Proppant Transport In Fractures Research Articles

Experiment and simulation of slurry flow in irregular channels to understand proppant transport in complex fractures

Slurry flow and proppant placement in irregular fractures are crucial to evaluate hydraulic fracturing stimulation but need to be better understood. This study aims to investigate how irregular fracture affects proppant transport and distribution using laboratory experiments and micro-scale numerical models. The unresolved method of the computational fluid dynamics (CFD) and the discrete element method (DEM) considers Saffman lift force, Magnus force, and virtual mass force to accurately capture the frequent interaction between proppant and slickwater. Experimental results validated the reliability of the optimized CFD-DEM model and calibrated primary parameters. The effects of crack height and width, bending angle, and distance between the crack and inlet on particle distribution were studied. The results indicated that the improved numerical method could rationally simulate proppant transport in fractures at a scale factor. The small crack height causes downward and upward flows, which wash proppant to the fracture rear and form isolated proppant dunes. A wider region in the fracture is beneficial to build up a large dune, and the high dune can hinder particle transport into the fracture rear. When the crack is close to the inlet, the primary fracture without proppants will close to hinder gas production. The smaller the bending angle, the smaller the proppant dune. A regression model can precisely predict the dune coverage ratio. The results fundamentally understand how complex fractures and natural cracks affect slurry flow and proppant distribution.

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.

Coupled Hydraulic Fracture and Proppant Transport Simulation

This paper focuses on the study of proppant transport mechanisms in fractures during frac-packing operation. A multi-module, numerical proppant, reservoir and geomechanics simulator has been developed, which improves the current numerical modeling techniques for proppant transport. The modules are linked together and tailored to capture the processes and mechanisms that are significant in frac-pack operations. The proposed approach takes advantage of a robust and sophisticated numerical smeared fracture simulator and incorporates an in-house proppant transport module to calculate propped fracture dimensions and concentration distribution. In the development of software capability, the propped fracture geometry and proppant concentration, which are the output of the proppant module, are imported to the hydraulic fracture simulator through mobility modification. Complex issues of proppant transport in fractures that are addressed in the literature and captured by the current model are: hindered settling velocity (terminal velocity of proppant in the injection fluid), the effect of fracture walls, proppant concentration and inertia on settling (due to extra drag forces applied on particles, compared to single-particle motion in Stokes regime in unbounded medium), possible propped fracture porosity and also mobility change due to the presence of proppant, and fracture closure or extension during proppant injection. A sensitivity analysis is conducted using realistic parameters to provide guidelines that allow more accurate predictions of the proppant concentration and fluid flow. The main objective of this study is to link a numerical hydraulic fracture model to a proppant transport model to study the fracturing response and proppant distribution and to investigate the effect of proppant injection on fracture propagation and fracture dimensions.

Study on Proppant Transport in Fractures of Supercritical Carbon Dioxide Fracturing

Supercritical carbon dioxide (SC-CO2) fracturing is a promising stimulation treatment in the unconventional oil and gas industry, which combines the utilization of greenhouse gas and the development of energy resources. Aiming to study the proppant transport in fractures, which is one of the most important issues in this technology, a generalized and pragmatic numerical method coupled with multiphase flows, physical property models of CO2, and heat and mass transfer in the formation rock has been established to calculate the proppant transport in fractures of SC-CO2 fracturing. Experimentally, experimental validation of the proppant transport was conducted, which verifies the accuracy of the simulation results. Combining experimental and simulation results, the flow condition of proppant–CO2 two-phase flow was divided into a stationary layer, a rolling layer, a saltatory layer, a suspending layer, and a pure liquid/gas layer. Numerically, movement laws of proppant beds in fracture and influence factors on the distribution of proppant beds were analyzed. Proppant bed was formed near the inlet of the fracture at the initial stage of fracturing, and the height and length of the bed would increase concurrently until reaching a steady state. The equilibrium height has positive correlations with the injection temperature, proppants’ volume fraction, density, and diameter and negative correlations with the injection pressure and displacement. Nevertheless, the equilibrium time has negative correlations with all these factors except pressure. Additionally, the proppant transports of SC-CO2 and water were compared, which indicated that water has better proppant-carrying capacity with lower proppant bed height and longer proppant bed length at the same fracturing time. The distribution of the proppant bed with the structure of a nozzle inlet is relatively homogeneous in the near-wellbore zone of the fracture compared to the cuboid structure, which could reduce the risk of fracture closure in the near-wellbore zone. Results obtained in this paper could provide references for SC-CO2 fracturing design which would promote the development of this technology.

A Novel Hydraulic Fracturing Method Based on the Coupled CFD-DEM Numerical Simulation Study

For the development of tight oil reservoirs, hydraulic fracturing employing variable fluid viscosity and proppant density is essential for addressing the problems of uneven placement of proppants in fractures and low propping efficiency. However, the influence mechanisms of fracturing fluid viscosity and proppant density on proppant transport in fractures remain unclear. Based on computational fluid dynamics (CFD) and the discrete element method (DEM), a proppant transport model with fluid–particle two-phase coupling is established in this study. In addition, a novel large-scale visual fracture simulation device was developed to realize the online visual monitoring of proppant transport, and a proppant transport experiment under the condition of variable viscosity fracturing fluid and proppant density was conducted. By comparing the experimental results and the numerical simulation results, the accuracy of the proppant transport numerical model was verified. Subsequently, through a proppant transport numerical simulation, the effects of fracturing fluid viscosity and proppant density on proppant transport were analyzed. The results show that as the viscosity of the fracturing fluid increases, the length of the “no proppant zone” at the front end of the fracture increases, and proppant particles can be transported further. When alternately injecting fracturing fluids of different viscosities, the viscosity ratio of the fracturing fluids should be adjusted between 2 and 5 to form optimal proppant placement. During the process of variable proppant density fracturing, when high-density proppant was pumped after low-density proppant, proppants of different densities laid fractures evenly and vertically. Conversely, when low-density proppant was pumped after high-density proppant, the low-density proppant could be transported farther into the fracture to form a longer sandbank. Based on the abovementioned observations, a novel hydraulic fracturing method is proposed to optimize the placement of proppants in fractures by adjusting the fracturing fluid viscosity and proppant density. This method has been successfully applied to more than 10 oil wells of the Bohai Bay Basin in Eastern China, and the average daily oil production per well increased by 7.4 t, significantly improving the functioning of fracturing. The proppant settlement and transport laws of proppant in fractures during variable viscosity and density fracturing can be efficiently revealed through a visualized proppant transport experiment and numerical simulation study. The novel fracturing method proposed in this study can significantly improve the hydraulic fracturing effect in tight oil reservoirs.

Lubrication model of suspension flow in a hydraulic fracture with frictional rheology for shear-induced migration and jamming.

We developed a model for suspension flow in a hydraulic fracture, taking into account frictional rheology to capture the effects of shear-induced particle migration, jamming and transition to close packing. One of the key issues with the existing slurry rheology models is that each of them diverges near the close packing limit, which is typically resolved in numerical simulations via a pragmatic (and mostly unjustified) regularization. Another drawback of the family of existing models for proppant transport in fractures is the assumption of a uniform cross-flow concentration profile, which neglects the effects of shear-induced migration. We developed a self-consistent model for slurry flow with a constitutive relation for suspension rheology, which is applicable in the entire range of particle volume concentration, from dilute suspension through dense suspension to the close packing limit. In addition, we investigated the influence of various constitutive relations for the suspension rheology on the final model for the slurry flow. The selected model for slurry flow was implemented into a two-dimensional lubrication model of proppant transport in a fracture (based on the two-continua approach), and illustrative simulations were conducted in comparison with the family of existing suspension rheology models (having a singularity). Validation against laboratory experiments is discussed.

Evaluation of numerical schemes for capturing shock waves in modeling proppant transport in fractures

In petroleum engineering, the transport phenomenon of proppants in a fracture caused by hydraulic fracturing is captured by hyperbolic partial differential equations (PDEs). The solution of this kind of PDEs may encounter smooth transitions, or there can be large gradients of the field variables. The numerical challenge posed in a shock situation is that high-order finite difference schemes lead to significant oscillations in the vicinity of shocks despite that such schemes result in higher accuracy in smooth regions. On the other hand, first-order methods provide monotonic solution convergences near the shocks, while giving poorer accuracy in the smooth regions. Accurate numerical simulation of such systems is a challenging task using conventional numerical methods. In this paper, we investigate several shock-capturing schemes. The competency of each scheme was tested against one-dimensional benchmark problems as well as published numerical experiments. The numerical results have shown good performance of high-resolution finite volume methods in capturing shocks by resolving discontinuities while maintaining accuracy in the smooth regions. These methods along with Godunov splitting are applied to model proppant transport in fractures. It is concluded that the proposed scheme produces non-oscillatory and accurate results in obtaining a solution for proppant transport problems.