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

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

A mechanistic interpretation of potential CO2 leakage through shaly caprocks

Global warming brought upon by anthropogenic CO2 emissions into the atmosphere is causing significant impacts on the Earth and represents one of the major concerns of the current century. To be controlled, it is widely accepted that huge amounts of CO2 at the gigatonne scale have to be captured and injected back into the underground in a process known as Carbon Capture and Storage (CCS). As CO2 is less dense than the in-situ brine, it tends to flow upward out of the storage reservoir by buoyancy and the injection overpressure. A laterally-extensive and thick nonfractured caprock possessing low permeability and high entry capillary pressure is commonly expected to keep CO2 within the host reservoir. However, the potential risks of CO2 leakage through the intact caprock need thorough assessment. This contribution brings together experimental observations and numerical simulations to inspect the sealing capacity of an intact shaly caprock and render an in-depth understanding of the governing flow mechanisms. Reproducing the subsurface conditions of CO2 intrusion and flow through the caprock, breakthrough experiments are conducted on Opalinus Clay as a representative caprock for CO2 storage. The adopted approach consists of injecting supercritical CO2 into the caprock sample lying between two permeable porous disks, all initially saturated with brine. Supplementary experiments are also performed to characterize the pore structure and hydromechanical properties of the specimen. The extracted properties are used to parameterize a two-phase flow model in deformable porous media and simulate the breakthrough experiment carried out on Opalinus Clay to make a mechanistic interpretation of the experimental observations. Simulation results reveal three concomitant CO2 flow mechanisms into and through the caprock: molecular diffusion, bulk volumetric advection, and transported CO2 dissolved in the advected brine. It is inferred that the high entry pressure and low effective permeability prevents free phase CO2 penetration deep into the caprock. The drainage path is followed by the imbibition of brine back into the pores from the downstream until recovering the initial state of being completely saturated with brine. While the contribution of brine advection to CO2 transport is found to be negligible, we find that CO2 flow through the caprock is mainly governed by molecular diffusion, whose effects on the potential leakage of CO2 during geological time scales have to be taken into account.Peer reviewe

CO2 capillary breakthrough is unlikely to occur and compromise the caprock sealing capacity

Carbon capture and geologic storage, mainly in deep saline aquifers, is extensively considered as an essential component of any strategy to achieve carbon neutrality and effectively mitigate climate change. At pressure and temperature conditions relevant to CO2 storage in sedimentary formations, CO2 is less dense than the resident brine and tends to float, threatening the long-term storage of CO2 underground [1]. Therefore, successful deployment of geologic CO2 storage and its public acceptance is primarily a matter of ensuring the sealing capacity of caprock overlying the storage reservoir and yet to be investigated. Bringing together experimental methods and a numerical interpretation scheme, we aim at shedding light on the processes governing CO2 intrusion and flow through low-permeability shaly caprock. We perform CO2 injection experiments on Opalinus Clay samples retrieved from the Mont Terri underground rock laboratory in Switzerland. Two types of Opalinus Clay are examined: intact specimen, representing an ideal caprock for CO2 storage, and remolded shale, representing the potential shear zone in the caprock [2]. The latter is found to possess relatively higher intrinsic permeability and lower capillary entry pressure. We parameterize a two-phase flow model in deformable porous media using appropriate hydromechanical properties and replicate experimental observations. Simulation results highlight three concomitant flow mechanisms: molecular diffusion of CO2, bulk volumetric advection of CO2, and brine advection transporting dissolved CO2. The bulk CO2 intrusion is confined to the lowermost portion of the specimen and remains unable to trigger an effective increase in the relative permeability to CO2. Therefore, advective CO2 migration is minor. We conclude that rapid capillary breakthrough of CO2 is unlikely to take place and compromise the sealing capacity of nonfractured caprock. The relatively slow diffusive flow appears to purely dominate leakage in the long term. Yet, diffusive CO2 leakage may occur over geological time scales and have to be assessed through field-scale numerical simulations.Peer reviewe

Coupled HM modeling assists in designing CO2 long-term periodic injection experiment (CO2LPIE) in Mont Terri rock laboratory

We are performing a series of coupled hydro-mechanical (HM) simulations to model CO2 flow through Opalinus Clay at the Mont Terri rock laboratory in the CO2 Long-term Periodic Injection Experiment (CO2LPIE). CO2LPIE aims at inter-disciplinary investigations of the caprock sealing capacity in geologic CO2 storage in a highly monitored environment at the underground laboratory scale. Numerical modeling allows us to gain knowledge on the dynamic processes resulting from CO2 periodic injection and to assist the experimental design. The cyclic injection parameters (i.e., the period and the amplitude) have to be optimized for the field experiment and therefore different values are taken into account. Opalinus Clay is a claystone with nanoDarcy permeability that contains well developed bedding planes responsible for its anisotropic HM behavior. The hydraulic anisotropy is defined by a permeability parallel to the bedding planes being three times the one perpendicular to it. Additionally, the drained Young’s modulus is measured to be 1.7 GPa parallel and 2.1 GPa perpendicular to bedding. Excavation reports by swisstopo document a SSEdip of 45° for the bedding planes at the experiment location. CO2 injection generates a mean overpressure of 1 MPa into the brine that propagates into the formation. The differential pressure between CO2 and formation water, i.e., capillary pressure, is lower than the entry pressure and thus, CO2 diffuses through the pores but does not advect in free phase. The liquid overpressure distribution is distorted by the hydraulic anisotropy, preferentially advancing along the bedding planes, as the associated permeability is higher than the one perpendicular to the bedding. The pore pressure buildup induces a poromechanical stress increase and an expansion of the rock that leads to a permeability enhancement of up to two orders of magnitude. The cyclic stimulation propagates trough the domain faster and with a lag time and an attenuation, both of which increase with distance from the source with, their values being dependent on permeability, porosity and stiffness of the rock. As a result of the model orthotropy, the attenuation and the lag time change with direction, i.e. they are higher in the direction perpendicular to the bedding and lower in the direction parallel to the bedding.Peer reviewe