Proppant Settling Research Articles

Extreme Limited-Entry Perforating Enhances Bakken Completions

This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper SPE 189880, “Mining the Bakken II—Pushing the Envelope With Extreme Limited-Entry Perforating,” by Paul Weddle, SPE, Larry Griffin, SPE, and C. Mark Pearson, SPE, Liberty Resources, prepared for the 2018 SPE Hydraulic Fracturing Technology Conference, The Woodlands, Texas, USA, 23–25 January. The paper has not been peer reviewed. This paper presents the evolution of a Bakken advanced completion design with the added enhancement of extreme limited entry (XLE) perforating. With this strategy, an operator has consistently stimulated more than 11 perforation clusters per stage. The high number of active clusters, or fracture-initiation points, has been measured directly with radioactive tracers and fiber-optic diagnostics and is validated through improved production relative to offset completions. XLE The technique, as the name suggests, pushes the level of perforation friction past 1,500 psi. Additional sources of pressure variations have been identified and can be summed up as the fracture-entry pressure. Fracture-entry pressure, defined as the pressure from immediately outside the perforations to the fracture tip, includes, but is not limited to, near-wellbore tortuosity, net pressure, stress shadowing, and fracture extension. To advance the goal of evenly diverting stimulation fluid to all the fracture-initiation points (clusters), the designed perforation friction must moderate perforation-/cluster-level pressure variations in the fracture-entry pressure. To achieve this, the magnitude of the sources of variability should be quantified and addressed with the completion and perforating design. Additionally, one must account for the dynamic fracturing process, within which most of the fracture-entry-pressure parameters, as well as the perforations, are changing as the stimulation progresses. The primary sources of variability to be considered when designing for XLE are as follows: Minimum horizontal stress variability along the lateral; 90% of a Bakken lateral is within a 750-psi range. Near-wellbore friction variations, stage to stage and perforation to perforation. Stepdown tests demonstrate a P50 of 625 psi for starting near-wellbore friction. Stress shadowing between active clusters. The magnitude of stress shadowing is dependent on formation properties, completion-design parameters, and cluster spacing. For design purposes, it is assumed to be approximately 200 psi. Fracture-extension-pressure variability based on changes to net pressure. Perforation-friction initial-condition variability may limit ability to reach design pump rate. Perforation diameters at all possible orientations must be considered, along with the number of perforations open vs. the number shot. Perforation friction changes during a fracturing stage of approximately 500 psi caused by rounding and erosion of perforations and possible loss of clusters because of proppant settling in the liner. For the Bakken, a minimum 2,000-psi perforation friction is targeted at the beginning of the job, with the expectation that, as perforations round and erode, it will remain greater than 1,500 psi at the end of pumping to account for all possible fracture-entry pressure variations.

Modeling of hydraulic fracture closure on proppants with proppant settling

The placement of proppants in a hydraulic fracture is governed by slurry flow, proppant transport and settlement. The proppant distribution can significantly affect the conductivity of the hydraulic fracture, and hence will impact the productivity of hydraulic fractured wells. Past studies have investigated fracture closure on proppants by envisioning hydraulic fractures as two parallel plates with uniformly distributed proppants. However, in reality, hydraulically induced fractures are wider in the middle and narrower near the fracture edges. In addition, a proppant dune is likely to be accumulated at the bottom of the fracture because of proppant settling. As a result, fracture closure on proppants is controlled by both fracture geometry and the distribution of proppant in the fracture. This is a dynamic process where fracture geometry and the proppant pack will evolve as a function of pressure, and the associated surface contact problem is notoriously challenging to solve because of its nonlinear and nonlocal nature. In this study, we proposed a general approach to model hydraulic fracture closure. The residual fracture width profile for both propped and un-propped sections can be obtained at different drawdown pressures for rocks with different clay content. The maximum stress acting on proppants can be calculated to guide the selection of proppants with appropriate strength. Most importantly, the effect of stress concentration and stress amplification can be quantified when a proppant dune or bank is formed. We also show that the traditional method of estimating the maximum stress on a proppant pack is an underestimation.

Peculiarities of the hydraulic fracture propagation caused by pumping of proppant-fluid slurry

A new numerical model for hydraulic fracturing has been developed. This model takes into account several simultaneous processes: pumping of proppant-laden slurry and its flow through the fracture, fracture growth with variable height and length, proppant settling, forming of proppant packing, and fluid filtration through this packing. Simulation experiments demonstrated that proppant particle diameter has significant influence on forming the proppant packing, fluid filtration through the packing, and, finally, on the fracture length and ultimate distribution of fracture width.

A new model for simulating particle transport in a low‐viscosity fluid for fluid‐driven fracturing

Fluid‐driven fracture (i.e., Hydraulic fracturing) is an important way to stimulate the well productivity in the development of unconventional reservoirs. A low‐viscosity fluid called slickwater is widely used in the unconventional fracturing. It is a big challenge to simulate the particle (or proppant) transport in the low‐viscosity fluid in a field‐scale fracture. A new model to simulate particle transport in the low‐viscosity fluid in a field‐scale fracture is developed. First, a new parameter is defined, called the rate of proppant bed wash‐out, by which we incorporated the mechanism of proppant bed wash‐out into Eulerian‐Eulerian proppant transport model. Second, we proposed a novel way to consider the effect of proppant settling on the proppant concentration in the upper layer (or suspending layer) to accurately simulate the proppant transport. Additionally, a dimension reduction strategy was used to make the model quickly solved. Our simulation results were compared with published experimental data and they were consistent. After validation, the effect of fluid viscosity, injection rate, fracture height, and proppant concentration on the proppant distribution in a fracture is investigated. This study provides a new model to simulate particle transport. Meanwhile, it gives critical insights into understanding particle transport in the field‐scale fracture. © 2018 American Institute of Chemical EngineersAIChE J, 64: 3542–3552, 2018

A New Friction-Reducing Agent for Slickwater-Fracturing Treatments

Summary Friction reducers (FRs) represent an essential component in any slickwater-fracturing fluid. Although the majority of previous research on these fluids has focused on evaluating the friction-reduction performance of chemical components, only a few studies have addressed the potential damage these chemicals can cause to the formation. Because of the polymeric nature of these chemicals—typically polyacrylamide (PAM)—an FR can either filter out onto the surface of the formation or penetrate deeply to plug the pores. In addition, breaking these polymers at temperatures lower than 200°F remains a problem. The present study introduces a new FR that replaces the linear gel with an enhanced proppant-carrying capacity and reduced potential for formation damage. Friction-reduction performance, proppant settling, breakability, and coreflood experiments using tight sandstone cores at 150°F were conducted to investigate a new FR (FR1). The performance of the new FR was compared with two different FRs: a salt-tolerant polymer that is a copolymer of acrylamide and acrylamido-methylpropane sulfonate (FR2), and a guar-based polymer (FR3). Different breakers were used to examine the breakability of the three FRs, including ammonium persulfate (APS), sodium persulfate (SPS), hydrogen peroxide (HP), and sodium bromate (SB). The friction reduction of the new chemical was higher than 70% in fresh water or 2 wt% potassium chloride (KCl) in the presence of calcium chloride (CaCl2) or choline chloride. The presence of 1 lbm/1,000 gal of different types of breakers did not affect the friction-reduction performance. The friction reduction of 1 gal/1,000 gal of the new FR1 was also higher than that of the guar-based FR3 at a load of 4 gal/1,000 gal at the same conditions. The results show that the new FR is breakable with any of the tested breakers. Among the four tested breakers, APS is the most-efficient breaker. Static and dynamic proppant-settling tests further indicated a superior performance of FR1 for proppant suspension compared with a PAM FR (FR2). Coreflood experiments showed that FR1 did not cause any residual damage to the core permeability when APS was used as a breaker, compared with 10% and 9% damage when FR2 and FR3 were tested, respectively. Coreflood tests also showed that FR1 is breakable using SB with only 2.5% damage, whereas FR2 and FR3 resulted in 47% and 41% damage, respectively. The results also show that higher salinity does not affect the breakability of the new FR. The proposed FR shows higher friction-reduction performance and better proppant-carrying capacity with no formation damage, compared with the conventional counterparts. Hence, FR1 is a viable choice for application in fracturing formations with proppants.

Pillar Fracturing a Sandstone Reservoir Shows Benefit Over Conventional Fracturing

This article, written by Special Publications Editor Adam Wilson, contains highlights of paper SPE 186199, “Unlocking Depleted and Low-Modulus Telisa Sandstone Reservoir With Pillar-Fracturing Technique: Well-Performance-Improvement Comparison With Conventional Fracturing,” by M. Azhari, SPE, N.F. Prakoso, and D. Ningrum, Medco E&P Indonesia, and L. Soetikno and A. Makmun, Schlumberger, prepared for the 2017 SPE/IATMI Asia Pacific Oil and Gas Conference and Exhibition, Bali, Indonesia, 17–19 October. The paper has not been peer reviewed. The Kaji Semoga Field in South Sumatra consists of three main reservoirs—Telisa sandstone (TLS), Baturaja limestone, and Talangakar sandstone. The successful development of TLS with hydraulic fracturing led to further efforts to maximize oil recovery. After a study with suitable samples and cases, pillar fracturing was considered. This method is similar to conventional fracturing techniques where fluid and proppant are used to create conductive paths in the reservoir layer; however, pillar fracturing relies on open flow channels. Pillar Fracturing A major step change in the development of the Kaji Semoga Field was to use the hydraulic-fracturing technique referred to as pillar fracturing. The pillar-fracturing technique creates stable voids within the proppant pack that serve as infinite-conductivity channels for fluid flow rather than the intragranular flow of conventional fracturing techniques (Fig. 1). Hydrocarbon will flow preferentially through the channels rather than through the proppant pack. In addition, pillar fracturing allows for better fracture cleanup, longer fracture effective half-lengths, and lower pressure drops along the fracture. Consequently, production after pillar fracturing is greater than that following conventional fracturing. Another advantage of the pillar-fracturing technique is a low screenout rate compared with conventional fracturing. The prevention of screen out is related to the different bridging characteristics of conventional- and pillar-fracturing slurries. The stable voids within the proppant pack are created through a combination of specific pumping schedules, specific equipment, and fracturing-fluid design. The voids are achieved by alternate pumping of gelled fluid in two types of pulses, proppant-laden pulses and proppant-free pulses. Fibers are added continuously during pumping to mitigate the dispersion of the proppant-laden pulses as they are conveyed throughout the surface lines and the wellbore and within the fracture. A dispersed pulse can create narrow fracture widths and a reduction in the number and quality of the channels, which leads to a reduction of fracture conductivity. The fiber also enhances the proppant-carrying capacity and prevents proppant settling within the fracture. The pulse stage is then followed by a continuous proppant tail-in stage near the end of the treatment before the flush stage. This tail-in stage ensures good connectivity between the wellbore and the channels created during the treatment. Specially engineered blending equipment was needed to ensure stable and consistent pulse delivery.

Evaluation and optimization of degradable-fiber-assisted slurry for fracturing thick and tight formation with high stress

Economically producing hydrocarbon from tight formations requires fracturing with a large volume of water and proppants to ensure a highly conductive fracture network. Proppant settling can hinder proppant transportation within fractures, limiting dimensions of the propped fractures and well production rates. This problem is more serious in thick formations that tall fractures are needed. In deep reservoirs with high closure stresses, using high pumping rate or viscous cross-linked gel have their own limitations, while adding degradable fibers can solve the problem. However, no visualized flow experiments have been conducted that can fill the gap between the bench-top measurements and the field tests, thus providing reliable mechanisms to guide the optimization of this technique for a chosen reservoir.In this study, a systematic screening and evaluation procedure is developed to quantify the effectiveness of degradable fibers on suspending proppants and enhancing fracture conductivity, so as to explore the mechanisms behind this technique. Results indicate that a dilute concentration of fibers in proppant slurry can double proppant placement efficiency without blocking perforation holes on the wellbore; fiber degradation leaves no residue that damages fracture conductivity; fiber degradation does not cause the closure of propped fractures even at high closure stresses.

Particle Image Velocimetry (PIV) Analysis of Particle Settling in Narrow Fracture Experiments

Abstract This paper presents the results of an experimental study of dense sand particles’ settling micromechanics in narrow smooth and rough fractures. Particle Image Velocimetry (PIV) is used for the analysis of velocities of individual particles and groups of particles and their relative paths, collisions, and agglomerating in viscous Newtonian fluid. The displacement vectors obtained through PIV analysis show the movements and velocities of individual and groups of particles and the global velocity trends of the observed area. Experiments were conducted in a relatively narrow 2-mm slot compared to the main particle size of 0.66 mm across. Smooth and rough fracture walls are considered, in which acrylic plates and 3-D–printed hydraulic fracture replica were used. The measured results from this experimental study give new insights into the effects of particle and agglomerate size and shape as well as fluid dynamic viscosity on slurry settling velocity. It was found that an increase in fluid dynamic viscosity promotes the formation of larger agglomerated particles of sand, which, in return, affect overall slurry settling velocity. Observations from rough fracture wall experiments yielded significantly different slurry behavior, with increased erratic particle motions and fewer agglomerations compared to the smooth, idealized fracture walls. Experimental results give new insights into slurry settling in narrow fractures, which has applications to proppant settling in fractures for the oil, gas, and geothermal industries.

Numerical simulation of proppant transport in hydraulic fractures

A central issue in hydraulic fracturing treatment in petroleum wells is the transport of proppant particles by the injection fluid. In this paper, we present an innovative proppant transport model in a fixed rectangular- and elliptic-shaped slots. The proposed model is an improvement to the current modeling of proppant transport by applying a non-oscillatory numerical scheme which has high accuracy everywhere in solution domain, even close to the steep gradients. In addition, inertia, fracture wall, and concentration effects on proppant settling along with slurry evolution as a function of proppant concentration has been considered.This paper introduces the mathematical equations that govern the proppant transport phenomenon and discusses special front capturing numerical techniques, boundary conditions, coupling between proppant and slurry mass conservation equations and time stepping restrictions required for the solution stability. We incorporated published correlations obtained from proppant transport laboratory experiments in our numerical model to better capture the physics of the problem. 5th- order WENO scheme was used to avoid oscillation and diffusion at the proppant front since traditional finite difference discretization was found to be insufficient in solving the hyperbolic transport partial differential equations. Results show that the technique used in this study can capture the proppant distribution with minimum oscillation and diffusion.A series of sensitivity analysis was conducted to explore the legitimacy of these assumptions and to provide guidelines that allow more accurate predictions of the proppant and fluid transfer. Numerical results are presented to show how proppant distribution is impacted by the injection fluid viscosity, density difference between proppant particles and injection fluid, proppant size, and fluid flow injection rate. Results of the sensitivity analysis illustrate the significance of choosing appropriate viscosity of the injection fluid as small changes in the viscosity may cause noticeable effects on the concentration distribution. In addition, we found that variation of proppant size and density within a reasonable range have a modest effect on proppant concentration distribution.Furthermore, we also investigated the amount of gravity driven vertical motion of proppant which is driven by density differences (convection) and compare it to a second gravity driven motion which is proppant settlement. Both of these two well recognized mechanisms can occur inside a fracture during proppant placement, however, the importance of each mechanism as a function of proppant injection design parameters is not fully understood.

Feedback control of proppant bank heights during hydraulic fracturing for enhanced productivity in shale formations

In hydraulic fracturing of shale formations, compared to conventional reservoirs, the fracturing fluid injected is of low‐viscosity and hence during pumping the proppant settles significantly, forming a proppant bank. Motivated by this consideration, we initially develop a high‐fidelity process model of hydraulic fracturing to describe the dominant proppant settling behavior during hydraulic fracturing process. Second, a novel remeshing strategy is developed to handle the high computational requirement due to moving boundaries. Third, a section‐based optimization method is employed to obtain key fracture design parameters for enhanced productivity in shale formations subject to given fracturing resources. Fourth, a reduced‐order model is constructed to design a Kalman filter and to synthesize a real‐time model‐based feedback control system by explicitly taking into account actuator limitations, process safety and economic considerations. We demonstrate that the proposed control scheme can regulate the uniformity of proppant bank heights along the fracture at the end of pumping. © 2017 American Institute of Chemical EngineersAIChE J, 64: 1638–1650, 2018

An X-FEM Formulation for the Optimized Graded Proppant Injection into Hydro-fractures Within Saturated Porous Media

In this study, a two-dimensional fully coupled computational model is developed for simulation of proppant settlement in hydro-fractures with the use of the extended finite element framework. The porous domain is governed by the well-known \((\mathbf{u}-p)\) formulation, which consists of the momentum balance equation of the bulk, in conjunction with the momentum balance and continuity equations of the pore fluid. The hydro-fracture inflow is modeled as a 1D flow on the basis of the Darcy law, in which fracture permeability is incorporated by means of the cubic law. Contact constraints are elaborated to eliminate the overlap of fracture edges and the leak-off flow. Proppant settlement is conducted on the basis of Stokes’ law for particle terminal velocity, in which the effects of fracture walls, concentration, viscosity and bridging are incorporated into the model. A fixed-point algorithm is introduced to achieve the optimum combination for the proppant injection. Using the extended finite element method, the strong discontinuity in the displacement field due to crack body, as well as the weak discontinuity in the pressure field due to leakage, is included in the model with the use of the Heaviside and modified level set enrichment functions, respectively. The robustness and versatility of the proposed numerical algorithm in determining the optimum proppant injection is examined through several numerical simulations.

Experimental and numerical investigation into nano-stabilized foams in low permeability reservoir hydraulic fracturing applications

Hydraulic fracturing is one of the most effective and efficient methods to enhance oil and gas recovery from reservoirs. Well productivity is mainly determined by the propped dimensions of the fracture. However, the propped dimensions are controlled by proppant transport and proppant placement, of which settling and convection are the controlling mechanisms. In this paper, a new concept is described to overcome the problem of proppant settling during a foam fracturing treatment and prior to fracture closure. This new concept consists of introducing nanoparticles into the base fracturing fluid system to stabilize the foam structure and reduce proppant settling. The results show that foam stability directly influences proppant placement and final fracture conductivity. When foam fracturing fluids experience long closure times, most often as a result of low leakoff, foam breakage occurs. This breakage results in proppant settling and accumulation at the bottom of the fracture, reducing propped dimensions. Whilst, high pumping rates and foam quality provide greater initial propped area, foam stability is major factor controlling the final propped area and productivity. Experimental results show that foam stability increases 2–3-fold in presence of 0.8-wt% silica nanoparticles under 90 °C. The enhancement of foam stability using nanoparticles, particularly foam life-time, allows better proppant suspension, maintains post-fracture conductivity, and minimizes productivity losses. This work is intended to guide the practitioner in the use of nano-stabilized foam to improve unconventional reservoir stimulation.

Experimental and simulation study of foam stability and the effects on hydraulic fracture proppant placement

Foam has previously been used as fracturing fluid; however, there have not been enough study on foam stability and its effectiveness on proppant placement during hydraulic fracturing. In this paper, an experimental study was performed using free drainage method at 90 °C. Then, the rheological characterization of foam was produced based on dynamic foam quality change during foam drainage experiments and also based on viscosity breakdown by disproportionation. Subsequently, a 3-D hydraulically fracturing simulation was developed to evaluate the foam performance as a fracturing fluid using different vertical well scenarios. The results show that foam stability is dependent not only on the overall treatment time but also to fracture closure on proppant. For example, longer closure time accelerate proppant settling and accumulation at the bottom of the fracture, lowering propped area, and reducing productivity. The simulation results indicate that this lower productivity can be attributed to the final propped area, proppant distribution confirming the relationship between foam stability, foam rheology, proppant transport and fracture effectiveness.

Is Magnus effect relevant for proppant settling in narrow fractures?

Resolved Computational Fluid Dynamics-Discrete Element Method is an attractive approach for simulating multiphase flows of fluid-solid mixtures, as it includes a 4-way coupling. However, many of its modernimplementations neglect the torque coupling between the fractions. Here we augment the CFDEM®coupling toolkit to allow it to capture this interaction and use it to simulate sedimentation of spherical grains in two-dimensional narrow fractures, with and without grain rotation taken into account. While the rotational effects turns out to change the dynamics of individual grains, it does not affect the average settling velocity significantly.

Numerical simulation of in-pipe particle transportation in CO2 foam

Proppant-carrying foam fracturing fluids have complex rheological and transportation properties. Current studies on these fluids often focus on experimental phenomena. However, due to the limitation of experimental research, only macroscopic properties, such as the critical settling velocity, can be obtained. Transportation mechanisms and volume fraction distributions are poorly understood as well. In our study, the liquid-solid drag coefficient is corrected, and the mathematical physical model of non-Newtonian fluids of the particle-foam multiple phase is established by using a two-phase model. Proppant settling and transport properties in foam fracturing fluids are numerically studied, particle distribution on pipe cross section is obtained at various conditions, and a criterion for full development of fluid flow in pipe is set. We also find that when the Reynolds number (Re) is less than 190, the critical point of full development of flow increases with Re, whereas when Re is greater than 190, the critical point of full development decreases exponentially with the increasing of Re before stabilizing at approximately 45.

Synthesis and evaluation of a novel clean hydraulic fracturing fluid based on star-dendritic polymer

This paper presents a novel clean hydraulic fracturing fluid using a star-dendritic polymer as thickening agent. The star-dendritic polymer consists of two components, a core and the beams. The core was prepared from the thioxanthone (TX) and the polyamidoamine (PAMAM) by a divergent synthesis procedure using the reagent excess method starting from ethylenediamine (EDA). Then the photopolymerization process was carried out to make the PAMAM-8-TX and monomers reacts. After the synthesis, the dendrimer star polyacrylamide (DSPAM) was characterized by several tests to classify the monomers involved in the reaction and the architecture of the polymer. The results show that the DSPAM is in star-shaped.The hydraulic fracturing fluid formulation was designed using the DSPAM as thickening agent. For the purpose of evaluating the performance of the new fracturing fluid, shear resistance, thermal stability, rheology property, viscoelasticity, proppant transportation and breaking ability were tested by several experiments. The results show that the fracturing fluid has an excellent shear resistance under high temperature. The apparent viscosity remains 140 mPa s after 2hr shearing when the testing temperatures are 90 °C and 140 °C. So the fracturing fluid has an acceptable thermal stability. The static proppant sedimentation velocity is from 0.038 cm/min∼0.095 cm/min when the testing temperatures are 60 °C and 90 °C. The sedimentation velocity increases with the rising temperature when the sand proportion remains constant. In addition, the increasing of sand proportion could cut short the time of proppant settlement. The residual contents after breaking are hardly measured in various testing temperatures. A star-dendritic polymer was synthesized as a novel fracturing fluid thickening agent. The properties are acceptable to meet the requirement as the thickening agent due to the thermal resistance, shear resistance and proppant carrying ability of the fracturing fluid are quite effective in hydraulic fracturing treatment.

Fracture propagation laws in staged hydraulic fracturing and their effects on fracture conductivities

Completely taking into account the interferences between fractures as well as the friction effects on injection allocations, a fully coupled finite element method inherited from a verified one is proposed to discuss fracture propagation laws and analyze their impacts on fracture conductivities. Simulations show that although fractures have similar injection allocations that fluctuate around the allocation averaged by fractures, interferences between them lead to their different propagation rates and some fractures even stop propagating for a while. Shorter fractures generally have higher pressure and smaller pressure gradients than longer ones. The pressure differences between fractures result in long fractures having bottlenecking zones far away from the wellbore, and make them vulnerable to screen-out at the inlets and the bottlenecking zones. The effects of the propagation laws on fracture conductivities include: (1) the conductivities in short fractures are weakened by rapid proppant settlement in them; (2) long fractures may lost their conductivities due to screen-out near the wellbore; (3) the conductivities in long fractures decrease because of screen-out at the bottlenecking zones.

Experimental study on modified polyacrylamide coated self-suspending proppant

In recent years，different types of self-suspending proppants have come up. Among these proppants, “fracturing fluid coated” self-suspending proppant, which can adopt different types of proppants, is evaluated experimentally in this research. Evaluation standards and methods for this “fracturing fluid coated” proppant are discussed firstly. Suspension parameters are essential indicators of performance evaluation, the final proppant settlement volume and deposition time consumed from uniform mixture to the final volume under relative sensitive experiment conditions are selected as evaluation indexes. Influence of different factors on proppant suspension is evaluated by experiments, and corresponding results make the application scope of self-suspending proppant clear.

Numerical and Experimental Studies of Particle Settling in Real Fracture Geometries

Proppant is a vital component of hydraulic stimulation operations, improving conductivity by maintaining fracture aperture. While correct placement is a necessary part of ensuring that proppant performs efficiently, the transport behavior of proppant in natural rock fractures is poorly understood. In particular, as companies pursue new propping strategies involving new types of proppant, more accurate models of proppant behavior are needed to help guide their deployment. A major difficulty with simulating reservoir-scale proppant behavior is that continuum models traditionally used to represent large-scale slurry behavior loose applicability in fracture geometries. Particle transport models are often based on representative volumes that are at the same scale or larger than fractures found in hydraulic fracturing operations, making them inappropriate for modeling these types of flows. In the absence of a first-principles approach, empirical closure relations are needed. However, even such empirical closure relationships are difficult to derive without an accurate understanding of proppant behavior on the particle level. Thus, there is a need for experiments and simulations capable of probing phenomena at the sub-fracture scale. In this paper, we present results from experimental and numerical studies investigating proppant behavior at the sub-fracture level, in particular, the role of particle dispersion during proppant settling. In the experimental study, three-dimensional printing techniques are used to accurately reproduce the topology of a fractured Marcellus shale sample inside a particle-flow cell. By recreating the surface in clear plastic resin, proppant movement within the fracture can be tracked directly in real time without the need for X-ray imaging. Particle tracking is further enhanced through the use of mixtures of transparent and opaque proppant analogues. The accompanying numerical studies employ a high-fidelity three-dimensional particle-flow model, capable of explicitly representing the particles, the fracture surface and the interstitial fluid flow. Both studies reveal large-scale vortex motion during particle settling. For the most part, this behavior is independent of the fracture topology, instead driven by interactions between the sinking particles and the upwelling interstitial fluid. This motion results in large amounts of particle dispersion, significantly greater than might be expected from traditional slurry models. The competition between the particles and the fluid also results in a redistribution of particles toward the fracture walls, which has significant implications for the transport of proppant along the fracture.

Rheological behavior and particle suspension capability of guar gum: sodium tetraborate decahydrate gels containing cellulose nanofibrils

Guar gum (GG) fracturing fluids were studied by incorporating cellulose nanofibrils (CNFs) in anhydrous borax crosslinked guar gum gels. To fully understand the impact of CNF on the proppant suspension capability of developed fracturing fluids, their shear rate-dependent viscosity and viscoelasticity were investigated. The shear rate dependencies of fluids was fitted to the Carreau model. The zero shear rate viscosity and elasticity of fracturing fluids increased significantly by incorporating CNF in guar gum gels. On the other hand, the viscosity at high shear rates (>100 s−1) decreased as desired. The proppant settling velocities through fracturing fluids were evaluated by modeling the terminal falling velocity of proppants moving through a Carreau model fluid. The experimental results of the rheological behavior and the modeling results of the proppant settling rate indicated that the fracturing fluids containing CNF had better suspension capabilities. In addition, the lower viscosities of CNF formulated GG gels at higher shear rates will make them more pumpable.