Proppant Transport In Hydraulic Fractures Research Articles

On the correlation between proppant addition strategy and distribution

The proppant pumping schedule is critical in hydrofracking because it affects both fluid flow and proppant transport. In the study reported here, a validated CFD-DEM model was employed to study the impact of proppant pumping strategy on proppant transport in a rough rock fracture. Three injection sequences of multi-sized proppants were applied while varying other pumping parameters including injection velocity, the size of large proppants, and the concentration of large proppants. Both vertically and horizontally sheared rough, joint rough, and smooth fractures were adopted to enable a better understanding of the impact caused by fracture geometry. The results reveal that the variation in proppant distribution caused by adjusting the pumping sequence or a single pumping parameter is not absolute. Rather than pumping parameters, the final proppant distribution is governed directly by two competing effects caused by the alteration of proppant bridging effect. It was also found that the proppant volume retained in fractures is under-estimated if the CFD mesh size is too small to reach three times the particle size. The lateral conveyance of proppant could be enhanced by considering the Saffman lift force in CFD-DEM modeling. The results of this study shed light on proppant transport in hydraulic fractures under various addition strategies, which will assist the optimization of fracturing treatments in unconventional reservoirs.

Effect of roughness characteristics of hydraulic fractures on the proppant transport using supercritical CO2

The morphology of rough fractures significantly affects proppant transport in hydraulic fractures. The fracture roughness is even more critical in proppant transport using supercritical CO2 since supercritical CO2 produces rougher fractures than other conventional fracturing fluids. In this study, we used roughness characteristics of the fractures generated by using supercritical CO2 and real rock samples determined by direct measurement together with coupled CFD (Computational Fluid Dynamics)-DEM (Discrete Element Method) to investigate the effects of different roughness characterization parameters on the efficiency of proppant transport using supercritical CO2. Laboratory experimental data of proppant transport in supercritical CO2 within the fracture were used to verify the reliability of the model. The results show that as the ratio of proppant diameter to average width of rough fracture increases, the proppant is more likely to bridge or even form sand blockage when transported in rough fractures, while better transport can be obtained when the ratio is less than 0.4. The main form of proppant particle transport within the rough fractures are the scouring and throwing of particles as the slurry flows through the bridging locations. In addition, the variation of root mean square (RMS) of asperity height values and fractal dimensions can have dissimilar effects on the proppant transport in rough fractures. The distance of proppant transport within the fracture decreases with the increasing fractal dimension, but increasing the RMS of asperity height value will yield proppant transport to longer distance. The proppant is transported farther in rough fractures than in planar fractures, but also, the required pumping pressure is higher in rough fractures, and the pumping pressure is positively related to the fracture roughness. Since using supercritical CO2 for fracturing produces rougher fractures than that of water-based fracturing fluids, higher pump power is required for efficient proppant transport using supercritical CO2, otherwise sand plug may occur. The results of this study lead to a better insight into the proppant transport behavior using supercritical CO2 in rough rock fractures and, therefore, would be useful for the improved field design of waterless fracturing operations conducted by using supercritical CO2.

A coupled CFD-DEM numerical study of proppant transport in hydraulic fracture and natural fracture

Based on computational fluid dynamics (CFD) and discrete element method (DEM), the factors affecting proppant transport law in hydraulic fracture and natural fracture are analyzed. The results indicated that the smaller the angle between hydraulic fracture and natural fracture, and the larger the width of natural fracture, the more proppants enter the natural fracture. As the viscosity of the fracturing fluid increases, the length of the “no proppant zone” in fracture is longer, and the proppant particles can be transported to a farther position. Proppant will quickly reach the fracture tip and be screened out under too high injection rate.

Proppant transport in hydraulic fractures by creating a capillary suspension

Slick water fracturing fluids with high viscosity and minimal friction pressure losses are commonly employed in hydraulic fracturing technology. The conventional method of conducting hydraulic fracturing operations using high-viscosity fluids is not always suitable in complex geological conditions because hydraulic fracturing in low-productive formations creates fractures that resemble radial fractures, lowering the impact’s efficiency and profitability due to inefficient use of materials and reagents. As such, the goal is to limit the height of the fracture development and increase their length. In this work, we developed a low viscosity fracturing fluid with high proppant transport ability in order to minimize the specific pressure in the fracture and control its height. The hydrophobizing effect of surfactant was investigated, and criteria for employing it in hydraulic fracturing fluid formulation were created. We also developed a fracture model that we used to run simulation experiments in order to determine the nature of the fracturing fluid dispersion. Model simulations of hydraulic fracturing were performed utilizing proppant mesh sizes of 30/60, 20/40, and 16/20. Our experimental results have demonstrated that utilizing a capillary suspension, it is possible to generate a stable formulation of hydraulic fracturing fluid with an effective settling capacity by adding PLA fibers as a reinforcing filler to the formulation.

Drag coefficient modification for proppant transport in fractures considering wall retardation

Proppant transport is crucial in hydraulic fracturing, which determines fracture conductivity and well productivity. Proppant transport in hydraulic fractures is essentially the dense solid−liquid flow. Unlike the widely studied fluid-solid flow in chemical engineering and fluvial sediment transport, the motion of proppants (particles) will be significantly affected by walls of narrow channels such as fractures and slots, because the drag force is changed. This paper studies the wall-retardation effect on particle movement, and modified the drag coefficient for simulating liquid−solid flow in narrow-width fractures. Both experiments and simulations on proppant transport are conducted. The drag coefficient considering wall-retardation effect is modified by the Lattice Boltzmann method (LBM), and is integrated into OpenFOAM for two fluid method (TFM) numerical simulation. Experiment and simulation results show a good agreement, and the improved drag model is more advanced than the Gidaspow model in reproducing the movement characteristics of suspended particles in narrow channels. Compared to previous research outcomes about this topic, this work can efficiently improve the accuracy on proppant transport simulation and be widely applied in industrial numerical simulations.

Fluid-driven particle transport patterns in a confined geometry: Effect of flow velocity and particle concentration

Fluid-driven granular transport is involved in many important geomorphological processes and industrial applications such as unconventional hydrocarbon recovery. Yet it remains challenging to fully understand the granular transport mechanisms in confined geometries. By performing simulations based on a coupled computational fluid dynamics and discrete element method (CFD-DEM) approach, we systematically investigate the particle transport patterns and mechanisms driven by fluid flow between two parallel plates. Depending on the local drag force, the particles can settle or suspend in the fluid, leading to fluid-driven particle transport by creeping or by suspension. In the case of settle particle layers, fluid velocity variation in the vertical direction contributes to the relative motion between particle layers. Fluid-induced fingering patterns are observed in the upper layer of settled particles during the sliding. It is shown that the average finger length increases linearly with time and is affected by the flow rate and particle volume fraction. Increasing the flow rate shift the particle migration from creeping and sliding to suspension, which generally improves the particle transport efficiency. The obtained understanding of particle transport patterns in the confined geometries may have practical implications for industrial scenarios such as proppant transport in hydraulic fractures and sand production in extraction systems.

Direct numerical simulation of proppant transport in hydraulic fractures with the immersed boundary method and multi-sphere modeling

In this paper, a resolved CFD-DEM method based on the immersed boundary method is proposed to simulate the proppant transport process, which is a multi-phase problem with strong fluid-particle coupling mechanisms in the oil and gas industry. A multi-sphere model is integrated into this method to describe complex particle shapes, in which Lagrangian points uniformly distributed on the particle surface are efficiently utilized for solving particle-particle interactions. This approach is validated by several benchmarks, including single-sphere and two-sphere settling tests. A modified driving pressure gradient is also adopted to satisfy bulk velocity constraints for simulating particle settling problems in periodic channels. Transport and settling behaviors of hundreds of sphere and cylinder proppant particles in periodic narrow channels with different widths are investigated, and settling laws and apparent viscosity models for proppant clouds with different shapes are then extracted from the simulation results. Benefiting from the features of multi-sphere modeling, this approach is demonstrated to be both robust and efficient for simulating fluid-particle coupling flow with complex particle shapes.

Effect of fracture roughness on the hydrodynamics of proppant transport in hydraulic fractures

The effect of fracture roughness is investigated on proppant transport in hydraulic fractures using Joint Roughness Coefficient and a three-dimensional multiphase modelling approach. The equations governing the proppant transport physics in the fracturing fluid is solved using the hybrid computational fluid dynamics model. The reported proppant transport models in the literature are limited to the assumption of a smooth fracture domain with no fluid leak-off or fluid flow from fracture to rock matrix interface. In this paper, a proppant transport model is proposed that accounts for the proppant distribution in rough fracture geometry with fluid leak-off effect to surrounding porous rock. The hydrodynamic and mechanical behaviour of proppant transport was found directly related to the fracture roughness and flow regime especially under the influence of low viscosity fracturing fluid typically used in shale gas reservoirs. For the proppant transport in smooth fractures, the fracture walls employ mechanical retardation effects and reduce the proppant horizontal velocity resulting in more significant proppant deposition. On the contrary, for the proppant transport in rough fractures, the inter-proppant and proppant wall interactions become dominant that adds turbulence to the flow. It results in mechanical interaction flow effects becoming dominant and consequently higher proppants suspended in the slurry and greater horizontal transport velocity. Furthermore, the mechanical interaction flow effects were found to be principally dependant on the proppant transport regime and become significant at higher proppant Reynolds number.

Numerical Modelling of Proppant Transport in Hydraulic Fractures

The distribution of proppant injected in hydraulic fractures significantly affects the fracture conductivity and well performance. The proppant transport in thin fracturing fluid used during hydraulic fracturing in the unconventional reservoirs is considerably different from fracturing fluids in the conventional reservoir due to the very low viscosity and quick deposition of the proppants. This paper presents the development of a threedimensional Computational Fluid Dynamics (CFD) modelling technique for the prediction of proppant-fluid multiphase flow in hydraulic fractures. The proposed model also simulates the fluid leak-off behaviour from the fracture wall. The Euler-Granular and CFD-Discrete Element Method (CFD-DEM) multiphase modelling approach has been applied, and the equations defining the fluid-proppant and inter-proppant interaction have been solved using the finite volume technique. The proppant transport in hydraulic fractures has been studied comprehensively, and the computational modelling results of proppant distribution and other flow properties are in good agreement with the published experimental study. The parametric study is performed to investigate the effect of variation in proppant size, fluid viscosity and fracture width on the proppant transport. Smaller proppants can be injected early, followed by larger proppants to maintain high propping efficiency. This study has enhanced the understanding of the complex flow phenomenon between proppant and fracturing fluid and can play a vital role in hydraulic fracturing design.

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.

Slickwater Proppant Transport in Hydraulic Fractures: New Experimental Findings and Scalable Correlation

Summary Slickwater hydraulic fracturing is an important technology that has enabled the oil and gas industry to economically develop enormous unconventional resources. Despite its great success, this technology faces challenges, especially with proppant transport in complex fractures. Very limited work exists in the literature regarding slickwater proppant flow in subsidiary fractures or predictive correlations to estimate settled proppant-dune heights. This paper provides a scalable correlation to predict dune height across a wide range of flow rates and proppant concentrations in the primary fracture. A unique feature of this correlation is its inclusion of the friction effect; roughness was introduced to the fracture-slot walls. A 30/70-mesh brown sand was used to conduct the slot-flow experiments and build the correlation. This paper also spotlights the proppant-transport mechanism during proppant-dune development in the primary fracture. Understanding this mechanism reveals key information regarding the horizontal and vertical settled-proppant-size distribution. In addition, experimental results are presented to answer the debatable question of whether slickwater can transport proppant into tertiary fractures. In fact, the data show that proppant in slickwater is not only capable of “turning the corner” but also developing high dune levels exceeding 97% of the tertiary fracture-slot heights.

Numerical simulation of proppant transport in hydraulic fracture with the upscaling CFD-DEM method

In this study, the coupled CFD (Computational Fluid Dynamics)-DEM (Discrete Element Method) method is employed to simulate the proppant transport process in a hydraulic fracturing system. The particle-particle and particle-wall interactions can be captured precisely in the DEM, which could not be fully considered in other methods. However, the DEM is time-consuming if every single proppant particle is considered as a discrete element. In order to reduce the computational efforts, the representative particle model (RPM) is adopted in this work for upscaling the CFD-DEM. Dynamic packing problems of both the uniform and bi-density cases are designed to illustrate the advantage of simulating proppant transport behaviors with the CFD-DEM, and the upscaling cases are performed with the RPM for comparison. In addition, upscaling issues regarding validation and large-scale applications of the RRM are also discussed. After upscaling, the main characteristics of the packing patterns can be captured, while the time cost is greatly reduced.

A Lagrangian Approach to Modelling Proppant Transport with Tip Screen-Out in KGD Hydraulic Fractures

This study introduces a continuum approach to model proppant transport in hydraulic fractures in a Lagrangian frame of reference. The model for the proppant transport is based on the recently obtained slurry flow solution inside a channel, where the latter utilizes a phenomenological constitutive relationship for a slurry. This approach allows us to describe the transition from Poiseuille flow with an effective viscosity to Darcy flow as the particle concentration increases towards the maximum value. The algorithm is presented for the one-dimensional case, for which propagation of a symmetric Kristinovich–Zheltov–Geertsma–De Klerk fracture is considered. To examine the effectiveness of the Lagrangian approach for proppant transport modelling, a set of parameters, for which proppant particles reach the fracture tip and cause the development of a proppant plug is selected. In this situation, the coupling between the hydraulic fracture propagation and proppant transport is the most significant. To estimate the accuracy of the Lagrangian proppant transport model, the results are compared to the predictions of an Eulerian proppant transport model, which utilizes the same algorithm for hydraulic fracture propagation. It is shown that, although both approaches have the same convergence rate, the error of the Lagrangian approach is three to five times smaller, which depends on the number of proppant elements used in the Lagrangian approach. This permits us to use a coarser mesh for hydraulic fracture propagation, and to obtain results with similar accuracy up to a hundred times faster.

Intrinsic anisotropy in fracture permeability

Contrary to the assumption in cubic law, the surface of fractures has some degree of roughness, which impacts their fluid dynamics. Incorporating the effect of roughness can improve the simulation of fluid flow in fractures and faults, as well as proppant transport in hydraulic fracturing. To investigate the effect of roughness on the fluid flow, we created a fracture using the Brazilian test, and its roughness was measured using a laser profilometer. Experimental permeability measurements showed a reduction in permeability as the effective stress increased. However, the unmatching surfaces of the fracture prevented its complete mechanical closure. Numerical simulations of the fluid dynamics were conducted on the measured fracture geometry. We determined that the hydraulic fracture aperture is less than the mechanical fracture aperture and that there was anisotropy in the fracture permeability. The ratio of hydraulic fracture aperture to mechanical fracture aperture, as well as anisotropy in fracture permeability, increased when the fracture aperture decreased. The anisotropy in fracture permeability was 45% at the lowest simulated fracture aperture. Integrating the experimental and numerical data, we estimated the fracture porosity and fracture permeability.

Proppant transport in hydraulic fracturing: Crack tip screen-out in KGD and P3D models

The aim of this study is to develop a model for proppant transport in hydraulic fractures capable of capturing both gravitational settling and tip screen-out effects, while prohibiting the particles from reaching the crack tips by imposing a width restriction based on the particle size. First, the equations that govern the propagation of hydraulic fractures and the proppant transport inside them are formulated. They are based on the solution for the steady flow of a viscous fluid, mixed with spherical particles, in a channel, which is obtained assuming an empirical constitutive model. This proppant transport model is applied to two fracture geometries – Khristianovich–Zheltov–Geertsma–De Klerk (KGD) and pseudo-3D (P3D). Numerical simulations show that the proposed method makes it possible to capture proppant plug formation and growth, as well as the gravitational settling for both geometries. A dimensionless parameter, whose magnitude reflects the intensity of the settling, is introduced for the P3D fracture.

Modeling non-Newtonian slurry flow in a flat channel with permeable walls

A model describing a shear-thinning slurry flow in a flat channel is developed. A diffusive flux modeling approach, which allows calculating particle migration to the channel center, is employed. The model developed accounts also for a fluid leak-off through permeable channel walls. Most of the calculation examples are presented for power-law slurry rheology and a few for Herschel–Bulkley slurries. An evolution of the cross-sectional solids concentration distribution along the channel is analyzed in dependence on slurry system parameters. The computations also revealed a significant effect of particle migration to the channel center on the pressure gradient. The results obtained demonstrate that the particle migration may be important for modeling particle transport in narrow flat channels and, therefore, this phenomenon must be taken into account in the modeling of technological processes, such as proppant transport in hydraulic fracturing.

Numerical Solution of Sand Transport in Hydraulic Fracturing

A numerical solution is developed for the deposition of a propping agent inside a hydraulic fracture. Such parameters as fluid leak-off into the formation, increase in sand concentration caused by leak-off, non-Newtonian fracturing fluids, hindered settling velocity, and an up-to-date geometry are taken into consideration. Three examples investigate the proppant deposition for low-, medium-, and high-viscosity fracturing fluids. Introduction Most hydraulic fracturing treatments are performed with a slurry composed of a fracturing fluid mixed with a propping agent. The propping agent usually consists of propping agent. The propping agent usually consists of sand particles varying in size from 4 to 60 mesh. High-strength glass beads of the same sizes also are used occasionally. The dominant use of sand as a propping agent has resulted in frequent substitution of the term "sand" for propping agent. The two terms will be used interchangeably in this paper. The reason for using a propping agent is simple. At the end of a fracturing treatment, the hydraulic fracture has to be kept open so that the reservoir fluid can flow through it. A propping agent serves this purpose and has to meet certain requirements. A proppant must be introduced in sufficient amounts, and individual particles should be strong enough so that they will not be crushed by the action of the in-situ stresses. Furthermore, the sand bed should be conductive enough to transmit the reservoir fluid flowing into it. Although particle transport is well established in chemical engineering, most research has been conducted on flow through cylindrical pipes. The available research for particle transport through parallel plates is scarce. One of particle transport through parallel plates is scarce. One of the fast investigations of proppant transport in hydraulic fractures was conducted by Kern et al., who considered bed buildup for vertical fractures. After several studies of proppant movement in horizontal fractures, bed buildup in vertical fractures was treated by Babcock et al. In their comprehensive study, these authors conducted a theoretical and experimental investigation of the proppant movement and deposition between two parallel plates with limited vertical extent. Schols and parallel plates with limited vertical extent. Schols and Visser investigated the mechanics of bed buildup and related it to particle size. Van Domselaar and Visser considered proppant concentration in a fracture created by a viscous gel in which there is no particle settlement. An attempt is made to consider proppant settlement in hydraulic fractures with conditions similar to those encountered in the field. Allowing for a change in either the proppant type or concentration during treatment, this proppant type or concentration during treatment, this study includesthe influence of fluid leak-off on fracture geometry and the concentration of the proppant in it, andconsideration of pseudoplastic as well as Newtonian fluids. The results were obtained by numerical computations carried out with a computer. Proppant Deposition in Hydraulic Fractures Proppant Deposition in Hydraulic Fractures The determination of a proppant schedule usually involves the fracture geometry. Suppose L denotes the length, omega max the wellbore width, and h the height of an induced vertical hydraulic fracture. Assuming that q, the fluid injection rate, stays constant during treatment, that the treatment fluid does not change, and that h is constant, then the variations of L and omega max with treatment time, t, can be expressed as (1) (2) JPT P. 132