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The paper provides details of two experimental methods that were developed during the study and performance characterization of fiber-assisted stimulation services. The first method is focused on measuring the change in terminal settling velocities of proppant, which changes are caused by fiber material loading to fracturing fluid. The experimental approach allows us to calculate hindered-settling functions and to predict fiber-assisted proppant settling control for some type of fracturing fluids (linear gels). The second method is focused on analysis of thermal stability of degradable fibers. The thermal stability is estimated by direct fiber performance examination immediately after exposure of material to fluid media under specific conditions. A static proppant settling test was applied here as a criterion of the fiber additive's ability to perform. The key output of the fiber stability test is the critical time, which is the limit of material stability time under given conditions. Mathematical processing was based on degradation rate calculations for set temperatures and calculation of critical time at varied temperature as a sum of fixed time stability functions. Validation was done by repeating calculated varied step temperature conditions experimentally and comparing actual fiber stability versus calculated stability.
Based on the obtained experimental data, it was possible to develop an empirical model suitable for integration into hydraulic fracture simulator. As a result, there is reliable tool for estimation of commercial fiber performance in hydraulic fracturing jobs.
Proppant is sometimes produced along with hydrocarbons in hydraulically fractured petroleum wells. Sometimes 10% to 20% of the proppant is backproduced, which can lead to damaged equipment and downtime. Furthermore, proppant flowback can lead to a substantial loss of fracture conductivity. A numerical study was conducted to help understand what conditions are likely to lead to proppant flowback. In the simulations, the mechanical interaction of a large number (several thousand) individual proppant grains was modeled with a distinct-element-type code. The numerical simulations show that hydraulic fractures propped with cohesionless, unbonded proppant fail under closure stress at a critical ratio of mean grain diameter to fracture width. This is consistent with published laboratory studies. The simulations identify the mechanism (arch failure) that triggers the mechanical instability and also show that the primary way that drawdowns (less than ˜75 psi/ft) affect proppant flowback is to transport loose proppant grains in front of the stable arch to the wellbore. Drawdowns >75 psi/ft are sufficient to destabilize the arch and to cause progressive failure of the propped fractures.
Proppant backproduction can significantly affect well economics because of (1) loss of fracture conductivity [unpropped sections of fractures tend to close under stress (Figs.1 and 2)], (2) the damage it inflicts on equipment (i.e., abrasion of pumps, casing, and wellhead), and (3) downtime and expense required to clean up the wellbore.
Refs. 1 through 3 address technical issues involved in the flowback of proppant. Ref. 1 presents experimental data that show how various properties and conditions (e.g., closure stress, fracture width, proppant embedment in the fracture walls, grain-size distribution, relative inclination of the fracture walls, and drawdown) affect the mechanical stability of cohesionless, unbonded proppant packs. It also raises fundamental questions regarding the process of proppant production (e.g., why does the transition from stability to instability occur over a narrow range of fracture widths and why do proppant packs in the laboratory fail at much smaller fracture widths than are known to exist in the field).
A numerical study presented in this paper addresses some of these fundamental questions. The objectives of the study are to help understand the mechanical process of proppant backproduction and to show how various field conditions and proppant properties affect the propensity for proppant flowback. This is accomplished by means of numerical simulations of proppant on a grain-size scale.
Abstract Hydraulic fracturing is often performed using resin-coated proppants to minimize proppant flowback during hydrocarbon production, whether the resin is precoated or coated on-the-fly as the treatment is pumped. Resin-fracturing fluid interaction can have a negative effect on fluid stability or resin consolidation, or both. This paper examines the effects of resin-fluid interactions on fluid stability, proppant consolidation strength, and strategies to mitigate the effects. Components of resins can change the fracturing fluid stability by interacting with crosslinker or breaker, or by changing the fluid pH. To offset the effect of a resin, the breaker/crosslinker/buffer concentration should be tuned while pumping resin-coated proppant. Similarly, resin-fluid interaction can decrease consolidation strength by disturbing resin-curing kinetics or reducing grain-to-grain contact, which can increase the possibility of proppant flowback during production. The influence of resins on fracturing fluid stability was evaluated by conducting rheology testing. The effect of fracturing fluids on the consolidation strength of resin was evaluated by comparing unconfined compressive strength (UCS) of proppant packs. The stability of zirconate and borate crosslinked guar fluids, when treated with coated on the fly liquid resin-coated proppant (LRCP), was lower than non-treated fluids at 260°F as a result of breaker activation by the resin components. The desired fluid stability was attained by lowering breaker concentration in liquid resin-treated fluid. During another round of testing, a second type of LRCP, based on different chemical functionality, increased the stability of synthetic polymer fluid at 400°F. Likewise, a rise in fluid stability was observed when guar fluid was treated with resin pre-coated proppant (RCP) at 200 and 250°F. The improved fluid stability is associated with reduction in active breaker concentration in the presence of furan resin and RCP. The UCS value of the proppant pack prepared from fracturing fluid-treated RCP was ~16 to 45% lower than the proppant pack without this fluid treatment. Additionally, the UCS value of proppant pack prepared using fracturing fluid-treated LRCP decreased by ~30%. However, the measured UCS value of LRCP pack with fracturing fluid exposure was higher than the RCP pack measured value even without exposure to this fluid. Incorporating LRCP instead of using RCP during fracturing operations could address the proppant flowback issue and possibly result in higher conductivity of propped fractures. It could help ensure economic production rates and prevent costs associated wellbore cleanup, downhole tool damage, erosion and damage to the tubular, chokes, valves and separators, and refracturing of the well. Ultimately, it could help maintain a lower cost per barrel of oil equivalent (BOE).
Factors controlling the stability of proppant in propped fractures were identified and investigated experimentally. The results indicate that the absolute size, distribution and type of proppant may affect stability, and hence the likelihood of proppant flowback.
The extent of embedment of the proppant into the rock determined by closure stress and rock hardness) was found to play a key role in stabilising the pack. in addition, channels, which may form due to proppant settling before fracture closure, were found to significantly reduce stability. These are, therefore, favourable sites for proppant flowback.
A numerical study has been used to determine lengths of unpropped fractures which may be maintained open due to the rock stiffness. The implications of these results for well productivity and the understood mechanisms of proppant flowback, in the field, are discussed.
The flowback of proppant from hydraulically fractured wells is of significant concern to the industry. In addition to problems of valve, line and choke erosion, there is the potential for a loss in near wellbore fracture conductivity. Furthermore, expensive separation facilities are required to filter out proppant from the hydrocarbons, and these may necessitate manning the rig. While substantial work has been carried out to identify means of reducing the amount of proppant flowed back, far less effort appears to have been directed towards understanding the mechanisms which determine whether or not a given well will back produce proppant.
Hence it is difficult to decide a priori what preventative measures should be taken. The uncertainty has been concisely put by Daneshy (1) who said of techniques to reduce flowback "None of these methods works all the time, and no criteria are available to know when to do what".
Various methods have been devised to reduce proppant flowback. These include resin-coating the proppant, installing mechanical screens and modifying the completion design. Resin-coated proppants have been used successfully in certain environments.
Abstract Loss of proppant from the near wellbore region of a fracture results in fracture pinch out and a noticeable decrease in well productivity. Downhole and surface equipment can be damaged when proppant flowback occurs as well. Resin coated proppant (RCP), fibers, deformable particles, resin on the fly, etc have been used to improve proppant pack stability. Selection of the appropriate proppant flowback control technology is made with consideration of engineering factors such as fluid compatibility issues, setting time, resistance to cycling stress-loading issues, and conductivity damage. The goal of the current work was to combine the beneficial features of mechanical proppant flowback control with chemical adhesive flowback control products. With mechanical features, the proppant pack stability is enhanced by blending fibers with proppant, thus increasing particle-particle interaction, and increasing the stability of proppant arches. This mechanism can enable aggressive flowback while providing an instantaneous, albeit a modest level of proppant flowback control. With the addition of an adhesive bonding mechanism to a mechanical flowback control material, the bicomponent material substantially increases proppant pack stabilization. Using a high temperature, high pressure proppant flowback control apparatus, we show the impact of particle bonding on the dosing required to achieve a specific level of proppant pack stability. We also show the impact of the flexible nature of bonded matrix on the proppant pack stability and tolerance to cyclic loading. A mechanistic proppant pack stability model was developed based on our experimental study. We discuss this model and its application towards the selection of the appropriate proppant flowback control technology for specific well conditions. We conclude the paper by discussing field cases of effective proppant flowback prevention techniques deployed as a result of model recommendation.