Phase-field Modeling of Hydraulic Fracturing under Heterogeneities and/or Layering of Rock Properties and In-situ Stress
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Energy - Geothermal↗
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The nature and origins of decametre-scale porosity in Ordovician carbonate rocks, Halahatang oilfield, Tarim Basin, China
At >7 km depths in the Tarim Basin, hydrocarbon reservoirs in Ordovician rocks of the Yijianfang Formation contain large cavities ( c . 10 m or more), vugs, fractures and porous fault rocks. Although some Yijianfang Formation outcrops contain shallow (formed near surface) palaeokarst features, cores from the Halahatang oilfield lack penetrative palaeokarst evidence. Outcrop palaeokarst cavities and opening-mode fractures are mostly mineral filled but some show evidence of secondary dissolution and fault rocks are locally highly ( c . 30%) porous. Cores contain textural evidence of repeated formation of dissolution cavities and subsequent filling by cement. Calcite isotopic analyses indicate depths between c . 220 and 2000 m. Correlation of core and image logs shows abundant cement-filled vugs associated with decametre-scale fractured zones with open cavities that host hydrocarbons. A Sm–Nd isochron age of 400 ± 37 Ma for fracture-filling fluorite indicates that cavities in core formed and were partially cemented prior to the Carboniferous, predating Permian oil emplacement. Repeated creation and filling of vugs, timing constraints and the association of vugs with large cavities suggest dissolution related to fractures and faults. In the current high-strain-rate regime, corroborated by velocity gradient tensor analysis of global positioning system (GPS) data, rapid horizontal extension could promote connection of porous and/or solution-enlarged fault rock, fractures and cavities. Supplementary material: Stable isotopic analyses and the velocity gradient tensor and principal direction and magnitude calculation are available at https://doi.org/10.6084/m9.figshare.c.4946046 Thematic collection: This article is part of the The Geology of Fractured Reservoirs collection available at: https://www.lyellcollection.org/cc/the-geology-of-fractured-reservoirs
Geology↗
Alternative, Less Water-Intensive, Fracturing Fluids for Enhanced Geothermal Systems
Four fracturing fluids; water, CO2, CO2 with water, and CO2 with an aqueous solution of a CO2-reactive polymer, poly(allylamine) (PAA, 1wt%) were evaluated using a high-temperature true-triaxial fracturing apparatus and ½ foot side granite cubic samples. All three CO2-based fracturing fluids, CO2, CO2 with water, and CO2 with aqueous PAA fractured granite at higher breakdown pressures, high transient flow rates, and produce higher-conductivity fractures as compared to water. Additionally, faster pressurization rates with CO2-based fracturing fluids (obtained when fracturing at constant flow rate mode) are found to be associated with higher fracture conductivities. When partially saturating the rock sample with PAA solution followed by fracturing with CO2, the volume expansion caused by CO2-induced cross-linking of PAA leads to a faster pressure increase due to the associated stress generated and increase in viscosity. It was also found that CO2 as a fracturing fluid injected in hot dry rock (HDR) attain the highest fracture conductivity only when injected at very high flow rates, followed very closely by the CO2/PAA fracturing fluid system that generates fractures with, on average, similarly high conductivity values though independently of injection flow rate and using 1/6 of the mass of CO2 as compared to CO2 in HDR. Breakdown pressures were also similar for CO2 stimulation in HDR and CO2/PAA fluid system under identical injection flow rates. It is concluded that CO2 (when injected in HDR) and CO2/PAA are the fluids of choice for stimulation of granitic rock samples under the studied geothermal P/T conditions. CO2/PAA, however, offer the following three additional advantages; 1) it requires significantly lower volumes of CO2, 2) fracture permeability is independent of injection flow rate, and 3) the reversible (previously reported) viscosity increase is beneficial to transport proppants if they are ever developed enhanced geothermal systems.
Jian, Guoqing↗
THE IMPACT OF ROCK LITHOLOGY AND MICROSTRUCTURAL PROPERTIES ON PROPPANT EMBEDMENT AND FRACTURE CONDUCTIVITY: A CASE STUDY OF THE CANEY SHALE, SOUTHERN OKLAHOMA, USA
THE IMPACT OF ROCK LITHOLOGY AND MICROSTRUCTURAL PROPERTIES ON PROPPANT EMBEDMENT AND FRACTURE CONDUCTIVITY: A CASE STUDY OF THE CANEY SHALE, SOUTHERN OKLAHOMA, USA
Katende, Alan↗
Geochemical Impact of Acid Spearhead and Slickwater Stimulation on Wolfcamp Shale from the Hydraulic Fracturing Test Site
The Hydraulic Fracturing Test Site 1 (HFTS-1) was a field study performed in the Wolfcamp Formation in the West Texas Permian (Midland) Basin, USA, with a focus on improving the efficiency of hydraulic fracturing. Investigating site-specific rock-fluid geochemical interactions during hydraulic fracturing is an important step to understanding the impact on formation shale porosity, permeability, and long-term shale gas production. During field operations in this region, hydraulic fracturing fluid (HFF) injection usually starts with a concentrated acid spearhead for rapid rock dissolution, followed by the injection of near-neutral pH slickwater containing chemicals and proppants. A multistep sequential injection approach was used to investigate different stages of rock-fluid interactions. The carbonate content in the host rock is important when acid spearhead is considered, as carbonate mineral dissolution is rapid and can result in porosity and permeability changes in the shale matrix. Here, in this study, we designed flow-through experiments using fractured carbonate-rich and clay-rich Wolfcamp shale cores with (1) a short-time acid soaking step and (2) a long-term slickwater flow-through step to simulate the injection method used at HFTS-1. The fluid chemistry was analyzed. A thorough mineralogical progression [e.g., Calcium (Ca) dissolution and iron (Fe) redox progression] in the cores during HFF injection was also characterized and imaged by synchrotron microprobe. Reactive transport modeling was performed based on the experimental setup. The results showed that the acid spearhead is a crucial step in creating a reaction front by mineral dissolution, especially in carbonate-rich shales. A slight layer of ferrihydrite precipitated during the slickwater flow-through period. This study provides insights into potential geochemical impact due to hydraulic fracturing operations in the Permian Basin.
clastic rock↗
Fluid Induced Elemental and Mineralogy Alterations of Caney Shale
ABSTRACT: Studies were conducted on cores and rock cuttings recovered from two wells drilled in Caney shale in the Southern Oklahoma. Mineralogical compositions of these samples were obtained by X-Ray Diffraction (XRD) measurements using two methods of sample preparation: conventional powder (by crushing and grinding of rock fragments) and spray-drying of micronized rock powder method. Microstructure and microanalysis were acquired by Scanning Electron Microscopy/Energy Dispersive Spectroscopy (SEM/EDS). Batch experiments were then conducted using selected rock samples and fracturing fluid whilst deionized water was used as control sample for fluid/rock interaction. These were conducted at 95°C and ambient pressure for various time periods to assess the geochemical reactivity of the Caney shale. The rock samples were subsequently recovered from fluid, dried and analysis repeated using identical XRD and SEM/EDS measurements, following the same conditions. Preliminary experimental results show rock mineralogical compositions are predominantly quartz with relatively high clay content. The clay minerals are mainly illite. Carbonate content is less than 10% by weight for most samples, with other non-clay minerals pyrite, albite, dolomite, and some apatite. In general, sprayed samples showed more intense peaks and less full width at half maxima (FWHM) relative to conventional rock-powder samples. Post-experimental mineralogical changes were observed in samples with clay portions breaking down to amorphous entities. As expected, dissolution of carbonates (calcite and dolomite) was observed and newly detected dissolution of biogenic micro quartz resulted in formation of amorphous silica. 1.0 INTRODUCTION Formation of sedimentary rocks involves processes where rock sediments are emplaced with fluids starting interactions that persist over long periods. During the lithification process, sediments and fluids react and attain equilibrium state. This equilibrium is disturbed during drilling, completion, hydraulic fracturing and other operations that involve the introduction of engineered fluids into the subsurface. To return to equilibrium, dissolution and precipitation of minerals may occur. These reactions have implications on the petrophysical properties of the reservoir. Whiles dissolution and precipitation of minerals can enhance porosity and permeability, the reverse can also be true. For example, when acidic fluids are introduced into formations with high carbonate contents, carbonate minerals dissolve into solution thus increasing the porosity and permeability of the formation. In contrast, when alkaline fluids with pH above 11 are introduced into an illite-rich shale formation, the rock-fluid interaction leads to de-flocculation of illite platelets causing substantial increase in clay fines migration, consequently significant reduction in permeability (Salles et al., 2008).
Awejori, G.↗
A Method to Represent a Well in a Three‐Dimensional Discrete Fracture Network Model
Abstract In discrete fracture network (DFN) modeling, fractures are randomly generated and placed in the model domain. The rock matrix is considered impermeable. Small fractures and isolated fractures are often ignored to reduce computational expense. As a result, the rock matrix between fractures could be large and intersections may not be found between a well introduced in the model and the hydraulically connected fracture networks (fracture backbones). To overcome this issue, this study developed a method to conceptualize a well in a three‐dimensional (3D) DFN using two orthogonal rectangular fractures oriented along the well's axis. Six parameters were introduced to parameterize the well screen and skin zone, and to control the connectivity between the well and the fracture backbones. The two orthogonal fractures were discretized using a high‐resolution mesh to improve the quality of flow and transport simulations around and along the well. The method was successfully implemented within dfnWorks 2.0 (Hyman et al. 2015) to incorporate a well in a 3D DFN and to track particles leaving an injection well and migrating to a pumping well. Verification of the method against MODFLOW/MODPATH found a perfect match in simulated hydraulic head and particle tracking. Using three examples, the study showed that the method ensured the connectivity between wells and fracture backbones, and honored the physical processes of flow and transport along and around wells in DFNs. Recommendations are given for estimating the values of the six introduced well parameters in a real‐world case study.
Pham, Hai (ORCID:0000000310164394)↗
Abstract for Joint Work Statement 5 (AGMT-1005) between National Energy Technology Laboratory (NETL) and ExxonMobil Research and Engineering Company (EMRE) to Umbrella CRADA (AGMT-0922)
NETL and EMRE will focus on fundamental questions aimed at understanding the controlling factors on tight oil production flow paths and how those flow paths are impacted by the hydraulic fracturing fluids and processes. Key questions for the Parties to explore include the following: Are there geochemical reactions between hydrocarbon producing tight reservoir rocks and water from the hydraulic fracturing process? To what extent does this alteration impact oil production? By further characterizing the flow paths near the fracture-matrix interface, what is the relative impact of geochemical process compared to geomechanical changes on production? This research focuses on rock types found in the Permian Basin. The research drives along two main thrusts: 1) Inorganic geochemistry studies involving representative rock types, connate fluids (hydrocarbons and water), and injected fluids (fracturing water) will be conducted in order to bound the kinetics of key mineral reactions; 2) Computed tomography (CT) for characterization of flow paths as a function of geochemical and geomechanical processes associated with hydraulic fracture creation and reservoir production. The objective of this characterization is to ultimately develop new insights into the role geochemical and geomechanical effects play on production. The work will include studying mechanical damage near the rock-fracture interface and how it impacts flow properties.
02 PETROLEUM↗
Gouge formation and dilation impacts to flow during fracture shearing
We report flow in fractures is of critical importance to fluid flow in low-permeability shales, particularly when they restrict vertical migration from reservoirs. The Mt. Simon Sandstone is a target for geologic CO 2 sequestration and the overlying Eau Claire Formation is the sealing formation capping this reservoir. The lower Eau Claire, a primary barrier to upward migration of CO 2 from the Mt. Simon, is lithologically heterogenous with a mixture of sandstone and shale. We use a series of flow-through experiments on fractured samples from the Eau Claire to document the morphological changes taking place in the fracture and surrounding rock as it is sheared in a stepwise fashion. The samples were imaged using computed tomography after each shearing episode and flow resistance was calculated based on the pressure differential across each core. We found that gouge accumulations in the fracture plane led to specific instances of self-sealing behavior despite the fact that most samples increased in average fracture aperture and transmissivity. Gouge limited the increase or, in some cases, decreased flow through these samples. Fractures adjacent to shale-rich zones and concordant with bedding were more likely to show evidence of self-sealing, while increased sandstone content and fracture orientation perpendicular to bedding planes was more likely to result in fracture dilation and fracture permeability increases. Reactivation and shearing of vertical fractures in lithologically heterogenous caprocks is more likely to pose risks to long term CO 2 storage scenarios.
58 GEOSCIENCES↗
Study of the stress state around dynamically pressurized underground cavities in porous saturated rock formations and its implication for the fracture growth analysis
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Geosciences↗
Study on Foam Fracturing of Granite for the Development of Enhanced Geothermal Systems
Foam fracturing has been considered as a potential technical approach to address water concerns with hydraulic stimulation in the development of enhanced geothermal systems (EGS). This paper reports the recent progress of a project sponsored by the U.S. DOE GTO Waterless Stimulation Initiative. The project investigates the foam fracturing as an alternative approach to the hydraulic fracturing. The paper describes the development of foam fracturing testing system at Oak Ridge National Laboratory (ORNL). The system can be used to perform the hydraulic fracturing of rock-like materials with both water and foamed fluids under pressure to 6,000 psi (41.4 MPa). The system features the sections of foam generation and foam injection and has the capability of testing materials in both monotonic and cyclic injection modes.Experimental results of foam fracturing on Charcoal Black granite will be reported. Cylindrical granite specimens with a blind hole were studied along with water and nitrogen-gas-in-water foam as the fracturing fluids. The effects of fracturing fluids and injection modes on the breakdown pressure and failure responses of the material were examined. The observations from the experimental work and the implications to the EGS application will be presented and discussed.
Wang, Hong↗
An Integrated Multiscale Modeling Framework for Unconventional Stimulation and Production (Final Report)
The production of oil and gas from unconventional reservoirs largely depends upon two main features operating at different scales: (1) the establishment of a reservoir scale stimulated fracture network that effectively communicates with the rock volume, enhancing permeability and transport to the wellbore and (2) the coupled multi-phase flow, chemical and mechanical processes affecting the migration of hydrocarbons from the low permeability country rock adjacent to the stimulated fracture network. To date, there has been no simulation framework that allows seamless and integrated prediction of these features across spatial scales extending from the pore structure of the reservoir rock to the volume of the reservoir. In addition, there has been a lack of suitable field measurements to test such models, as stimulation and production data are often proprietary and not freely available to national laboratories and academic institutions. New multi-scale simulation capabilities are needed that are validated against suitable field-based research experiments on hydraulic fracturing and shale production.
02 PETROLEUM↗
Lattice modeling of excavation damage in argillaceous clay formations: Influence of deformation and strength anisotropy
This paper presents modeling of mechanical anisotropy in argillaceous rocks using an irregular lattice modeling approach, namely the rigid-body-spring network. To represent the mechanical anisotropy, new schemes are implemented in the modeling framework. The directionality of elastic deformation is resolved by modifying the element formulation with anisotropic elastic properties. The anisotropy of strength and failure characteristics is facilitated by adopting orientation-dependent failure criteria into the failure model. The verification of the improved modeling procedures is performed against theoretical model predictions for unconfined compression tests with various bedding orientations. Furthermore, excavation damage and fracturing processes in rock formations are simulated for different geomechanical configurations, such as rock anisotropy and tectonic heterogeneity. The simulated excavation damage characteristics are realistic and comparable with the actual field observation at a tunnel located in an argillaceous clay formation. The simulation results provide insights into the excavation damage zone phenomena with an explicit representation of fracturing processes.
58 GEOSCIENCES↗
Crystalline Disposal R&D at LBNL: FY20 Progress Report
Within the Spent Fuel & Waste Science and Technology (SFWST) Program, research work continues further with the goal to better understand long-term performance of disposal systems in three main geologic rock types: clay/shale, salt, and crystalline rock. This report documents LBNL’s research activities related to investigations of crystalline host rock according to the scope of two work packages: SF-20LB01030207 “Crystalline International Collaborations – LBNL,” and SF-20LB01030203 “Crystalline Disposal R&D – LBNL.” These research activities correspond are related to key Features, Events, and Processes (FEPs), ranked in importance from medium to high, as listed in Table 7 of the Used Fuel Disposition Campaign Disposal Research and Development Roadmap (FCR&D-USED-2011-000065 REV0) (Nutt, 2011). Specifically, these research activities address FEP 2.2.01, Excavation Disturbed Zone (EZD). The results of these research activities provide important insights into understanding and predicting flow and transport processes that could occur in low-permeability crystalline rocks, in which fractures might serve as main conduits for fluid flow and radionuclide transport. The evolution of the EDZ during the excavation of the tunnel as well as the evolution of microcrack growth within the EDZ after the emplacement of backfill are critical for predicting the long- term behavior of the EDZ. A number of factors including stress, temperature, water activity, capillary pressure, chemistry, and mineralogy can affect the rock evolution, which require advanced experimental tools to study it.
11 NUCLEAR FUEL CYCLE AND FUEL MATERIALS↗
Utah FORGE 5-2428: Coupled Investigation of Fracture Permeability Impact on Reservoir Stress and Seismic Slip - 2024 Annual Workshop Presentation
This is a presentation on the Coupled Investigation of Fracture Permeability Impact on Reservoir Stress and Seismic Slip Behavior by Lawrence Livermore National Laboratory, presented by Kayla Kroll. This presentation addresses testing and modelling related to the dependency of EGS production on sustained permeability and sufficient fluid flow through pre-existing or induced fractures in hot rock. This presentation was featured in the Utah FORGE R&D Annual Workshop on August 13, 2024.
15 GEOTHERMAL ENERGY↗
Accessing the Subsurface Biosphere Within Rocks Undergoing Active Low-Temperature Serpentinization in the Samail Ophiolite (Oman Drilling Project)
The Oman Drilling Project established an “Active Alteration” multi-borehole observatory in peridotites undergoing low-temperature serpentinization in the Samail Ophiolite. The highly serpentinized rocks are in contact with strongly reducing fluids. Distinct hydrological regimes, governed by differences in rock porosity and fracture density, give rise to steep redox (Eh +200 to –750 mV) and pH (pH range 8.5–11.2) gradients within the 300–400 m deep boreholes. The serpentinites and fluids host an active subsurface ecosystem. Microbial cell abundances in serpentinite vary at least six orders of magnitude, from ≤3.5 × 10 1 to 2.9 × 10 7 cells/g. Low levels of biological sulfate reduction (2–1,000 fmol/cm 3 /day) can be detected in rock cores, particularly in rocks in contact with reduced groundwaters with pH < 10.5. Thermodesulfovibrio is the predominant sulfate reducer identified via metagenomic sequencing of adjacent groundwater communities. Here, we infer that transport and reaction of microbially generated sulfide with the serpentine and brucite assemblages gives rise to optical darkening and sulfide overprinting, including the formation of tochilinite-vallerite group minerals, potentially serving as an indicator that this system is inhabited by microbial life. Olivine mesh-cores replaced with ferroan brucite and minor awaruite, abundant veins containing hydroandradite garnet and polyhedral serpentine, and late-stage carbonate veins are suggested as targets for future spatially resolved life-detection investigations. The high-quality whole-round core samples that have been preserved can be further probed to define how life distributes itself and functions within a system where chemical disequilibria are sustained by low-temperature water/rock interaction, and how biosignatures of in situ microbial activity are generated.
58 GEOSCIENCES↗
Injection Parameters That Promote Branching of Hydraulic Cracks
Fluid injection into rock formations can either produce complex branched hydraulic fractures, create simple planar fractures, or be dominated by porous diffusion. Currently, the optimum injection parameters to create branched fractures are unknown. We conducted repeatable hydraulic fracturing experiments using analog-rock samples with controlled heterogeneity to quantify the fluid parameters that promote fracture branching. A large range of injection rates and fluid viscosities were used to investigate their effects on induced fracture patterns. Paired with a simple analytical model, our results identify the threshold at which fracture transitions from an isolated planar crack to branched cracks when closed natural fractures exist. These results demonstrate that this transition can be controlled by injection rate and fluid viscosity. In relation to the field practices, the present model predicts slickwater and lower viscosity fluid injections promote fracture branching, with the Marcellus shale used as an example.
58 GEOSCIENCES↗
Fe Oxidation and Species Distribution at the Rock–Fluid Interface of Marcellus Shale Reacted with Hydraulic Fracturing Fluid
Hydraulic fracturing of shale reservoirs resulted in significant opportunity for increased oil and gas production in the United States. Rock-fluid interactions can cause mineral dissolution and precipitation reactions that lead to permeability changes in the shale matrix, which ultimately may affect transport pathways and hydrocarbon production. Understanding the distribution of secondary precipitates, such as barite and Fe(III) (hydro)oxides, and cation leaching at the rock-fluid interface is an important step to further investigate how these geochemical processes can change permeability and transport pathways. In this study, thin sections of the fracture-matrix interface were made from reacted Marcellus shale cores. The thin sections were characterized using synchrotron X-ray fluorescence imaging and synchrotron X-ray absorption spectroscopy. Fe species with different oxidation states were identified in the maps, together with barite and Ca distribution. The results show that ferrihydrite, as newly formed Fe(III)-bearing precipitates, aligned well with the border of the Ca (e.g., calcite) leaching region in the reaction front. Some Fe-containing clay also dissolved, but the dissolution region for the clay was not as deep as the calcite. Further, the reaction front is about three times deeper in the direction parallel to the shale bedding than that perpendicular to the bedding. The Ca leaching region can be an index for reaction front detection for Marcellus shale. Reactive transport modeling was conducted and the predicted Ca leaching boarder align well with ferrihydrite precipitation, consistent with the experimental observation. The carbonate mineral dissolution can be crucial to promote fluid access into the shale matrix. Together with our previous study on the shale reactive surface, this follow-up study showed similar Ca leaching region and Fe(III) precipitates distribution in the reaction front regardless of barite precipitation on the surface, indicating that the barite coatings on the surface may not pose a significant impact on reactive transport at the shale-fluid interface.
04 OIL SHALES AND TAR SANDS↗