# Daniel Birdsell Publications Content

## Publications

Underlined names are links to current or past GEG members

### REFEREED PUBLICATIONS IN JOURNALS

5.
Mindel, J.E., P. Alt-Eppig, A.A. Les Landes, S. Beernink, D.T. Birdsell, M. Bloemendal, V. Hamm, et al , M.O. Saar, D. Van den Heuvel, and T. Driesner, Benchmark study of simulators for thermo-hydraulic modelling of low enthalpy geothermal processes, Geothermics, 96/102130, 2021. [View Abstract]In order to assess the thermo-hydraulic modelling capabilities of various geothermal simulators, a comparative test suite was created, consisting of a set of cases designed with conditions relevant to the low-enthalpy range of geothermal operations within the European HEATSTORE research project. In an effort to increase confidence in the usage of each simulator, the suite was used as a benchmark by a set of 10 simulators of diverse origin, formulation, and licensing characteristics: COMSOL, MARTHE, ComPASS, Nexus-CSMP++, MOOSE, SEAWATv4, CODE_BRIGHT, Tough3, PFLOTRAN, and Eclipse 100. The synthetic test cases (TCs) consist of a transient pressure test verification (TC1), a well-test comparison (TC2), a thermal transport experiment validation (TC3), and a convection onset comparison (TC4), chosen to represent well-defined subsets of the coupled physical processes acting in subsurface geothermal operations. The results from the four test cases were compared among the participants, to known analytical solutions, and to experimental measurements where applicable, to establish them as reference expectations for future studies. A basic description, problem specification, and corresponding results are presented and discussed. Most participating simulators were able to perform most tests reliably at a level of accuracy that is considered sufficient for application to modelling tasks in real geothermal projects. Significant relative deviations from the reference solutions occurred where strong, sudden (e.g. initial) gradients affected the accuracy of the numerical discretization, but also due to sub-optimal model setup caused by simulator limitations (e.g. providing an equation of state for water properties).

4.
Birdsell, D. T., B. M. Adams, and M. O. Saar, Minimum Transmissivity and Optimal Well Spacing and Flow Rate for High-Temperature Aquifer Thermal Energy Storage, Applied Energy, 289/116658, pp. 1-14, 2021. [View Abstract]Aquifer thermal energy storage (ATES) is a time-shifting thermal energy storage technology where waste heat is stored in an aquifer for weeks or months until it may be used at the surface. It can reduce carbon emissions and HVAC costs. Low-temperature ($<25$ \degree C) aquifer thermal energy storage (LT-ATES) is already widely-deployed in central and northern Europe, and there is renewed interest in high-temperature ($>50$ \degree C) aquifer thermal energy storage (HT-ATES). However, it is unclear if LT-ATES guidelines for well spacing, reservoir depth, and transmissivity will apply to HT-ATES. We develop a thermo-hydro-mechanical-economic (THM\$) analytical framework to balance three reservoir-engineering and economic constraints for an HT-ATES doublet connected to a district heating network. We find the optimal well spacing and flow rate are defined by the reservoir constraints'' at shallow depth and low permeability and are defined by the economic constraints'' at great depth and high permeability. We find the optimal well spacing is 1.8 times the thermal radius. We find that the levelized cost of heat is minimized at an intermediate depth. The minimum economically-viable transmissivity (MEVT) is the transmissivity below which HT-ATES is sure to be economically unattractive. We find the MEVT is relatively insensitive to depth, reservoir thickness, and faulting regime. Therefore, it can be approximated as$5\cdot 10^{-13}$m$^3\$. The MEVT is useful for HT-ATES pre-assessment and can facilitate global estimates of HT-ATES potential.

3.
Birdsell, D., S. Karra, and H. Rajaram, On the Representation of the Porosity‐Pressure Relationship in General Subsurface Flow Codes, Water Resources Research, 54/2, pp. 1382-1388, 2018. [View Abstract]The governing equations for subsurface flow codes in a deformable porous media are derived from the balance of fluid mass and Darcy's equation. One class of these codes, which we call general subsurface flow codes (GSFs), allow for more general constitutive relations for material properties such as porosity, permeability and density. Examples of GSFs include PFLOTRAN, FEHM, TOUGH2, STOMP, and some reservoir simulators such as BOAST. Depending on the constitutive relations used in GSFs, an inconsistency arises between the standard groundwater flow equation and the governing equation of GSFs, and we clarify that the reason for this inconsistency is because the Darcy's equation used in the GSFs should account for the velocity of fluid with respect to solid. Due to lack of awareness of this inconsistency, users of the GSFs tend to use a porosity‐pressure relationship that comes from the standard groundwater flow equation and assumes that the relative velocity is already accounted for. For the Theis problem, we show that using this traditional relationship in the GSFs leads to significantly large errors. We propose an alternate porosity‐pressure relationship that is consistent with the derivation of the governing equations in the GSFs where the solid velocity is not tracked, and show that, with this relationship, the results are more accurate for the Theis problem. The purpose of this note is to make the users and developers of these GSFs aware of this inconsistency and to advocate that the alternate porosity model derived here should be incorporated in GSFs.

2.
Birdsell, D., H. Rajaram, and G. Lackey, Imbibition of hydraulic fracturing fluids into partially saturated shale, Water Resources Research, 51/8, pp. 6787-6796, 2015. [View Abstract]Recent studies suggest that imbibition of hydraulic fracturing fluids into partially saturated shale is an important mechanism that restricts their migration, thus reducing the risk of groundwater contamination. We present computations of imbibition based on an exact semianalytical solution for spontaneous imbibition. These computations lead to quantitative estimates of an imbibition rate parameter A with units of LT−1/2 for shale, which is related to porous medium and fluid properties, and the initial water saturation. Our calculations suggest that significant fractions of injected fluid volumes (15–95%) can be imbibed in shale gas systems, whereas imbibition volumes in shale oil systems is much lower (3–27%). We present a nondimensionalization of A, which provides insights into the critical factors controlling imbibition, and facilitates the estimation of A based on readily measured porous medium and fluid properties. For a given set of medium and fluid properties, A varies by less than factors of ~1.8 (gas nonwetting phase) and ~3.4 (oil nonwetting phase) over the range of initial water saturations reported for the Marcellus shale (0.05–0.6). However, for higher initial water saturations, A decreases significantly. The intrinsic permeability of the shale and the viscosity of the fluids are the most important properties controlling the imbibition rate.

1.
Birdsell, D., H. Rajaram, D. Dempsey, and H. Viswanathan, Hydraulic fracturing fluid migration in the subsurface: A review and expanded modeling results, Water Resources Research, 51/9, pp. 7159-7188, 2015. [View Abstract]Understanding the transport of hydraulic fracturing (HF) fluid that is injected into the deep subsurface for shale gas extraction is important to ensure that shallow drinking water aquifers are not contaminated. Topographically driven flow, overpressured shale reservoirs, permeable pathways such as faults or leaky wellbores, the increased formation pressure due to HF fluid injection, and the density contrast of the HF fluid to the surrounding brine can encourage upward HF fluid migration. In contrast, the very low shale permeability and capillary imbibition of water into partially saturated shale may sequester much of the HF fluid, and well production will remove HF fluid from the subsurface. We review the literature on important aspects of HF fluid migration. Single‐phase flow and transport simulations are performed to quantify how much HF fluid is removed via the wellbore with flowback and produced water, how much reaches overlying aquifers, and how much is permanently sequestered by capillary imbibition, which is treated as a sink term based on a semianalytical, one‐dimensional solution for two‐phase flow. These simulations include all of the important aspects of HF fluid migration identified in the literature review and are performed in five stages to faithfully represent the typical operation of a hydraulically fractured well. No fracturing fluid reaches the aquifer without a permeable pathway. In the presence of a permeable pathway, 10 times more fracturing fluid reaches the aquifer if well production and capillary imbibition are not included in the model.

### PROCEEDINGS REFEREED

4.
Birdsell, D., and M. Saar, Modeling Ground Surface Deformation at the Swiss HEATSTORE Underground Thermal Energy Storage Sites, Proceedings World Geothermal Congress, 2020. [View Abstract]High temperature (>25 °C) aquifer thermal energy storage (HT-ATES) is a promising technology to store waste heat and reduce greenhouse gas emissions by injecting hot water into the subsurface during the summer months and extracting it for district heating in the winter months. Nevertheless, ensuring the long-term technical success of an HT-ATES project is difficult because it involves complex coupling of fluid flow, heat transfer, and geomechanics. For example, ground surface deformation due to thermo- and poro- elastic deformation could cause damage to nearby infrastructure, and it has not been considered very extensively in the literature. The Swiss HEATSTORE consortium is a group of academic and industrial partners that is developing HT-ATES pilot projects in Geneva and Bern, Switzerland. Possible target formations at the Geneva site include: (a) fractured Cretaceous limestone aquifers interbedded within lower-permeability sedimentary rock and (b) Jurassic reef complex(es), also potentially fractured. In this work we offer numerical modeling support for the Geneva site. A site-specific, hydro-mechanical (HM) model is created, which uses input from the energy systems scenarios and 3D static geological modeling performed by other Swiss consortium partners. Results show that a large uplift (> 5 cm) is possible after one loading cycle, but a sensitivity analysis shows that uplift is decreased to ≤ 0.3 cm if the aquifer permeability is increased or an auxiliary well is included to balance inflow and outflow. Future work includes running coupled thermo-hydro-mechanical (THM) models for several loading and unloading cycles. The THM framework can help inform future decisions about the Swiss HT-ATES sites (e.g. the final site selection within the Geneva basin, well spacing, and operating temperature). It can also be applied to understand surface deformation in the context of geothermal energy, carbon sequestration, and at other ATES sites worldwide.

3.
Guglielmetti, L., P. Alt-Epping, D. Birdsell, F. de Oliveira, L. Diamond, T. Driesner, O. Eruteya, P. Hollmuller, et al., and M.O. Saar, HEATSTORE SWITZERLAND: New Opportunities of Geothermal District Heating Network Sustainable Growth by High Temperature Aquifer Thermal Energy Storage Development, World Geothermal Congress, 2020. [View Abstract]HEATSTORE is a GEOTHERMICA ERA-NET co-funded project, aiming at developing High Temperature (~25°C to ~90°C) Underground Thermal Energy Storage (HT-UTES) technologies by lowering the cost, reducing risks, improving the performance, and optimizing the district heating network demand side management at 6 new pilot and demonstration sites, two of which are in Switzerland, plus 8 case studies. The European HEATSTORE consortium includes 24 contributing partners from 9 countries, composing a mix of scientific research institutes and private companies. The Swiss consortium, developing HEATSTORE in Switzerland, involves of two industrial partners (Services Industriels de Geneva - SIG and Energie Wasser Bern - EWB) and four academic partners (Universities of Geneva, Bern, Neuchâtel and ETH Zurich), with support from the Swiss Federal Office of Energy. The aims are to develop two demonstration projects for High Temperature Aquifer Thermal Energy Storage (HT-ATES) in the cantons of Geneva and Bern such that industrial waste heat can be converted into a resource. This paper presents the results of the first year of activities in the Swiss projects. The activities planned cover subsurface characterization, energy system analysis, surface implementation design, legal framework improvement and business modelling to ensure the sustainability of the projects. This approach is supported by large industrial investments for subsurface characterization. Two wells, down to 1200m below surface level (bsl) are being drilled in the Geneva area to tap potential targets in the carbonate Mesozoic units and at least three additional wells, down to 500m bsl will target the Molasse sediments in the Bern area next year. These wells allow subsurface exploration and characterization and will provide data, used for detailed THMC modelling to assess the thermal energy storage potential at the two sites in Switzerland. The results of such numerical modelling are combined with energy system analysis to quantify the waste heat availability and heat demand and hence optimize the production and injection operations. The outcomes of the coupled assessments will aid in designing the integration of the new installations into the district- heating network. Legal framework improvements, based on complete technical evaluation and on the best-practice sharing with the other European partners, will be an enabling tool to accelerate the implementation of the HT-ATES systems, while business modelling helps calibrate the economic feasibility of the projects and helps industrial partners to plan future investments.

2.
Birdsell, D., H. Rajaram, and S. Karra, Code development for modeling induced seismicity with flow and mechanics using a discrete fracture network and matrix formulation with evolving hydraulic diffusivity, 52nd US Rock Mechanics/Geomechanics Symposium and Discrete Fracture Network Engineering Conference, ARMA 18-565, 2018. [View Abstract]Injection-induced seismicity (IIS) depends on pore pressure, in-situ stress state, and fault orientation; generally occurs in basement rock that contains fractures and faults; and moves away from the injection well as a nonlinear diffusion process. Therefore, to numerically model IIS a code should incorporate flow and geomechanics, the presence of fractures and faults, and the capability for hydraulic diffusivity to evolve with effective stress and failure history. In this work, we introduce and verify a modeling framework that allows hydraulic diffusivity to evolve as fractures open and close. Details and challenges in code development are discussed, including how the Bandis model for normal fracture deformation can be used to calculate hydraulic diffusivity as a function of effective normal stress. The discrete fracture network and matrix (DFNM) model is implemented in PFLOTRAN such that hydraulic diffusivity has different constitutive relationships for fracture and matrix grid cells. This model is applied to understand the recent IIS near Greeley, Colorado, and its results are compared to: (a) a traditional DFNM model where hydraulic diffusivity cannot evolve and (b) an equivalent porous media (EPM) model where the effect of the fractures are averaged over a large region of rock. The new DFNM model predicts critical pressure will propagate farther from an injection well. This modeling framework shows promise for applications where fracture and matrix flow are important and hydraulic diffusivity is a function of pressure, stress, and/or shear failure history.

1.
Birdsell, D., H. Rajaram, D. Dempsey, and H. Viswanathan, Numerical Model of Hydraulic Fracturing Fluid Transport in the Subsurface with Pressure Transient and Density Effects, 49th US Rock Mechanics/Geomechanics Symposium, 2015. [View Abstract]Understanding the transport of hydraulic fracturing (HF) fluid that is injected into the deep subsurface for shale gas extraction is important to ensure that shallow drinking water aquifers are not contaminated. Pressure gradients, permeable pathways such as faults or improperly abandoned wellbores, and the density contrast of the HF fluid to the surrounding brine could encourage upward HF fluid migration. In contrast, very low shale permeability and well production may work to keep HF fluid at depth and remove it from the subsurface. Single-phase flow and transport simulations are performed to quantify how much HF fluid is removed via the wellbore and how much reaches overlying aquifers. If a permeable pathway connects the shale reservoir to the overlying drinking water aquifer, the pressure transient due to injection and the density contrast allows rapid upward plume migration at early times, but well production reverses the direction of flow and removes a large amount of HF fluid from the subsurface. We present estimates of HF fluid migration to shallow aquifers during the first 1,000 years and show that the pressure transient from well operations should be included in subsequent numerical models while buoyancy may be neglected depending on depth and permeability.

### THESES

1.
Birdsell, D., An Investigation of Subsurface Fluid Injections Related to Oil and Gas Development: Modeling Hydraulic Fracturing Fluid Migration and Induced Seismicity, Dissertation, 161 pp., 2018. [View Abstract]Unconventional oil and gas development is made economically feasible by horizontal drilling and hydraulic fracturing, and it can produce large volumes of wastewater that are injected into disposal wells. In the first part of this dissertation, we investigate the potential for fracturing fluid to migrate from the target formation to contaminate shallow drinking water aquifers. We present semianalytical calculations for capillary imbibition into shale, which can sequester up to 95% of the fracturing fluid, thus preventing migration to aquifers. Next, we present a numerical model of fracturing fluid migration, which is the first to combine injection and production, imbibition, and buoyancy. In the absence of a permeable pathway between the injection formation and the aquifer, no contamination occurs. In the presence of a permeable pathway, well suction and capillary imbibition significantly reduce the amount of fracturing fluid reaching the aquifer compared to scenarios that do not account for suction and imbibition. In part two, we present a new framework for modeling basin-scale injection-induced seismicity (IIS). The framework incorporates flow and geomechanics, the presence of fractures and faults, and the capability for hydraulic diffusivity to evolve with effective stress and earthquake history, which is modeled by the Mohr-Coulomb failure criteria. The model is implemented in the massively-parallel code PFLOTRAN, which is important to capture the large length scales ($\sim$10 km) and many fractures and faults (100-1000s). Applications of this model: (a) put constrains on the hydraulic diffusivity in basement rock, which may have been too large in previous modeling studies; (b) explain the heterogeneity of earthquake locations; and (c) capture the variations in critical pressure that cause earthquakes, based on stress state and fault orientation. In part three, we show work that verifies and increases the accuracy of subsurface simulators, which is important for the continued investigation of fluid migration, IIS, and other subsurface phenomena. First, we derive a porosity-pressure relationship for general subsurface flow codes (GSFs) which accounts for the relative velocity between pore fluid and rock matrix. Simulations using this relationship show excellent agreement between GSFs and the groundwater flow equation. Next we verify the fully-coupled flow and geomechanics implementation in PFLOTRAN by comparing to an analytical solution.