caprock that prevents CO2 escaping from the storage formation. Meanwhile, the caprock sealing capacity is of particular significance and yet to be assessed in more detail. In this presentation, we aim at shedding light on the flow processes governing potential CO2 leakage through shaly caprocks by combining experimental observations and numerical simulations. We present breakthrough experiments on Opalinus Clay, which is a representative caprock for CO2 storage. These experiments reproduce supercritical CO2 intrusion and flow through the caprock sample under representative reservoir conditions. Next, we address numerical simulation of the breakthrough experiments using a twophase flow model in deformable porous media to provide a mechanistic interpretation of experimental observations. Overall, we conclude that CO2 leakage through the caprock is dominated by molecular diffusion rather than by rapid bulk volumetric advection.Peer reviewe