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We present the digital slot — a tool for the development of new hydraulic fracturing technologies via digitization of slurry flow in narrow channels. We consider slurry containing fluid, proppant, and fiber components. The flow is described by a continuum mathematical model based on the lubrication theory. The numerical algorithm utilizes Lagrangian approach with finite volume pressure solver. We present the results of laboratory validation and simulation examples showing the key effects affecting solids transport in hydraulic fracturing: settling, bridging, gravity slumping, materials degradation, viscosity contrast, and bank formation.

Oil-saturated strata of Western Siberia fields are represented by laminated low-permeability sandstone separated by shale layers. Therefore, when designing hydraulic fractures, it is important to create longer propped fracture half-length and provide coverage of oil-saturated layers along the entire net height. Implementation of high-volume proppant fractures in combination with high-viscosity crosslinked fluids leads to excessive fracture height growth. In some cases it results in ineffective proppant distribution in the target layer and, moreover, to unwanted water production if the water contact or water bearing formation is close. To overcome these issues, it was proposed to use a novel hydraulic fracturing fluid that is a viscous slickwater based on synthetic polymer-polyacrylamide (also known as HiVis FR or HVFR). The low viscosity of HVFR (about 10 times lower than that of a crosslinked gel) allows a long fracture to be created and restricts height growth. Additionally, use of polyacrylamide instead of guar gives a larger value of retained conductivity. The full workflow for implementing HVFR for hydraulic fracturing in conventional formations includes candidate evaluation, HVFR laboratory testing, an integrated engineering approach to fracture modeling, operational considerations, and post-fracturing production analysis. The workflow evolved during the technology implementation cycle in a specific oil field, particularly the modeling step, which used a new high precision multiphysics (MP) model. The MP model provides an advanced, high-quality high- precision fracturing design to properly evaluate fracture geometry and proppant distribution by accounting for proppant settling in viscoelastic fluid and an accurate simulation of proppant placement when using a pulsing schedule. During the 2-year project, considerable success was achieved in expanding of the technology implementation scope. Several records were achieved on Kondinskoe oil field - a 150-t of ceramic proppant (SG, specific gravity,~3.1) were placed in a conventional reservoir by low-viscosity fracturing fluid and the first worldwide combination of viscous slickwater with channel fracturing technology was successfully performed. The use of HVFR, due to ability of fracture growth control, prevented breakthrough into the water-bearing zone. In addition, considerable improvement of operational efficiency was achieved due to use of cold water, lower amounts of additives, and less equipment, which resulted in a smaller location and environmental footprint. This first implementation of the viscous slickwater in conventional wells in Western Siberia enabled evaluating its effect on production rate. Increasing demand for maximizing production from low- permeability formations makes the result of this viscous slickwater implementation campaign of special interest. The application of a full engineering workflow, including design, execution, and evaluation of the Viscous slickwater treatments is a key to successful technology implementation and production optimization.

We introduced fracture hydrodynamics and in-situ kinetics model capable of simulating particle size distribution of propping agent. We demonstrated on several cases that accounting for particle size distribution in numerical simulation of hydraulic fracturing results in a noticeable difference of predicted fracture geometry and conductivity, as compared to the modeling approach where propping agent is represented by one effective mean diameter.

It is often thought that in ultra-low-permeability unconventional reservoirs, proppant does not play a significant role in productivity because any proppant effectively results in a relatively infinite fracture conductivity. Although with over 75% of the treatment jobs pumped with slickwater in the shale reservoirs due to its cost benefit, it is also thought that slickwater has limited solids carrying capacity. Overflushing may compromise productivity by creating near-wellbore pinchouts. The study presented here aims to test some of these conventions through the use a new high-resolution slurry transport model in unconventional reservoirs. Hydraulic fracture propagation and solids transport are simulated across multiple wells and reservoir settings to measure the production performance of unconventional reservoirs. Wells completed in the Eagle Ford formation are studied using an integrated earth model built to capture the reservoir geology, petrophysics and geomechanics. A pseudo 3D model for fracture propagation has been coupled with a fine grid numerical simulation of proppant and fluid transport in hydraulic fracture. The transport model can distinguish and demarcate the corresponding bridging resolution. This allows capturing the effect of slurry with proppant bypassing bridged 2D elements. Multivariate analysis of over 50 cases with various combinations of hydraulic fracturing fluid with viscosity ranging from 1.5 cP (slickwater) to 362 cP (crosslinked gel) and various proppants ranging from 100 mesh to 20/40 proppants are evaluated using the 2D transport model to determine the impact on production. Additionally, various pumping schedules ranging from 750 lbm/ft to 3,000 lbm/ft are evaluated. Parametric sensitivity of the overflush fluid type and volume has been studied to measure the impact on proppant dislodgement in the near-wellbore area. Production performance for all the scenarios is studied through numerical simulation and an economic analysis workflow to evaluate the matrix for fracturing fluid and proppant selection.

The present study demonstrates a very helpful role of computer algebra systems (CAS) for deriving and testing new numerical methods. We use CAS to construct and test a new numerical method for solving boundary – value problems for the 2D Navier — Stokes equations governing steady incompressible viscous flows. We firstly describe the core of the method and the algorithm of its construction, then we describe the implementation in CAS for deriving formulas of the method and for testing them, and finally we give some numerical results and concluding remarks.

We demonstrate the advantages of a new hydraulic fracturing simulator comprising a fine-scale fracture hydrodynamics and in-situ kinetics model. In contrast to existing commercial modeling tools, it has a sufficient resolution and other functionality for adequate representation of modern stimulation technologies: pulsing injection of proppant, mixtures of multiple fracturing materials (fluids, proppants, fibers, etc.), materials degradation, etc. This simulator accounts for the influence of materials distribution on fracture propagation and calculates fracture conductivity distribution. We coupled it with a production simulation model and established a complete framework for hydraulic fracturing treatment design. In addition to the selection of the pumping schedule, this model can be used to define specifications for novel hydraulic fracturing materials. This is a step change tool for wellbore stimulation and production forecast.

We propose a new model and workflow to predict, quantify and mitigate undesired flowback of proppant from created hydraulic fractures. We demonstrate several field cases in which we predict significant proppant flowback and propose options for mitigation. Mitigation of proppant flowback is based on case- specific changes in the fracturing treatment design and modifications in the well startup schedule that preserve near-wellbore conductivity. The presented workflow integrates four key components of proppant flowback study: (A) simulation of the fracturing job with a high-resolution model of proppant placement inside a fracture; (B) subsequent simulation of flowback from the created fractures equipped by a validated proppant flowback model; (C) a series of laboratory experiments, which quantify the proppant flowback for a wide range of commercial proppants; and (D) an accurate mathematical model, which is validated by the results of laboratory experiments and integrated into a flowback simulator to predict the behavior of injected proppant. Each component is presented with sufficient details to demonstrate its necessity for accurate modeling of a coupled solid-and-fluid flow inside a fracture. The presented theory of proppant pack mobilization is based on the concept of a proppant pack erosion process evolving from free boundaries of proppant packs. The theory confirms that proppant flowback critically depends on flow rate, proppant, fracture, and reservoir parameters. Laboratory experiments on proppant flowback in a cell support these theoretical predictions. The theoretical model of proppant flowback is integrated into the numerical simulator of early-time production from a fractured reservoir and predicts flowback of both injected solids and fluids from a fracture. We show that the combination of the proppant flowback model, laboratory experiments, hydraulic fracturing design tool, and early-time production simulator result in a useful workflow for prediction and mitigation of issues with proppant flowback and production decline. The entire workflow was validated using field cases where proppant flowback was observed. Modeled amounts of flowed back proppant are in good agreement with amounts of proppant observed in the field. Hydraulic fracturing design optimization was performed to minimize or eliminate proppant flowback. The novelty of the proposed study is related to the model of proppant flowback, which accounts for erosion of the proppant pack and is calibrated against unique laboratory experiments. The presented model and proppant flowback mitigation workflow can assist in understanding and mitigating proppant flowback events that can occur during wide range of oilfield operations.