Proppant In Hydraulic Fractures Research Articles

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.

Community airborne particulate matter from mining for sand used as hydraulic fracturing proppant

Field and laboratory studies were conducted to evaluate the impact of proppant sand mining and processing activities on community particulate matter (PM) concentrations. In field studies outside 17 homes within 800m of sand mining activities (mining, processing, and transport), respirable (PM4) crystalline silica concentrations were low (<0.4μg/m3) with crystalline silica detected on 7 samples (2% to 4% of mass). In long-term monitoring at 6 homes within 800m of sand mining activities, the highest daily mean PM concentrations observed were 14.5μg/m3 for PM2.5 and 37.3μg/m3 for PM10, although infrequent (<3% of time), short-term elevated PM concentrations occurred when wind blew over the facility. In laboratory studies, aerosolized sand was shown to produce respirable-sized particles, containing 6% to 19% crystalline silica. Dispersion modeling of a mine and processing facility indicated that PM10 can exceed standards short distances (<40m) beyond property lines. Lastly, fence-line PM and crystalline silica concentrations reported to state agencies were substantially below regulatory or guideline values, although several excursions were observed for PM10 when winds blew over the facility. Taken together, community exposures to airborne particulate matter from proppant sand mining activities at sites similar to these appear to be unlikely to cause chronic adverse health conditions.

Simulation of the transport and placement of multi-sized proppant in hydraulic fractures using a coupled CFD-DEM approach

The productivity of fractured wells is mainly governed by propped fractures, so it is of significant importance to find out where the injected proppants go during hydraulic fracturing treatments, as this is essential to scheduling proppant injection in fracturing design. Using a coupled CFD-DEM model, the transport and placement of multi-sized proppants in fractures in vertical and horizontal wells were systematically investigated, and the effects of having multi-sized particles relative to uniformly-sized ones on the proppant placement were quantitatively characterized. When a proppant-laden fluid is injected into a fracture in vertical wells, a small proppant bank quickly forms. The injected large and small proppant particles are almost uniformly mixed, with just a small-proppant region at the back side of the bank. In comparison in horizontal wells, a proppant dune first forms near the wellbore in a fracture, and the large proppant particles are more likely to accumulate near the wellbore while the small particles are transported deeper into the fracture. The main transport mechanisms of proppant particles are settlement and fluidization, which cause a three-layer flow pattern (stationary proppant bed, fluidization layer and clean fluid layer) to form. In addition, vortex is also an important proppant transport mechanism, especially in a fracture in horizontal wells, where the vortex drags the injected proppant particles to different locations causing a dual-dune profile. The effect of fracture tip screen-out on the proppant placement was investigated. Screen-out can significantly change the flow field in a fracture and this will subsequently affect final proppant placement. Ultimately, the process of graded proppant injection was realistically modeled, which shows small proppants to be transported deeper into the fracture, while large proppants accumulate near the wellbore.

Experimental study of permeability and its anisotropy for shale fracture supported with proppant

Shale gas is an important unconventional natural gas resource, but shale has extremely low permeability. Production of shale gas can be improved by using proppants for hydraulic fracturing and maintaining fracture conductivity, and a better understanding of the permeability and its anisotropy of proppant-supported fractures would be useful in optimising gas production. This paper described experiments on shale permeability and its anisotropy with respect to gas pressure, effective stress and gas type for a natural fracture supported with two sizes of proppant. A cubic sample from the Silurian Longmaxi formation in the Sichuan Basin, China, was used in the study; the testing direction of the sample was altered, and both helium (non-sorbing) and methane (sorbing) were tested. Microscopic X-ray computerised tomography (μ-CT) scanning was used to reveal the proppant distribution and fracture shape. Finally, an analytical model was applied to describe the permeability with respect to pore pressure and effective stress and the results were used to determine the relationships between initial fracture compressibility, Klinkenberg coefficient and absolute permeability. The permeabilities of propped fractures were found to be a few hundred or even a few thousand times higher than those of the natural fracture under the same experimental conditions, with both the proppant size and the amount of proppants added affecting this increase. The permeability was anisotropic in two horizontal directions. The direction and ratio of permeability anisotropy of the proppant-supported fracture differed from those of the natural fracture, depending on the amount of proppants added and their distribution in the fracture. The model indicated that permeability decreased with effective stress at a given pore pressure, and with pore pressure at a given effective stress. It also suggested that adding proppants will significantly change the absolute permeability but not the initial fracture compressibility. Although the permeability had strong anisotropy, the initial fracture compressibility relationships with absolute permeability were independent of flow direction and followed the same trend. The relationship between the Klinkenberg constant and absolute permeability was found to be well described by a cubic law function.

Numerical simulation of transport and placement of multi-sized proppants in a hydraulic fracture in vertical wells

The placement of proppant in hydraulic fractures, which is governed by slurry flow, proppant transport and settlement, can significantly affect the conductivity of the fracture, and this will subsequently affect the productivity of the hydraulically fractured wells. To investigate proppant transport mechanisms and placement profiles in a hydraulic fracture, computational fluid dynamics coupled with discrete element method is used to model slurry flow and micro-mechanical interactions of proppant particles in a fracture. The effect of the solid phase is introduced in terms of volumetric porosity and particle interaction forces, and a contact model is used to simulate the particle-to-particle and the particle-to-wall interactions. The particles are modelled as real spheres, and for a two-dimensional computational model, an out-of-plane length of the maximum particle diameter is assumed. The proppants are injected into the fracture through seven perforations to mimic the slurry flow in a fracture in vertical wells. When the proppants are injected into a fracture driven by thin fracturing fluids, they quickly settle out of the fluid and accumulate on the fracture bottom, forming a proppant dune. A three-layer flow pattern forms in the fracture consisting of: (1) a stationary proppant bed at the bottom, (2) a proppant-fluid mixture layer above it, and (3) a clean fluid layer at the top. During the early stages, the motion of the proppants is governed by suspension, settlement and fluidization, and the injected proppants mainly settle and accumulate near the wellbore. When the equilibrium height of the dune is reached, the dune changes to a proppant bank. Then the later injected proppants are transported to overshoot the bank and dragged deeper into the fracture. A new proppant transport mechanism, vortex, is observed, which governs the proppant motion after they leave the proppant bank.

Characterization of agro-waste resources for potential use as proppant in hydraulic fracturing

Utilization of deformable substrates as proppant with sparse distributing for Hydraulic Fracturing (HF) is contributed to the improvement of fracture conductivity. Agro-waste resources, specifically shells of tropical plants such as coconut shell and palm kernel shell are two environmentally friendly and potential renewable resources that can be utilized as propping agents in HF. Nevertheless, they are not yet widely studied as such. Given this lack of particular technical and reliable data, their conversion into proppant cannot be designed with confidence as it happens with other studied biomass feedstock such as walnut-hull. This paper includes a study on the experimental characteristics of two agro-wastes or biomasses, coconut shell and palm kernel shell, as possible resources as proppants in HF. The main characteristics envisaged are particle size distribution, roundness and sphericity, density, specific gravity, bulk density, turbidity, crush resistance, single particle compression test, and fracture conductivity. The results obtained from the analyses are compared to the commonly known walnut-hull particles. These biomasses show promising results and meet some of established API/ISO standards. This paper is intended to provide technical information on the characteristics of new agro-wastes resources and to emphasize their major strengths; that have not been highlighted in the literature. These results are of key importance for the consequent development of high-strength composite proppants. Recommendations are also proposed for future improvement in increasing the quality of the results.

Uniaxial compression of dense granular materials: Stress distribution and permeability

A uniaxial compression apparatus designed to investigate the dynamics of proppant particle packing is described. This dynamic compression device (DCD) uses a small charge of particles, approximately 3.5–7 cm 3 depending upon the configuration of an experiment. The DCD allows simultaneous measurement of boundary stresses on the packing and permeability of the packing to fluid flow. Visualization of the packing at compressive stress levels up to σ A = 7100 psi (49 MPa) is also demonstrated. The stresses measured are the normal stress at both walls on the axis of compression, with their difference providing a measure of the shear (or friction) force from the walls, and the transverse normal stress on one orthogonal axis, σ T . Permeability may be determined by either a fixed pressure drop or fixed flow rate experiment, with results presented from the fixed pressure drop approach by imposing a small gravity head of the flowing liquid; liquid flow is gravimetrically measured. The behaviors of packings of hard particles, both uncoated and when coated with a very viscous and tacky polyamide resin, are reported. The packing materials include a ceramic material used as a proppant in hydraulic fracturing of petroleum wells, and glass beads. The proppant and glass beads have mean diameters of approximately 700 μm and 500 μm, respectively, with the proppant less spherical and more polydisperse than the glass beads. The proppant particles undergo only limited breakage up to the maximum stress imposed here. The glass beads undergo significant breakage. Protocols for the compression can be varied. The reported results use a single loading and unloading at the same speed in each segment (increasing and decreasing stress), as well as a cyclic loading of differing numbers of cycles. The rate of loading is specified and can be varied from 0.56 μm/s–1.7 mm/s, with results here focusing on the range of 1–30 μm/s. Measured stress and permeability are shown as a function of axial load and loading rate, as well as the behavior in cyclic loading. The results are shown to be highly reproducible, establishing the ability to determine the properties of a proppant pack in a small charge device with imposed deformation. The influence of the resin coating on the properties is reported. Direct visualization is one of the key features of the DCD. Local motions and overall rearrangements of particles can be monitored by digital camera. These have been presented using particle image velocimetry (PIV).

New Proppant for Deep Hydraulic Fracturing

Summary Much work has focused on developing and evaluating various materials for use as proppants for hydraulic fracturing. Sand is used most often as a fracturing proppant in shallow wells. Deep wells with high closure stresses require a proppant, such as sintered bauxite, that will not crush under adverse conditions. Ceramic and zirconium oxide beads and resin-coated sand proppants also have been developed for deep hydraulic fracturing. A new fracturing proppant has been developed that exhibits the properties necessary for use in deep hydraulic fracturing. This proppant is produced by precuring a specially modified phenolformaldehyde resin onto sand. The new proppant maintains conductivity and resists crushing much better than does sand. The new proppant was compared to intermediate-density sintered bauxitic proppants and cured-in-place proppants and the tests were confirmed by an independent laboratory. Introduction Sintered bauxite commonly is used as a proppant in deep wells, where materials such as sand or glass beads would be crushed. Sintered bauxite resists crushing much better than sand does at high closure stresses, but it is much more expensive and harder to place because of its high bulk density. Several types of materials are used as proppants. Sand is one of the more common materials used because of its availability and cost. However, sand is limited to use in relatively shallow wells where closure stresses of 8,000 psi [55.2 MPa] or less are encountered. When sand does collapse as a result of closure stress, a great volume of fines is generated, which causes an irreversible decrease in conductivity of the fracture. This phenomenon also is observed with glass beads. A new proppant has been developed that resists crushing and maintains conductivity much better than regular fracturing sand. The new proppant is made from sand coated with a modified phenolformaldehyde resin. The resin is cured around each of the sand grains. When the sand grains are coated with the proper amount of resin and cured to the proper degree, the resin adds substantial strength to the sand grain because of the physical characteristics of the resin once it is cured. Results presented in this paper show that sintered bauxite and the new proppant generate very few fines when exposed to the same closure stress. Flow tests also show that the precoated sand grains, which are cured individually, show much less decrease in conductivity than does uncoated sand when confined at closure stresses greater than 6,000 psi [41.3 MPa]. Theory The use of sands coated with phenolformaldehyde resins (i.e., thermoset) has been under development for several years. The basic application has been placement of such a material into a wellbore in an uncured state. The natural temperature of the well or injected steam then causes the resin on the sand to "flow" and set, thus producing a high-strength, consolidated porous medium. This type of material has been used in conjunction with screens to produce a strong, consolidated porous medium in a fracture. The application of sand coated with uncured thermoset resin for both gravel packing and fracturing has met with varied success. This type of process has several limitations to successful application. Sand coated with an uncured thermoset resin must be placed at a relatively low temperature. Exposure to elevated temperatures (i.e., greater than 160 to 200 deg. F [greater than 71 to 93 deg. C]) for long periods during placement may cause the resin to cure partially before placement is complete. Exposure of the sand coated with an uncured thermoset resin to elevated temperatures during placement could be detrimental to the conductivity of the final fracture or gravel pack. This could result from the resin-coated sand becoming "tacky" during placement, thus allowing small particles within the placement fluid to adhere to the coated sand grain. Placement of this type of "dirty" particle within a fracture or gravel pack could decrease the final conductivity substantially. One major concern with using sand coated with an uncured thermoset resin as a proppant is how it is cured once it is in place in the fracture. Test results show that crushing similar to that observed for sand occurs for certain resincoated sands if the closure stress is not kept off the proppant until it is fully cured (Fig. 1). Tests with certain modified thermoset resins have shown that even though a strongly consolidated porous medium can be obtained by using sand coated with the uncured resin, as previously described, a unique property is imparted to sand if the coated sand grains are cured individually under proper conditions. Once the resin-coated sand grains are cured individually, the resulting material has a resistance to crushing similar to that of sintered bauxite. JPT P. 98^

Interaction of Proppants with Crack Formation and Propagation in Hydraulic Fracturing: ABSTRACT

In many wells, productivity can be increased by repeated hydraulic fracturing. Repeated flow cycling has also been shown to increase productivity, sometimes significantly above what can be obtained with a single cycle. The increased productivity from repeated flow cycling suggests that a predictive capability for treating specific wells could be developed if the controlling parameters and their interactions in the flow/cycle treatment process were better understood. Although the primary role of the proppant in hydraulic fracturing is to maintain fracture opening, the proppants may have other effects such as altering the pressure distribution along the fracture (e.g., by blocking the tip) and hence significantly affecting the fracture mechanics. Although proppant transport by fluids has been studied intensively, the coupled interaction problem of fracture propagation, fluid flow, and proppant transport has not been previously investigated. In SRI International's program to analyze the coupled interactions of proppants and fracture mechanics, proppant distributions are being determined for the coupled problem of fluid-proppant-fracture interaction, and the effects of the proppant distributions on fracture production are being evaluated for the flow/cycle treatment. Scaled experiments in several media (PMMA and rock simulant) will check the correlation of fluid penetration and fracture propagation rate with a calculational model for the fluid-fracture interactions. The scaled experiments will also constrain the relation between the proppant distributions and the fluid-fracture interactions. The computational model will be verified by comparing calculations of the proppant distributions in the scaled experiments, for which viscous or gravitational effects are dominant, with the scaled experiments. End_of_Article - Last_Page 1583------------