The main objective in this study is to experimentally investigate how two types of shales different in swelling characteristic and organic carbon content behave in terms of the mechanical and infiltration properties when being subjected to changing osmosis with varying brine salinity under in-situ stress conditions. This research also aims to determine the swelling tendency of thePierre shale and replicate the effect of selection of brine salinity on oil recovery in liquid-rich shales. A series of coupled measurements from the triaxial experiments at reservoir conditions was conducted on Pierre shale core samples to determine rock properties and behavior under fluid pore pressure intrusion. The main use of the measured properties is to demonstrate the impact of fluid transport induced by low salinity water injection accompanied by osmosis, clay swelling, and rock elasticity change on oil recovery from rock matrix. The Young's modulus, bulk modulus, and Poisson's ratio of the Pierre shale were determined from measurements on the two types of Pierre shale samples obtained from an outcrop having high 65 wt% of smectite content and well core having lean clay content and 3.5 wt% TOC (total organic carbon). The first two experiments were performed on the outcrop samples comprising of similar characteristics of 65 wt% smectite with negligible TOC while the last experiment was performed on the well core sample containing negligible smectite content with 3.5 wt% TOC. The membrane efficiencies of the shale samples were experimentally determined to range from 5% to 28% depending on clay content and stress. The outcrop shale showed 1% of swelling that is significantly higher than 0.07% of swelling developed in the well core shale lean in clay content. One of the most important findings of this research is that the membrane coefficient representing the stress sensitivity of membrane efficiency of organic-rich well core shale was found to be 11.5%/kpsi which is significantly higher than that of the swelling shale ranging from 2.2 to 3.5%/kpsi. This is interesting because the approximately 5.5 times stiffer matrix of organic-rich shale shows the significantly higher stress sensitivity in terms of the membrane efficiency. The Young's modulus of swelling shale declined with water saturation as well as the water transfer driven by osmotic pressure with low salinity brine. In contrast, the well core shale showed significant increase in the dynamic Young's modulus. The experimental data were used to evaluate the mass exchange between the fractures and rock matrix using a coupled fluid flow and geomechanics model accounting for shale swelling and osmosis transport to evaluate how fluid and rock interaction affect the oil recovery from the rock matrix of the Pierre shale during low salinity water injection (LSWI). A 25% increase in oil recovery factor was determined after considering the effects of changing membrane efficiency due to clay swelling resulting from LSWI. The modeling results therefore suggest that the variation of membrane efficiency due to swelling can be an important factor affecting the oil recovery from matrix block during LSWI enhanced oil recovery.

Capillary condensation is the condensation of the gas inside nano-pore space at a pressure lower than the bulk dew point pressure as the result of multilayer adsorption due to the high capillary pressure inside the small pore throat of unconventional rocks. The condensation of liquid in nano-pore space of rock changes its mechanical and acoustic properties. Acoustic properties variation due to capillary condensation provides us a tool to monitor phase change in reservoir as a result of nano-confinement as well as mapping the area where phase change occurs as well as characterize pore size distribution. This is particularly important for tight formations where confinement has a strong effect on phase behavior that is challenging to measure experimentally. Theoretical studies have examined the effects of capillary condensation; however, these findings have not been verified experimentally. The main objective of this study is to experimentally investigate the effect of capillary condensation on the mechanical and acoustic properties of shale samples. The mechanical and acoustic characterization of the samples was carried out experimentally using a state-of-the-art tri-axial facility at the Colorado School of Mines. The experimental set-up is capable of the simultaneous acquisition of coupled stress, strain, resistivity, acoustic and flow data. Carbon dioxide was used as the pore pressure fluid in these experiments. After a comprehensive characterization of shale samples, experiments were conducted by increasing the pore pressure until condensation occurs while monitoring the mechanical and acoustic properties of the sample to quantify the effect of capillary condensation on the mechanical and acoustic properties of the sample. Experimental data show a 5% increase in Young's Modulus as condensation occurs. This increase is attributed to the increase in pore stiffness as condensation occurs reinforcing the grain contact. An initial decrease in compressional velocity was observed as pore pressure increases before condensation occurs which is attributed to the expansion of the pore volume when pore pressure increases. After this initial decrease, compressional velocity slightly increases at a pressure around 750 - 800 psi which is close to the condensation pressure. We also observed a noticeable increase in shear velocity when capillary condensation occurs, this could be due to the immobility of the condensed liquid phase at the pore throats. The changes of geomechanical and acoustic signatures were observed at around 750 - 800 psi at 27°C, which is the dew point pressure of the fluid in the nano-pore space of the sample at this temperature. While the unconfined bulk dew point pressure of carbon dioxide at the same temperature is 977 psi. Hence, this study marks the first measurement of the dew point of fluid in nano-pore space and potentially leads to the construction of the phase envelope of fluid under confinement.

This paper presents analytical and experimental studies of the effects of supercritical CO2 injection on the seismic velocity of sandstone initially saturated with saline water. The analytical model is based on poroelasticity theory, particularly the application of the Biot-Gassmann substitution theory in the modeling of the acoustic velocity of porous rocks containing two-phase immiscible fluids. The experimental study used a high pressure and high temperature triaxial cell to clarify the seismic response of samples of Berea sandstone to supercritical CO2 injection under deep saline aquifer conditions. Measured ultrasonic wave velocity changes during CO2 injection in the sandstone sample showed the effects of pore fluid distribution in the seismic velocity of porous rocks. CO2 injection was shown to decrease the P-wave velocity with increasing CO2 saturation whereas the S-wave velocity was almost constant. The results confirm that the Biot-Gassmann theory can be used to model the changes in the acoustic P-wave velocity of sandstone containing different mixtures of supercritical CO2 and saline water provided the distribution of the two fluids in the sandstone pore space is accounted for in the calculation of the pore fluid bulk modulus.

It is well known that shear wave propagates slower across than parallel to a fracture, and as a result, a travelling shear wave splits into two directions when it encounters a fracture. Shear wave splitting and permeability of porous rock core samples having single fracture were experimentally investigated using a high-pressure triaxial cell, which can measure seismic shear wave velocities in two directions mutually perpendicular to the sample axis in addition to the longitudinal compressive wave velocity. A single fracture was created in the samples using a modified Brazilian split test device, where the cylindrical sample edges were loaded on two diametrically opposite lines by sharp guillotines along the sample length. Based on tilt tests and fracture surface profilometry, the method of artificially induced tensile fracture in the sample was found to create repeatable fracture surfaces and morphologies. Seismic velocities of the fractured samples were determined under different levels of stress confinement and fracture shear displacement or mismatch. The effective confining stress was varied from 0.5 MPa to55 MPa, while the fractures were mismatched by 0 mm, 0.45 mm and 1 mm. The degree of matching of the fracture surfaces in the core samples was evaluated using the joint matching coefficient（JMC）. Shear wave splitting, as measured by the difference in the magnitudes of shear wave velocities parallel(V S1 )and perpendicular(V S2 ) to the fracture, is found to be insensitive to the degree of mismatching of the fracture joint surfaces at 2 MPa, and decreased and approached zero as the effective stress was increased.Simple models for the stress-and JMC-dependent shear wave splitting and fractured rock permeability were developed based on the experimental observations. The effects of the joint wall compressive strength（JCS）, JMC and stress on the stress dependency of joint aperture were discussed in terms of hydro-mechanical response. Finally, a useful relationship between fractured rock permeability and shear wave splitting was found after normalization by using JMC.

Technological advancement of laboratory testing methods for investigating comprehensive geomechanical and transport properties of reservoir rocks is of great importance to the development of comprehensive geomechanical models that can improve operational risk assessment and production optimization. The in situ stress state can largely change due to drilling, completion, fracturing, and production, depending on the initial complexity of the geological setting, intrinsic rock anisotropy, and the degrees of artificial geomechanical and hydraulic disturbances. The versatility of a geomechanical model is therefore assured by reflecting the effects of stress magnitude and anisotropy on the considered properties. The purpose of this study was to investigate the effects of intermediate principal stress on the constitutive and flow behavior of Berea sandstone under true triaxial stress state. We present results of experiments with independent variation of intermediate principal stress under an array of octahedral normal and shear stresses, performed in dry and water-saturated cylindrical Berea sandstone core samples using a novel true triaxial testing apparatus. Data analysis indicated presence of microstructural behavior suggestive of opening and closing of microfractures, and revealed the effect of intermediate principal stress on deformation, wave velocities, and permeability. Vertical permeability displayed reduction to a smaller extent under increased vertical stress than under increased radial stress while exhibiting a steady decline under higher mean stress.

Abstract Decreasing fracture effectiveness due to conductivity decay is a strong contributor to the steep production decline commonly observed in shale plays. The conductivity of a fracture is determined experimentally by measuring the pressure drop of a fluid flowing through a uniformly distributed proppant bed in a core with fixed length and height. Fracture conductivity degradation results from damage mechanisms and fluid interactions that occur during hydraulic fracturing operations. Rock softening and proppant embedment are some of these damage mechanisms. The impact of these interactions can be observed by measuring fracture conductivity in the laboratory under stress states similar to field conditions. This study is based on experiments performed on fractured and propped Niobrara core plugs. The samples were characterized using X-ray Diffraction (XRD), and X-ray Fluorescence (XRF), and helical CT-scans. The experiments were performed on a triaxial stress test assembly to monitor the chemical and mechanical alterations in the formation, proppant, and fluid under reservoir conditions. To achieve this, fluid chemical composition, dynamic and static moduli, and conductivity were obtained. The setup was used for the simultaneous acquisition of stress, ultrasonic compressional and shear wave velocities, flow data and fluid sampling. The results from this study indicate that stress-dependent, long-term fracture conductivity shows the sharpest decline in the early stages of the experiment. The associated fluid sample analysis indicates that the highest physicochemical dissolution of most of the elements is happening at the early contact of the fluid with the rock and is later enhanced by the pressure increase in the system. A comparison with the conductivity measurements performed on Vaca Muerta samples shows a similar behavior, yet a steeper initial decay than that observed in the Niobrara samples. The difference observed between the two samples is related to the mineralogy of the formation and the high proppant embedment observed in the Vaca Muerta samples. Although higher softening occurred in the Niobrara samples, larger embedment was observed in the Vaca Muerta sample. This experimental observation is an indication that the conductivity damage varies not only with the mineralogical content of the formation, but also with the distribution of minerals along the fracture face. Geomechanical, geochemical, and flow data integration provided a better understanding of proppant embedment and mineral distribution of the rock. It is the conclusion of this study that even if the intact core sample contains an average mineralogical composition, the heterogeneity caused by variations in the mineralogy at where the fracture is induced has the biggest impact on embedment.

Summary The objective of this research is to determine the physicochemical processes underlying water and solute transport in organic-rich source rocks. To achieve this goal, a custom-designed experimental apparatus was constructed to conduct flow tests, founded on a high-pressure triaxial assembly. The apparatus is capable of maintaining core samples at reservoir pressure, temperature, and confining stress. We conducted several 120-day low-salinity osmotic tests in low-clay, organic-rich Eagle Ford carbonate-shale samples. Test results showed gradual, slow increase of pressure within the samples. Because this pressure behavior could not be explained properly with classical models, we formulated a mass-transport mathematical model that relies on fundamental chemical osmosis principles driving low-salinity brine into high-salinity core samples. Our mathematical model was articulated to simulate flow into the core as a 3D porous medium rather than transport across a thin, molecule-selective membrane. The model is dependent on the following principles: The low-salinity brine selectively enters the pores by diffusion mass transport, and the pre-existing, ionized dissolved salt molecules within the core are restrained by internal electrostatic forces to counterdiffuse in the direction opposite to that of the low-salinity-brine molecules entering the pore network. Critical model input data, such as permeability, porosity, and rock compressibility, were obtained from flow experiments on twin cores, and the diffusion coefficient was chosen by history matching. The strengths of the numerical simulation include reliance on mass-transport fundamental principles; not requiring the use of an ambiguously defined membrane-efficiency term; and relying on chemical-potential gradient as the driving force for the low-salinity brine to enter the high-salinity core, generating osmotic pressure within the pore network. The latter implies that osmotic pressure is the consequence of water entering the cores, not the cause. Results of this research have provided a more plausible explanation of pore-scale mass transport in organic-rich shales, and provide useful insights for design of effective enhanced-oil-recovery (EOR) processes.

The production in organic-rich shale reservoirs typically decline very rapidly as a result of decay in fracture conductivity. Proppant embedment into fracture walls results in reduction in the width of the proppant pack impacting the fracture conductivity. Spalling can take place as small particles break off, lowering the proppant pack permeability and porosity. The results of these two damage mechanisms have been investigated experimentally in this chapter for Niobrara and Vaca Muerta shale core samples using simultaneous measurements of compressional and shear wave velocities, conductivity, geomechanical properties, and geochemical composition changes during core flow experiments under triaxial stress state initially measuring intact sample properties, then using Brazilian tests to fracture the samples, place proppants in the fractures, and measure the variation of fracture conductivity. Niobrara samples presented about 20%–25% Young's modulus decrease when 2% KCl was used, whereas Vaca Muerta samples' Young's modulus reduction with the same fluid was 5%, yet conductivity reduction was larger than Niobrara. This variation was attributed to local mineralogical changes in the samples and the choice of higher strength calcite-filled fracture location in Niobrara sample.

The continuous changes of the stress-dependent permeability, compressibility and poroelasticity of a tight oil and gas shale were characterized by using the Constant Rate of Strain (CRS) consolidation test. The CRS consolidation test compresses a thin disk test specimen at a given constant rate of strain under one-dimensional consolidation with one-sided drainage condition and measurement of excess pore pressure at the undrained end. Permeability is calculated from the one-dimensional consolidation equation assuming oedometric loading, incompressible solid grains, and an idealized excess pore pressure distribution in the sample. The CRS test method has been widely used for the determination of the stress-dependent permeability of soft sediments, but has not been utilized for very stiff and low permeability shale as well as to obtain their stress-dependent compressibility and poroelasticity parameters. To test its appropriateness to shale, CRS tests were performed on thin disk samples of Mancos shale using a high pressure and high temperature triaxial cell under high isotropic confining stresses. Two modifications were done to make the CRS test applicable to determination of the continuous stress-dependent hydro-mechanical properties of tight shale: (1) the test method was modified for isotropic loading, and (2) nonlinear poroelastic effects were accounted for in the solution of the pore pressure dissipation equation. The permeability values from the CRS tests with the modified analytical solution were found to be in agreement with those obtained from the Constant Pressure Gradient Permeability test, and Pressure-pulse Decay Permeability test using nitrogen as pore fluid and corrected for Klinkenberg and non-Darcy flow effects.

This paper presents the results of a study on the use of nanoindentation test to characterize the strain rate-dependent compressive response of asphalt concrete. Nanoindentation is now widely used for characterization and testing of composite as well as single-phase materials. Using a small piece of sample, nanoindentation tests can evaluate material behavior and structure in terms of the elasticity, time-dependent response, yield strength, damage, crack advance, debonding, and fatigues. In this study, a mixture of asphalt and calcium carbonate filler powder filling the intergranular void space of the asphalt concrete was characterized in terms of strain rate sensitivity at room temperature. The indentation hardness is observed to continuously decrease during constant indentation strain rates, but the hardness response clearly indicates positive strain rate dependency when compared at the same indentation depths. Following the constant strain rate tests, indentation creep response of the asphalt–filler mixture was tested at constant load conditions. The strain rate sensitivity values characterized from double logarithmic relationships between indentation hardness and strain rate during constant strain rate and constant load tests are comparable with that determined from uniaxial compression test of cylindrical asphalt concrete samples. The observed indentation size effect on hardness value was analyzed based on an existing size effect model. The size effect in the asphalt–filler mixture, which is stronger than that defined by the model, could be attributed to a plastically graded surface of asphalt–filler sample.

The paper presents theoretical and experimental investigations of the effects of supercritical CO2 injection on the seismic velocity of sandstone initially saturated with saline water. The Biot-Gassmann theory is used in the modeling of the acoustic velocity of porous rocks containing two-phase immiscible pore fluids. The experimental study used a high-pressure and high-temperature triaxial cell with P&S wave measurement capability to clarify the seismic response of samples of Berea sandstone to supercritical CO2 injection under deep saline aquifer conditions. The results confirm that the Biot-Gassmann theory can be used to model the changes in the acoustic P-wave velocity of sandstone containing different mixtures of supercritical CO2 and saline wate,r provided that the distribution of the two fluids in the sandstone pore space is accounted for in the calculation of the pore fluid bulk modulus.