Proppant Performance Research Articles

Influences of Proppant Concentration and Fracturing Fluids on Proppant-Embedment Behavior for Inhomogeneous Rock Medium: An Experimental and Numerical Study

Summary Proppant plays a vital role in hydraulic fracturing in tight oil/gas production because it helps to keep the fractures open during the production process. However, it is common for proppant embedment, the main type of proppant degradation, to occur under high compression load, which greatly reduces the fracture conductivity, and consequently reduces the production rate. During the process of hydraulic fracturing, the fracturing fluid only has the chance to contact and infiltrate the fractures that are in the top surface of the rock medium because of ultralow rock permeability and the short time of fluid existence, whereas the condition of other parts of the rock remain unchanged, creating inhomogeneity within the rock medium. Therefore, the present study conducted a comprehensive experimental and numerical evaluation to investigate the behavior of proppant for inhomogeneous rock media, considering the factors (effective stress, proppant concentration, and fracturing fluid) that affect proppant performance. According to the experimental results, increasing the proppant concentration reduces the proppant embedment, and, interestingly, the optimal proppant concentration is approximately 150% coverage. Furthermore, the influence of fracturing fluid on proppant embedment is more significant for high proppant concentrations, and the embedment under water-saturated conditions is higher than that under oil-saturated conditions. The numerical simulation achieved the same result as the experimental study, showing that 150% proppant coverage is the optimal proppant concentration to achieve the minimum proppant embedment. In addition, numerical modeling indicated that the inhomogeneity of the rock formation can also considerably enhance proppant embedment through differential settlement during compression.

Thermally-induced mechanical behaviour of a single proppant under compression: Insights into the long-term integrity of hydraulic fracturing in geothermal reservoirs

With the increasing demand persisting for energy extraction from geothermal resources, many scientific research studies have been carried out on the performance of proppants to improve the energy extraction process. Knowledge of the after-effects of proppants exposed to realistic geothermal reservoir conditions is crucial. Therefore, the aim of the experimental study reported here was to investigate the mechanical behaviour of a single proppant under incremental loading conditions, and the mineralogical and microstructural alterations due to exposure to elevated temperatures (100 °C, 200 °C, 300 °C and 400 °C) and different cooling conditions (slow cooling and quenching). Significant mechanical weakening, overall strength reduction of 52.19% and 69.74% and Young’s modulus reduction of 43.64% and 55.45% for the slow cooling and quenching cooling techniques were observed in a single proppant when the temperature increased from room temperature (25 °C) to elevated temperatures. Scanning electron microscopy revealed the occurrence of thermal cracks inside the proppant microstructure, together with the alteration of the mineral structure, and significant changes in zeolite, Na-feldspar and K-feldspar were observed upon exposure to elevated temperatures. The post-failure behaviour of a single proppant was studied conducting 3-D X-ray computed tomography (CT) scanning. Under normal loading conditions, proppants cleave and generate large fragments like a flower, and this happens suddenly and quite violently through the material. Interestingly, post-failure analysis revealed that the failure mechanism of a single proppant consists of three major stress levels, where initially proppant fails at a high stress level and gains some crushing-associated strength at later stages.

Downward flow of proppant slurry through curving pipes during horizontal well fracturing

The transport of proppant-fracturing fluid mixture in a fracturing pipe can significantly affect the final proppant placement in a hydraulic fracture in horizontal well fracturing. To improve the understanding of the hydrodynamic performance of proppants in a curving fracturing pipe, a modified two-layer transport model was proposed by taking the viscoelastic properties of carrier fluid into consideration. Fluid temperature was determined by an energy equation in order to accurately characterize its rheological properties, and the Chang–Darby model was used to represent the viscosity-shear rate relationship. The flow pattern of particle-fluid mixture in a curving fracturing pipe was investigated, the effects of particle and fluid properties and injection parameters were analyzed, and a flow pattern map was established. Three transport stages are observed: (1) particles keep suspended in the carrier fluid at small inclined angle; (2) a small number of particles settle and accumulate on pipe bottom to form a particle bed load flow at intermediate inclined angle; (3) numerous particles settle out of carrier fluid and the particle bed quickly develops in an approximate horizontal pipe. The transition processes between different stages were observed, and the transition velocity from particle bed load flow to full suspension flow increases with the increase in inclined angle. However, an inverse transition phenomenon occurs at intermediate inclined angle, where the full suspension flow inversely turns into particle bed load flow with the increase in injected velocity.

Effect of MnO2 on the Properties of Mullite-based Ceramics

Low-density mullite-based ceramic proppants were prepared from recycled coal fly ash, bauxite, and additives (feldspar, talcum, BaCO3, and MnO2). The effects of MnO2 content and sintering temperature on properties and morphologies of resultant proppants were investigated. Results showed that the performance of proppants have approved significantly at sinter temperature around 1380 °C when the content of MnO2 equals to 4 wt %, such as low apparent density(2.70 g cm−3) and low breakage ratio (3.10% under 52 MPa pressure). The low-density mullite-based ceramic proppant therefore is assumed to be a promising proppant material for the hydraulic fracturing production of petroleum.

Mechanical-Damage Characterization in Proppant Packs by Use of Acoustic Measurements

SummaryThe strength and conductivity of proppant packs are key parameters for assessing proppant-pack performance. Mechanical damage in the propping agents, which leads to compaction and crushing, significantly reduces the conductivity of the proppant pack. Mechanical damage of proppants is usually analyzed by use of crush tests. However, measurements from these tests remain questionable because of discrepancies in procedures and test results. Therefore, a need emerges to develop techniques for characterizing the properties and mechanical damage in proppant packs.In this paper, we introduced a new technique that is based on interpretation of acoustic measurements from a granular effective-media model to quantify mechanical damage in propping agents. We performed uniaxial compression tests in the laboratory and measured the compressional- and shear-wave velocities in proppant packs loaded at axial stresses ranging from 10 to 110 MPa. After unloading the tests in which maximum axial stresses of 28, 55, 69, 97, and 110 MPa were applied, we conducted sieve analysis on the proppant packs. We applied an effective-medium theory modeled after the Hertz-Mindlin granular contact model to approximate the effective elastic properties. We then calibrated the model by use of the elastic properties estimated from the experimental measurements to characterize the mechanical damage of the proppant packs.We observed that the increase in grain-to-grain contact as the axial stress increases results in compaction and crushing in the proppant pack. We showed that the compaction effects and elastic and plastic behaviors in the stress–strain profile of the proppant pack were in agreement with the analysis of fines generated at different stress levels. The combined effect of compaction and crushing resulted in a reduction of porosity and, consequently, decreased the compressional- and shear-wave velocities of the proppant pack. The Hertz-Mindlin model showed a good approximation of the effective elastic properties estimated from the acoustic-wave velocities when calibrated with the pressure-dependent grain contact and the fraction of nonslipping grains as parameters. We demonstrated that the calibrated parameters can be correlated with the mechanical damage in the proppant pack. The characterization of mechanical damage in proppant packs can improve the design of the propping agents and quantification of proppant performance. Furthermore, the laboratory procedure can be extended to the use of borehole acoustic measurements in providing a real-time in-situ assessment of proppant performance.

Proppant's performance with reservoir rock under variable closure pressure: Results of experiments with a newly developed experimental set-up

Before Hydraulic-fracturing operations, a pre-analysis of proppant's interaction with cored reservoir-rock can significantly contribute in selection of proppant type. In this study, a new equipment and corresponding test method is discovered for onsite testing of a monolayer proppant pack in fracture. Experimental study of reduction in fracture width, permeability and fracture conductivity of a fracture containing monolayer proppant pack with increase in closure-pressure has been carried out. The developed apparatus maintains and measures the fluid flow rate through the apparatus, while a required closure-pressure up-to 65.5MPa is developed on the proppant pack. Variations in parameters with respect to increasing closure-pressure are determined and analyzed.

Improved Conductivity and Proppant Applications in the Bakken Formation

This article, written by Special Publications Editor Adam Wilson, contains highlights of paper SPE 163849, ’Investigation of Improved Conductivity and Proppant Applications in the Bakken Formation,’ by Bethany Kurz, SPE, Energy and Environmental Research Center, Darren Schmidt, SPE, and Phil Cortese, SPE, Weatherford, prepared for the 2013 SPE Hydraulic Fracturing Technology Conference, The Woodlands, Texas, USA, 4-6 February. The paper has not been peer reviewed. Production from the Bakken and Three Forks formations within the Williston basin is continuing to climb as a result of applied horizontal drilling and hydraulic fracturing. Key to increased oil production is the evolution of reservoir-stimulation techniques, such as fracturing-fluid systems and proppant types. This study evaluated the key factors that may result in conductivity loss within the Bakken and Three Forks reservoirs. The results of this work suggest that certain fluids may affect both rock and proppant strength and, therefore, require consideration. Introduction This work was conducted in an effort to answer questions regarding reservoir stimulation in the Bakken petroleum system, specifically with respect to conductivity loss in hydraulic fractures over time. The key questions that were evaluated include Can conductivity loss be attributed to proppant degradation, rock strength, or both? What role do fluids have in affecting propped-fracture conductivity? To what extent does fluid exposure affect the degradation of rock strength and proppant performance? How do various proppants perform relative to one another under stress using actual core samples from the Bakken petroleum system? Laboratory tests were conducted to evaluate the sensitivity of Bakken and Three Forks formation cores to various fluids; to evaluate the strength of Ottawa sand, an unspecified premium precured resin-coated sand (RCS), and a lightweight ceramic proppant (Econoprop) with respect to various fluids; and to measure the relative laboratory conductivity performance of propped fractures using actual rock core. Fluids used in experiments to examine potential strength degradation for both rock and proppant included the following: Slickwater—a mixture of polyacrylamide and fresh water Crosslinked gel—guar-polymer thickening agent, borate crosslinker, and fresh water Gelled diesel—diesel fuel and phosphate ester Bakken-formation crude—crude oil Bakken-formation brine—highly concentrated saltwater The fluids were selected on the basis of the most common fluids expected to be encountered in the Bakken formation. Embedment Brinell-hardness-index testing is a measure of rock strength that is determined by embedding a metal ball into formation rock at a given applied load. This type of index testing was used to gauge the relative strength of Bakken and Three Forks samples before and after exposure to formation and hydraulic-fracturing fluids. Core samples used to study rock strength originated from the North Dakota Industrial Commission (NDIC) #16771 well, located in the Manitou field of Mountrail County. Rock strength was measured with a Brinell-hardness-test apparatus. Publicly available hardness data were available from two wells in proximity, including NDIC #15986, Robinson Lake field, Mountrail County, approximately 10 miles to the southeast NDIC #16083, Capa field, Williams County, approximately 10 miles to the southwest

Long-Term Hydrothermal Proppant Performance

Summary The hydrothermal degradation of alumina-based proppants, which can lead to significant loss of fracture conductivity, has been the topic of recent papers. Most of these studies were necessarily compromised either by being performed on too short of a time scale to produce geochemical effects or by using unrealistic temperature conditions to accelerate the geochemical reactions. Those studies, however, provided insight that geochemical reactions likely occur during production from hydraulically generated fractures and gave evidence of possible long-term proppant instability. This paper presents proppant-pack-permeability and proppant-crush-strength data collected from a selection of hydrothermal tests performed in sealed test cells packed with proppant and formation material, with no flow and no mechanical closure stress. Long-term proppant performance was determined by evaluating the proppant after 0, 15, 45, 90, and 180 days of hydrothermal exposure at 300 and 450°F, resulting in verification that proppants degrade continuously with time at all temperatures. A typical longterm proppant-testing result was 80% loss in proppant-pack permeability and 40% loss in proppant crush strength, with just 180-days exposure at 300°F at typical reservoir pH. These tests were performed both with and without formation material present to demonstrate the impact of reservoir/proppant compatibility. Scanning-electron-microscope (SEM) and energy-dispersive X-ray (EDX) spectroscopy analyses verified that dynamic molecular rearrangement occurs over the entire temperature range, though at an accelerated rate as temperature increased. Extensive effort was made to quantify both the accuracy and repeatability of testing procedures used in this study and will be presented. This involves measurement of pack permeability using water before and after hydrothermal exposure, and single-proppant-grain crush-strength determination using Weibull statistical analyses. This paper presents substantial data to support the importance of proppant compatibility when selecting the best proppant for long-term fracture conductivity. It also suggests that methods in addition to American Petroleum Institute (API) crush and conductivity procedures need to be developed and implemented to rank proppant performance properly.

Proppant Diagenesis—Integrated Analyses Provide New Insights Into Origin, Occurrence, and Implications for Proppant Performance

SummaryIn the application of hydraulic fracturing of oil and gas reservoirs, the objective is to create a conductive pathway for hydrocarbons to flow into the wellbore. This is accomplished by placing a proppant into the created fracture that will prevent fracture closure and maintain a high conductivity for an extended time period. A number of mechanisms have been identified that can degrade the fracture conductivity, including mechanical failure of the proppant grains, liberation of formation fines, proppant embedment, formation spalling, damage from the fracturing fluid, stress cycling, asphaltene deposition, proppant dissolution, and others. These factors in combination can reduce the effective conductivity by orders of magnitude as compared with the typically published conductivity data measured under reference conditions.Another mechanism for degradation of proppant over time has recently been postulated. This mechanism has been labeled "diagenesis" and refers to a dissolution and reprecipitation process that may reduce the porosity, permeability, and strength of the proppant pack as precipitants are deposited. Factors that were believed to control the occurrence and degree of diagenesis include closure stress, reservoir temperature, proppant type, and mineralogy of the rock formation. While much work has gone into evaluating this phenomenon by the industry, there remains uncertainty as to the prevalence of this mechanism and the prediction of its occurrence.This paper will summarize results from high-temperature static tests, extended-duration flow tests at reservoir conditions, detailed analyses of precipitates, the effects of this environment on the mechanical properties of proppants, chemical and mineralogical analysis of various reservoir shales, and evaluation of actual proppant samples that have been retrieved from producing wells.The paper will conclusively demonstrate that crystalline precipitates can be formed on the surface of all proppant types, including ceramic, sand, resin-coated materials, and even inert steel balls or glass rods. The nature of these precipitants indicate that they may be classified as zeolites. The chemistry and environment leading to the formation of zeolite precipitates will be reviewed. Testing indicates that zeolites can be formed without the presence of alumina-bearing proppant and appear to be largely dependent on the characteristics of the formation material and fluid. Conductivity tests carefully simulating reservoir conditions with actual reservoir shale core samples indicate that if diagenetic precipitation does occur it does not appreciably affect conductivity performance in the flowing conditions evaluated. Furthermore, other conditions known to occur in oil and gas reservoirs would naturally prevent the formation of zeolites.The results of this work will aid the stimulation engineer in proppant selection and treatment design. While there are many mechanisms contributing to the degradation of proppant performance, these studies indicate that zeolite precipitation is unlikely to be a dominant concern in most wells.

Production Acceleration or Additional Recovery? A Look Back at Three Published Field Trials - Long-Term Benefits of Improved Fracture Treatments

This article, written by Senior Technology Editor Dennis Denney, contains highlights of paper SPE 144349, ’Production Acceleration or Additional Recovery? A Look Back at Three Published Field Trials To Determine the Long-Term Benefits of Improved Fracture Treatments,’ by K. Blackwood, SPE, Highmount Energy; P. Handren, SPE, Denbury Resources; and M. Chapman, SPE, T. Palisch, SPE, and J. Godwin, SPE, Carbo Ceramics, prepared for the 2011 SPE North American Unconventional Gas Conference & Exhibition, The Woodlands, Texas, 14-16 June. The paper has not been peer reviewed. Improved fracture-treatment designs can increase production rates from many reservoirs. However, do these changes merely accelerate recovery, or is incremental production gained by changing the fracture design? The authors re-examined three tight gas case studies published in the past 10 years. The original studies were performed specifically to assess the effects of increased fracture conductivity on production. In all cases, the initial analyses documented that increasing conductivity appeared to increase production and recovery. Introduction Since the first commercial hydraulic-fracture treatments in 1949, treatment designs have evolved from experimentation with massive hydraulic fractures to proppantless water-fracture treatments. While the production increase in the first few months or even the first year typically repays the cost of the conductivity upgrade, and is robust enough to make the decision to upgrade relatively simple, it is important to know whether this increase can be translated into additional ultimate recovery. If the ultimate recovery is also increased, then the economics of the completion upgrade may be even more favorable because it leads to a potential increase in reserves and drainage area per well, which in turn can lead to a reduction in the number of wells needed to drain the reservoir effectively. In all cases, comparisons were made between high-quality ceramic proppants and either uncoated or resin-coated sand (RCS). In the past, conventional wisdom incorrectly suggested that hydraulic fractures in most tight gas wells behaved as “infinitely conductive.” This misperception arose from a poor understanding of proppant performance under downhole (realistic) conditions. Under realistic conditions, all proppants should be expected to lose more than 90% of their effective conductivity. By reanalyzing these case studies with longer term production results, the authors sought to determine whether the original conclusions still hold true, and whether confidence in the recovery projections can be determined.

Effects of Proppant Selection on Shale Fracture Treatments

This article, written by John Terracina, manager of fracturing technology at Momentive, contains highlights of paper SPE-135502-MS, Proppant Selection and Its Effect on the Results of Fracturing Treatments Performed in Shale Formations, by J.M. Terracina, SPE, J.M. Turner, SPE, D.H. Collins, SPE, and S.E. Spillars, SPE, of Hexion, prepared for the 2010 SPE Annual Technical Conference and Exhibition in Florence, Italy, 19-22 September. In October 2010, Hexion merged with Momentive. The oil and gas industry has strived to provide methods to test the proppants used in shale formation fracturing, but they still do not adequately address many of the factors that impact their effectiveness. There are many factors that occur downhole that need to be considered, such as: Proppant fines generation and migration in the fracture Proppant resistance to cyclic stress changes Effective conductivity vs. reference or baseline conductivity Proppant flowback and pack rearrangement in the fracture Proppant embedment in the fracture face Downhole proppant scaling The hypothesis of this study was that because of the formation characteristics in the three areas studied, curable resin-coated sand (CRCS) with its grain-to-grain bonding technology should provide higher downhole fracture conductivity leading to increased postfracture treatment well production. To verify the hypothesis, laboratory tests outside the traditional long-term baseline conductivity were conducted with proppants and formation core samples from each area. The objective was to more accurately simulate proppant performance under specific downhole conditions of temperature, pressure, fluid, and rock properties pertaining to each area. Proppant Selection Factors Studied Proppant fines generation and migration, as well as proppant flowback, were studied in the Fayetteville Shale of Arkansas. Proppant fines and embedment were studied in the Bakken Shale of North Dakota. And finally, proppant pack cyclic stress, embedment, and scaling were examined in the Haynesville Shale of Louisiana. Proppant Fines Generation and Migration Proppant fines are the small particles that break off from the proppant grain when subjected to fracture closure stress. The small broken pieces reduce pack porosity and permeability, and cause major degradation in the conductivity of proppant packs. When proppant fines migrate down the proppant pack toward the wellbore, they accumulate and reduce flow capacity. Proppant Pack Cyclic Stress When comparing proppants, one factor often overlooked is a proppant’s performance under closure stress changes. The forces of cyclic stress exerted on proppants downhole can cause them to fail. Events often occur multiple times throughout the life of a well, such as shut-ins because of workovers or connections made to a pipeline; in some cases, a well could be shut in because of pipeline capacity. These events lead to cyclic changes in fracture closure stress. This varying amount of pressure and stress can cause the proppants to shift or rearrange, resulting in a decrease in fracture width as well as additional proppant fines and proppant flowback.

Qualifying Proppant Performance

Summary Qualifying proppant performance of the delivered proppant before a frac job, or verifying proppant performance after a frac job, can add significant value to propped-fracture stimulations. Current field-testing protocol typically ensures that a proppant meets certain criteria with a simple yes or no answer. Seldom are strict collection and testing guidelines followed, as outlined by the American Petroleum Institute (API) and the International Organi-zation for Standardization (ISO). Qualifying proppant performance requires taking quality control a step further by considering the role that proppant characteristics play in the performance of proppant in the fracture. Applying a blend of established practices and new technology, quality-control data can be generated at the wellsite for comparison with the public domain (literature, websites, or fracturing simulators). These scientific data give an engineer insight into how delivered proppants are designed to perform and whether they do so. This eliminates the need to run expensive and time-consuming conductivity and permeability tests on every job. However, because proppant flow capacity or conductivity is a key measure of performance, some empirical results have been assimilated for wellsite and public data. Any differences in delivered proppant performance can be attributed to mining anomalies, manufacturing defects, transportation abuse, and/or contamination. This paper also introduces new patent-pending technology that enables wellsite-proppant sampling and evaluation before the fracturing treatment. Having prefrac data gives one the opportunity to make any necessary changes in fracture design and implementation to get the most from available proppant. It also provides for a detailed inspection of the wellsite-delivered prop-pant supply. For instance, one can isolate and sample each pneumatic trailer and monitor associated pneumatic discharge pressures. Case histories, onshore and offshore, support qualifying proppant performance.