SMART NANOFLUIDS FOR GEOTHERMAL APPLICATIONS AND METHODS OF USE

A method of formulating a nanofluid for use in a geologic formation for determining an average pore size of the geologic formation by selecting a desired flow rate of the nanofluid under a predetermined applied pressure, using the calculated average pore size and desired flow rate to estimate a desired average viscosity of the nanofluid, and producing the nanofluid having the desired average viscosity by combining a quantity of nanoparticles with an aqueous medium. A nanofluid made by said process. A method of treating a geologic formation, such as a geothermal formation, by injecting said nanofluid into the geologic formation.

GOVERNMENT SUPPORT

BACKGROUND

Enhanced Geothermal Systems (EGS) are engineered underground geothermal reservoirs created where there is hot rock (175-300° C.) but little to no natural permeability and/or fluid saturation [1]. EGS have the capacity to power tens of millions of American homes and businesses. The EGS provides efficient and economically feasible access to enormous clean energy resources. To provide adequate working fluid flow for the economic production flow and optimal heat extraction through the EGS, the reservoir should have a large network of fractures. However, if the fracture network is not uniform, the injected working fluid may take fast pathways where higher conductivity and high injectivity fracture exist, leading to short-circuiting.

During EGS development, subsurface permeability is enhanced via fluid injection, thermal rock-fluid interaction, chemical stimulation, or other safe, well-engineered stimulation processes that re-open pre-existing fractures or create new ones [2]. Successful stimulation in EGS depends on knowledge and characterizing of the subsurface. Tracers are recognized as a powerful method for characterizing the subsurface. In its simplest form, tracer testing can be defined as injecting one or more tracers, usually chemical compounds, into the subsurface in order to estimate its flow and storage properties [3]. Unlike the petroleum industry, the tracer development for the geothermal industry is at infancy levels because of numerous challenges, including high temperatures. There are literature examples of tracer development for geothermal applications. For instance, “a geothermal company in the 1990s conducted a number of tracer tests in a geothermal field. Fluorescein was used as the tracer, and the tests were generally considered a success. Further analysis of test results, however, suggested possible “problems” [3]. The Department of Energy has sponsored some projects in the past for tracer development in the geothermal industry.

Nanofluids, i.e., dispersions of nanoparticles in a liquid, have been proposed as heat transfer fluids. For example, dispersion of a few percent of a nanoparticle in ethylene glycol or oil can increase the thermal conductivity by 40% and 150%, respectively. If the concentration of nanoparticle in the nanofluid is high enough, a shear thickening behavior at high shear rates is observed. For example, Tseng and Wu and Chandrasekar et al. observed a transition from shear thinning or Newtonian behavior to shear thickening behavior for Al2O3/water nanofluids at concentrations beyond 2-5% [5-7]. Moldoveanu et al. found that while Al2O3nanofluids have higher viscosity than SiO2nanofluids, the latter has stronger shear thickening behavior [8]. Incorporation of nanoparticles into a fluid can affect heat transfer of the fluid by increasing the viscosity of the fluid. It should be noted that only an increase in viscosity by a factor higher than 4 compared to the increase in thermal conductivity can worsen the thermal performance of nanofluid compared to the base fluid [9].

DETAILED DESCRIPTION

The present disclosure is directed novel stimuli-responsive heat transfer nanofluids and systems for controlling the hydraulic conductivity of EGS reservoirs, and for uses as a tracer fluid. The disclosed nanofluids increase viscosity at high flow velocities in pores and form gels when reaching certain environmental conditions thereby reducing the flow through “fast paths” in the subsurface rock of the reservoir.

The surface properties of nanoparticles control the interparticle interactions in nanofluids and thus the rheological behavior. In the present disclosure, the surface treatment of nanoparticles is adjusted by (1) grafting responsive polymers, and/or (2) functional groups which affect the interparticle interaction of nanoparticles in response to temperature, pH, and/or salinity. Therefore, the disclosed nanofluids not only have enhanced heat transfer (which is essential for geothermal energy resource recovery) but also have responsive behavior in such a way that in response to a stimulus, (a) its non-Newtonian viscosity changes, and (b) it can undergo a sol-gel transition. The disclosed system addresses at least one of the following challenges in geothermal applications: (1) fluids or fluid additives that can increase bulk fluid viscosity if specific fluid flow velocities are reached, reducing mass transport through a targeted fracture network, (2) fluids or fluid additives that can solidify through jamming in small pore or in close proximity to other fluids to control fracture interference and potential “fast paths,” and (3) fluids or fluid additives that can control formation leak-off into the matrix rock.

Before further describing various embodiments of the compositions and methods of the present disclosure in more detail by way of exemplary description, examples, and results, it is to be understood that the embodiments of the present disclosure are not limited in application to the details of methods and compositions as set forth in the following description. The embodiments of the compositions and methods of the present disclosure are capable of being practiced or carried out in various ways not explicitly described herein. As such, the language used herein is intended to be given the broadest possible scope and meaning; and the embodiments are meant to be exemplary, not exhaustive. Also, it is to be understood that the phraseology and terminology employed herein is for the purpose of description and should not be regarded as limiting unless otherwise indicated as so. Moreover, in the following detailed description, numerous specific details are set forth in order to provide a more thorough understanding of the disclosure. However, it will be apparent to a person having ordinary skill in the art that the embodiments of the present disclosure may be practiced without these specific details. In other instances, features which are well known to persons of ordinary skill in the art have not been described in detail to avoid unnecessary complication of the description. While the compositions and methods of the present disclosure have been described in terms of particular embodiments, it will be apparent to those of skill in the art that variations may be applied to the compositions and/or methods and in the steps or in the sequence of steps of the method described herein without departing from the spirit, and scope of the inventive concepts as described herein. All such similar substitutes and modifications apparent to those having ordinary skill in the art are deemed to be within the spirit and scope of the inventive concepts as disclosed herein.

All patents, published patent applications, and non-patent publications referenced or mentioned in any portion of the present specification are indicative of the level of skill of those skilled in the art to which the present disclosure pertains, and are hereby expressly incorporated by reference in their entirety to the same extent as if the contents of each individual patent or publication was specifically and individually incorporated herein.

As utilized in accordance with the methods and compositions of the present disclosure, the following terms, unless otherwise indicated, shall be understood to have the following meanings:

Throughout this application, the terms “about” or “approximately” are used to indicate that a value includes the inherent variation of error for the composition, the method used to administer the composition, or the variation that exists among the objects, or study subjects. As used herein the qualifiers “about” or “approximately” are intended to include not only the exact value, amount, degree, orientation, or other qualified characteristic or value, but are intended to include some slight variations due to measuring error, manufacturing tolerances, stress exerted on various parts or components, observer error, wear and tear, and combinations thereof, for example. The term “about” or “approximately”, where used herein when referring to a measurable value such as an amount, a temporal duration, and the like, is meant to encompass, for example, variations of +20% or +10%, or +5%, or +1%, or +0.1% from the specified value, as such variations are appropriate to perform the disclosed methods and as understood by persons having ordinary skill in the art. As used herein, the term “substantially” means that the subsequently described event or circumstance completely occurs or that the subsequently described event or circumstance occurs to a great extent or degree. For example, the term “substantially” means that the subsequently described event or circumstance occurs at least 90% of the time, or at least 95% of the time, or at least 98% of the time.

The term “thermal conductivity” as used herein refers to the ability of a material to transfer or conduct heat. It is represented herein by the symbol κ (kappa) and has the units W/(m·K) i.e., watts per (meter×degrees Kelvin). Where used herein, the term “high thermal conductivity” refers to a κ>about 0.8 W/(m·K) up to a level such that viscosity, η, of the nanofluid is less than or equal to 4× the increase in thermal conductivity, κ, of the nanofluid.

The term “viscosity” as used herein refers to a material's resistance to flow. It is represented herein by the symbol η (eta) and has the unit Pa·s (Paschal second). In certain embodiments, the nanofluids of the present embodiments have a viscosity n in a range of about 0.001 Pa·s to about 0.1 Pa·s.

The term “nanoparticle” refers to a particle having a diameter in a range of about 1 nm to about 1000 nm, and more particularly about 1 nm to about 500 nm, and more particularly about 1 nm to about 100 nm.

A particular goal of the present disclosure is to develop smart nano-fluid based tracer and tracer interpretation tools to facilitate robust characterization of temperature distributions and surface area available for heat transfer in EGS. The tracer fluid system disclosed herein is a responsive non-Newtonian nanofluid (TRL 3) prepared by nanoparticles that are dispersed through ultrasonication in aqueous media comprised of, for example, water, electrolytes, alcohols such as glycols (e.g., ethylene glycol (EG), propylene glycol (PG), and glycerol), synthetic polymers such as, polyethylene glycol (PEG), polypropylene glycol (PPG), polyacrylamide (PAM), and polyethylenimine (PEI)), natural polymers such as guar gum and sodium alginate, and non-ionic and ionic surfactants, such as sodium dodecyl sulfate, dodecyltrimethylammonium bromide, Brij™-series non-ionic surfactants, and Pluronic™-type block copolymers (or others listed herein). In at least certain embodiments, the nanoparticles have high thermal conductivity.

Electrolytes of the aqueous medium may include but are not limited to salts of Na, K, Cl, Ca, Mg, carbonate, phosphate, sulfate, including for example NaCl, KCl, MgCl2, CaCl2), MgSO4, and CaSO4.

Surfactants that may be used in the present compositions include ionic surfactants (anionic, cationic, and amphoteric) and non-ionic surfactants.

The present disclosure includes nanofluids with different concentrations of electrolytes, alcohols, nanoparticles, polymer, and/or surfactants, and different mixtures thereof with different sizes and types of nanoparticles (see above). To control the flow behavior (viscosity and shear thinning/thickening), the composition of nanofluids is varied. Also, nanoparticles with different surface hydrophobicity/hydrophilicity are used. To prepare nanofluids, nanoparticles will be dispersed through ultrasonication in aqueous media comprised of, for example, water, and one or more electrolytes, alcohols, polymer, and surfactants.

The nanofluids can be formulated according to the following steps and parameters: (1) the average pore size of a geologic formation is estimated by injecting a mixture of, for example, an aqueous mixture containing water and EG, into the geologic formation, and measuring the flow rate of the aqueous mixture under applied pressure, (2) based on the calculated average pore size, an average media viscosity for the application is estimated from the required flow rate under applied pressure, (3) the typical volume fraction range of nanoparticles in the nanofluids of the present disclosure is 1 to 9%, wherein the shear thickening behavior is intensified as the volume fraction is increased. The maximum practical volume fraction of the nanofluid is chosen where the increase in viscosity, η, is less than or equal to 4× the increase in thermal conductivity, k,

Experimental (rheological measurements) and theoretical (e.g., Krieger-Dougherty model) approaches are used to determine viscosity and conductivity of nanofluids, (4) nanofluids with maximum practical volume fraction are prepared, and their flow curve is measured, and (5) the change in flow rate under dynamic change of pressure as well as the change in flow curve, temperature and composition of inlet and outlet nanofluids is used to determine the pore size distribution of rock/formation.

The shear-thickening behavior of nanofluids at high shear rates changes the fluid's response inside the reservoir fracture system. The response can be collected real-time with the injection pressure changes and then interpreted similarly to mini-frac tests to characterize the subsurface. The presence of polymer changes the interparticle interactions and induces the formation of clusters and gelation in nanofluids. High flow rates enhance the cluster formation and physical gelation. The interparticle interactions also change with the concentration of polymer, surfactant, salinity, temperature, and pH. Therefore, different formulations of nanofluids can be used as tracer fluids. The presence of depletants can induce strong depletion attraction, enhancing the potential for gelation. Therefore, nanofluids can transform to gel (undergo sol-gel transition) when reaching a certain salt concentration or pH, both of which can weaken the electrostatic repulsion between particles. The system and nanofluids of the present disclosure enable linkage of the flow pattern inside the fractures to pressure measurements and different responses that can be exploited for subsurface characterization.

REFERENCES