Fracture Conductivity Modelling in Experimental Shale Rock Interactions with Aqueous CO\u3csub\u3e2\u3c/sub\u3e

In large scale subsurface injection of carbondioxide (CO ) as obtainable in carbon sequestration programs and in environmentally friendly hydraulic fracturing processes (using supercritical CO ), long term rock-fluid interaction can affect reservoir and seal rocks properties which are essential in monitoring the progress of these operations. The mineralogical components of sedimentary rocks are geochemically active particularly under enormous earth stresses, which generate high pressure and temperature conditions in the subsurface. While geomechanical properties such as rock stiffness, Poisson\u27s ratio and fracture geometry largely govern fluid flow characteristics in deep micro-fractured formations. Simulation results suggested that influx-induced mineral dissolution/precipitation reactions within clay-based sedimentary rocks can continuously close micro-fracture networks, though injection pressure and effective-stress transformation first rapidly expand the fractures. This experimental modelling research investigated the impact of in-situ geochemical precipitation on conductivity of fractures under geomechanical stress conditions. Bulk rock geomechanical hardness was determined using Vickers\u27 micro-indentation. Differential pressure drop data across fractured composite core were also measured with respect to time over a five day period. This was used in estimating the conductivity of the artificially fractured cores with 25 μm-bore microtubings embedded. Three experimental runs per sample types were carried out in order to check the repeatability of observed changes. The results showed that most significant diagenetic changes in shale rocks after flooding with CO -brine, reflect in the effluent fluid with predominantly calcium based minerals dissolving and precipitating under experimental conditions. Micro-indentation results suggested slight reduction in the hardness of the shale rocks and this reduction appears dependent on diagenetic quartz content. Estimated fracture conductivity indicated that reactive transport of dissolved minerals can occlude micro-fracture flow paths, thereby improving caprock seal integrity with respect to leakage risk under CO sequestration conditions. 2 2 2

Experimental Evaluation of Wellbore Cement- Formation Shear Bond Strength in Presence of Drilling Fluid Contamination

The objective of this experimental study is to investigate the impact of physical and chemical mud contaminations on cement-formation shear bond strength for sandstone and shale formations. Physical contamination occurs when drilling fluids (mud) dehydrates on the surface of the formation, while chemical contamination on the other hand occurs when the drilling fluid (still in the liquid state) is mixed with cement slurry and reacts chemically with the cement during a cementing job. We investigated the impact of the contamination on the shear bond strength and the changes in the mineralogy of the cement at the cement-formation interface to quantify the impact of the contamination on the cement-formation shear bond strength. Wellbore cement has been used to provide well integrity through zonal isolation in oil & gas wells as well as geothermal wells. Cement failures could result from poor cementing, failure to completely displace the drilling fluids to failure on the path of the casing. A failed cement job could result in creation of cracks and micro annulus through which produced fluids could migrate to the surface leading to sustained casing pressure, contamination of fresh water aquifer and blow out in some cases. In addition, cement failures could risk the release of chemical substances from hydraulic fracturing into fresh water aquifer during the injection process. To achieve proper cementing, the drilling fluid should be completely displaced by the cement slurry. However, this is hard to achieve in practice, some mud is usually left on the wellbore which ends up contaminating the cement afterwards. For this experimental study, Berea sandstone and clay rich rock discs/cores had cement bonded with them to simulate cement-formation interfaces. These interface were contaminated either physically (dehydrated clays deposited on the surface) or chemically (by intermixing drilling fluids with cement slurry). Shear bond tests were performed on the composite cores after complete hydration of cement occurred (after 28days) in order to determine the shear bond strength. Preliminary results suggested that the detrimental impact of the contamination is higher when the cores are physically contaminated i.e. when we have mud cake present at the surface of the wellbore before a cement job is performed. Also, the results showed that shear bond strength is higher for sandstone formations when compared to shale formations, implying that the low permeability formations form much weaker bond with cement. This is of particular interest to wellbore integrity issues in hydraulic fracturing where high injection pressures of fracking fluids can easily cause de-bonding of weak rock-cement interface. Material characterization analysis was carried out to determine the micro structural changes at the cement-formation interface. Electron microscopy provided coupling of chemical/mineralogical composition with geomechanics of the interface. The phase compositions were characterized using a Jeol 8530F EPMA (with 5 wavelength dispersive spectrometers and a SDD energy dispersive spectrometer). Line transects were used to assess variations in the bulk composition. Abundances of phases were estimated using the Thermo NSS and Compass software on a Hitachi S3500N SEM with a energy dispersive spectrometer

Use of liquid pressure-pulse decay permeameter in experimental evaluation of permeability in wellbore cement under geopressured geothermal conditions

Geopressured reservoirs in the northern Gulf of Mexico basin along the coast of Louisiana have been determined to be viable source of geothermal energy and potential sites for carbon sequestration, where CO2 can be utilized to induce convective flow of geofluids and enhance heat harvesting. These reservoirs are made of unconsolidated sandstone capped by shale layers and possess temperatures as high as 140⁰C. At high temperatures, cement strength retrogression occur when calcium silicate hydrate phase in hydrated cement converts to alpha dicalcium silicate hydrate phase. The higher the temperature, the quicker the rate of transformation of calcium silicate hydrate. The conversion changes the structure of the hydrated cement leading to increased porosity, permeability and lowered compressive strength. The real problem lies in the great increase of permeability which makes the cement susceptible to chemical attack by low pH formation fluids which lead to loss of hydraulic barrier capability of cement, the most important function of cement in well bore system. The consequence of the loss of zonal isolation is the environmental release of previously contained geofluids, This study uses liquid pressure-pulse decay permeameter (PDPL) to measure the effect of increased temperature on cement permeability. PDPL is computer operated device capable of measuring permeability of cement to liquid (water) under reservoir conditions. Compared to conventional (steady state) methods, the liquid pressure-pulse decay permeameter cuts down the long time required to stabilize water fluxes from days or weeks to hours. This is very critical as cement permeability could change due to leaching or hydration during the time required in steady state methods. Permeability is calculated using pressure decay across a cement core sample over time. For the experiment, a range of chemical additive were added to portland cement slurry to counteract and curb strength retrogression, changing cement hydration products into chemically more stable phases, with favorable Ca to Si ratio. Four 13.1 pounds per gallon (ppg) (with water to solid ratio of 0.87) cement slurry designs with silica flour, calcined clay, silica sand, steel fiber and glass fiber and 13.1 ppg neat cement slurry were subjected to cycle thermal loading in salt brine. The results indicates that glass fiber and steel fiber cement can be added to the design to improve the permeability and increase the strength of the cement sheath for geopressured geothermal reservoirs in the Gulf of Mexico

Experimental investigations of caprock integrity in CO\u3csub\u3e2\u3c/sub\u3e sequestration

The geochemical reactivity of shale caprock during post-injection CO diffusive transfer should be included in the reservoir characterization of CO sequestration sites as slow reactive transport processes can either improve or degrade seal integrity over the long term. Simulation results reported by various researchers suggested that influx-induced mineral dissolution/precipitation reactions within shale caprocks can continuously close micro-fracture networks, while pressure and effective-stress transformation first rapidly expand then progressively constrict them. This experimental work applied specific analytical techniques in investigating changes in mineralogical and microstructural properties of crushed shale rocks after exposure (by flooding) to CO -brine for a time frame ranging between 30 days to 92 days at elevated pressure and fractional flow rate. The initial mineralogical composition in the samples showed bulk clay, quartz and feldspar as the major components of the formation. Initial capillary entry parameters for the shale were estimated from digitally acquired injection pressure profile. Flooding of the shale samples with CO -brine was followed by geochemical characterization of the effluent fluid and bulk shale rock through pH and XRD measurements. Nano-scale measurement of changes in internal specific surface area and cumulative pore size distribution (using the BET Technique) revealed that changes in the shale caprock due to geochemical interaction with aqueous CO can affect petrophysical properties. The naturally low permeability in shale may be altered by changes in specific surface area as the effective permeability of any porous medium is largely a function of its total pore geometry/volume, and particularly the pore throat-diameter. The geochemical reactivity of shale caprocks through acidic fluid transport in interconnected pore networks gave insight into the impact of diffusion and reaction rate on shale caprock integrity in CO sequestration. 2 2 2 2 2

Application of surface analysis in a study of the mechanism of alkali-carbonate reaction in concrete.

SIGLEAvailable from British Library Document Supply Centre-DSC:DXN028034 / BLDSC - British Library Document Supply CentreGBUnited Kingdo

Experimental Investigation of the Impact of Compression on the Petro-Physical and Micromechanical Properties of Wellbore Cement Containing Salt

Abstract In this study, we investigated the effect of compression on the micromechanical and the petrophysical properties of salted wellbore cement systems. The experiments were conducted using a customized bench scale model, which utilized an expandable tubulars simulating the compression of a previously cemented casing under field-like conditions. The "mini-wellbore model" sample consisted of a pipe inside pipe assembly with a cemented annulus. The cement samples were cured in a water bath for 28 days prior to the compression experiments to allow adequate hydration. The impact of compression on the cement's petro-physical and mechanical properties was quantified by measuring the porosity, permeability and hardness of salt cement cores drilled parallel to the orientation of the pipe from the compacted cement sheath. Permeability (Core-flood) experiments were conducted at 21˚C, 10,342 kPa confining pressure for a period of 120 minutes. During the core-flood experiments, conducted using Pulse-decay method, deionized water was flowed through cement cores to determine the permeability of the cores. The results obtained from these experiments confirmed that the compression of the cement positively impacted the cements ability to provide long term zonal isolation, shown by the effective reduction in porosity and permeability. Furthermore, the results confirm reduction in the detrimental effect of salt on the strength and stiffness in post-compression cement

Experimental study on a single cement-fracture using CO\u3csub\u3e2\u3c/sub\u3e rich brine

The efficiency of Carbon Capture and Storage (CCS) projects is directly related to the long term sealing efficiency of barrier systems and of wellbore cement in wellbores penetrating storage reservoirs. The microfractures inside the wellbore cement provide possible pathways for CO leakage to the surface and/or fresh water aquifers, impairing the long-term containment of CO in the subsurface. The purpose of this experimental study is to understand the dynamic alteration process in the cement caused by the acidic brine. The first experiment, at ambient temperature and pressure, was conducted by flowing CO -rich brine through 1 in. by 2 in. (25.4 mm by 50.8 mm) cement cores for 4 and 8 weeks durations. The second experiment was a 4 weeks long flow-through experiment conducted at ambient conditions using a 1 in by 12 in.(25.4 mm by 304.8 mm) cement core and CO -rich brine with a core flooding system under 600 psi (4.13 MPa) confining stress. Post-experiment material analysis from both experiments confirmed leaching of Ca from reacted cement, as reported in literature. However for the first time, porosity of the reacted regions was semi-quantified applying micro-CT images. © 2010 Elsevier Ltd. © 2011 Published by Elsevier Ltd. 2 2 2 2 2

Review of Geochemical and Geo-Mechanical Impact of Clay-Fluid Interactions Relevant to Hydraulic Fracturing

Shale rocks are an integral part of petroleum systems. Though, originally viewed primarily as source and seal rocks, introduction of horizontal drilling and hydraulic fracturing technologies have essentially redefined the role of shale rocks in unconventional reservoirs. In the geological setting, the deposition, formation and transformation of sedimentary rocks are characterised by interactions between their clay components and formation fluids at subsurface elevated temperatures and pressures. The main driving forces in evolution of any sedimentary rock formation are geochemistry (chemistry of solids and fluids) and geomechanics (earth stresses). During oil and gas production, clay minerals are exposed to engineered fluids, which initiate further reactions with significant implications. Application of hydraulic fracturing in shale formations also means exposure and reaction between shale clay minerals and hydraulic fracturing fluids. This chapter presents an overview of currently available published literature on interactions between formation clay minerals and fluids in the subsurface. The overview is particularly focused on the geochemical and geomechanical impacts of interactions between formation clays and hydraulic fracturing fluids, with the goal to identify knowledge gaps and new research questions on the subject

Diagenetic influence on fracture conductivity in tight shale and CO\u3csub\u3e2\u3c/sub\u3e sequestration

Most sedimentary rock formations (tight or highly porous) have geochemical characteristics that can lead to significant reactive ion exchange processes in aqueous media in the presence of carbondioxide. While geomechanical properties such as rock stiffness, poisson\u27s ratio and fracture geometry largely govern fluid flow characteristics in deep fractured formations, the effect of mineralization can lead to flow impedance in the presence of favorable geochemical and thermodynamic conditions. Shale caprock which seals more than two-thirds of oil and gas reservoirs have natural fractures that are unevenly distributed in the geosystem. Experimental works which employed the use of analytical techniques such ICP-OES, XRD, SEM/EDS and BET techniques in investigating diagenetic and micro-structural property of crushed shale caprock/CO -brine system concluded that net precipitation reaction processes can affect the distribution of petrophysical nanopores in the seal rock. XRD analyses indicated the presence of quartz, feldspar and bulk clays (muscovite, chlorite, kaolinite with the quantitative mineralogy estimates varing significantly with respect to quartz-bulk clay ratio in the six samples that were analyzed. Quartz and feldspar are reactive at low pH with the tendency to impact seal integrity. The presence of quartz in shale gives a reasonably high mechanical strength whereas clays make shale easily deformable with a potential to creep. The results showed that geochemical precipitates can be formed such that fluid flow through open micro and macro fractures may be constrained. Peclet-Damköhler reactive flow dimensionless number confirmed diffusion as the governing transport mechanism in aqueous CO -caprock interaction. Simulation results reported by various researchers suggested that influx-induced mineral dissolution/precipitation reactions within shale caprocks can continuously close micro-fracture networks, while pressure and effective-stress transformation first rapidly expand then progressively constrict them. The presence of traces of carbonate streaks which are soluble in low acidic pH environment is undesirable in caprocks. This experimental research investigated the impact of in-situ geochemical precipitation on conductivity of open micro-fractures under geomechanical stress conditions. Fracture conductivity in core samples of shale caprock with known mineralogical composition from different formations where CO injection is on-going are quantitatively evaluated under axial and radial stress using pulse-decay liquid permeametry/core flooding systems. This system incorporates high temperature and pressure conditions. The shale caprock cores were obtained during the drilling of vertical and short-radii injection wells in Alabama and South Louisiana as part of reservoir characterization for CO sequestration/enhanced oil recovery projects. Nano-indentation of multiple representative samples was applied to determine geomechanical properties evolution which can be correlated with the geochemistry of the shale caprock. This information will be useful as input data for simulation of subsurface CO plume in contact with overlaying shale caprock. Modeling of the diffusion controlled fluid flow and induced fracture diagenetic alterations in the shale caprocks are performed using CMG-GEM numerical simulators with imposed axial and radial geomechanical stress. The possibility of rock-fluid geochemical interactions constricting natural fracture conductivity in long term subsurface CO sequestration can lead to significant improvement in shale caprock seal integrity and mitigate injection induced perturbation. 2 2 2 2 2

MICROSTRUCTURE AND MICROMECHANICS OF SHALE ROCKS: CASE STUDY OF MARCELLUS SHALE

Shale rocks play an essential role in petroleum exploration and production because they can occur either as source rocks or caprocks depending on their mineralogical composition and microstructures. More than 60% of effective seals for geologic hydrocarbon bearing formations as natural hydraulic barriers constitute of shale caprocks. The effectiveness of caprock depends on its ability to immobilize fluids, which include a low permeability and resilience to the in-situ formation of fractures as a result of pressurized injection. The alteration in sealing properties of shale rocks is directly related to the differences in their mineralogical composition and microstructure. Failure of the shale starts with deterioration at micro/nanoscale, the structural features and properties at the micro/nanoscale can significantly impact the durability performance of these materials at the macroscale, therefore, study at micro/nanoscale becomes necessary to get better understanding of the hydraulic barriers materials to prevent failure and enhance long-term geologic storage of fluids. Indentation tests were conducted at both micro and nanometer level on Marcellus shale samples to get the mechanical properties of bulk and individual phase of the multiphase materials. The mechanical properties map were created based on the nano indentation results and the properties of each individual phase can be correlated with bulk response in the multiphase composite; the effect of each component on the microstructure and bulk mechanical properties can be better understood