Proppant-pack Damage Research Articles

Damage Potential, Proppant-Transport Traits of High-Viscosity Friction Reducers

This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper SPE 199736, “High-Viscosity Friction Reducers: Potential for Fracture Damage and Impact of Brines on Proppant-Transport Capability,” by Barry Hlidek, SPE, and Robert Duenckel, SPE, Stim-Lab. The paper has not been peer reviewed. High-viscosity friction reducer (HVFR) fracturing fluids are used widely for unconventional reservoir development. HVFRs are easy to apply and can reduce chemical costs. The objective of the study described in the complete paper is to measure polymer cleanup in both propped and unpropped fractures using multiple methods. Additionally, the study compares rheological measurements in proppant-transport observations in brines using a large-scale slot-flow device. Potential for Formation Damage High-polymer-concentration crosslinked fracturing-fluid systems often have been supplanted by linear (i.e., noncrosslinked) guar systems of lower-polymer loading (e.g. 20 lbm/1,000 gal or less) or slickwater systems often thought to be nondamaging. Recently, HVFRs have been used regularly not only to provide friction reduction during pumping of the fracture stimulation but also to offer some degree of proppant transport. These usually are polyacrylamides with concentrations of 2–3 gal/1,000 gal, but concentrations may be as high as 6 gal/1,000 gal. The potential for damage from these HVFRs is the subject of the authors’ study. The complete paper discusses evaluation of proppant-pack damage in conventional reservoirs as well, but this synopsis will focus on evaluation of proppant-pack damage in unconventional reservoirs.

Rheological and breaking characteristics of Zr-crosslinked gum karaya gels for high-temperature hydraulic fracturing application

The metal-crosslinked polymer gels play a vital role in stimulating oil and gas wells since many years, but the presence of large insoluble residues and incomplete breaking of the conventional guar-based fracturing fluids reduces the permeability of the proppant pack necessary for oil and gas flow. In the present study, a novel crosslinked gel has been synthesized using the low molecular weight acidic polymer gum karaya and zirconium crosslinker as an alternative to guar-based fracturing fluids to increase the proppant pack conductivity with competitive rheological performances. The polymer and crosslinker concentrations were optimized and the synthesized gel was studied for structure, morphology, time-independent and time-dependent rheological tests under simulated conditions of pressure and high temperature. The smaller amount of residues left by the fluid after breaking with oxidative breakers and higher regained permeability of the sand pack system compared to Guar confirm its ability to reduce the proppant pack damage.

A new novel crosslinker with space structure for low-polymer-loading fracturing fluid

Boron-based crosslinkers are used commonly to increase viscosity and to improve fluid-loss control and proppant transportability of guar and its derivative fluids. Boron crosslinkers are usually preferred because of their ability to reheal after shearing and their favorable environmental properties. In order to reduce both the formation and the proppant-pack damage from polymer residues and to reduce over-all fluid cost, more-efficient crosslinkers capable of crosslinking fluids with reduced polymer loading is of great interest. Previous studies demonstrated that polymer solutions have critical overlap concentration (C*), below which no intermolecular crosslinking leading to increased viscosity can occur. However, recent studies demonstrated that increased crosslinker size or length can lead to the crosslinking of polymer solutions well lower than the C* and can reduce polymer loading without compromising the rheology of the fracturing fluid.

New Propped-Fracture-Conductivity Models for Tight Gas Sands

Summary In this work, we developed two models of fracture conductivity in the presence of proppant-pack damage. The models are derived from the analysis of experiments relating dynamic fracture conductivity to flowback rate, reservoir temperature, polymer loading, presence of breaker, closure stress, and proppant concentration. The first model is purely empirical, and it is modeled after fracture-conductivity studies conducted in previous work. The experimental design and planning table were derived from fractional-factorial designs. This meant that we could uniquely quantify the effect of all the dependent variables and also ensure that the factors are not correlated. We tested the empirical model for adequacy—that is, ensuring the model residuals are normally distributed with no obvious trend and approximately constant variance. Because empirical models are generally constrained by the data on which they are built, we developed a semiempirical model dependent on dimensional analysis and nonlinear regression. Dimensionless products from the dimensional analysis procedure form the backbone of this semiempirical model. We used subset-regression analysis to determine the optimal regression model and found that model was adequate. We also found two dimensionless groups and used them to develop the semiempirical model. We thereafter corrected the semiempirical model for the effect of temperature. Both the purely empirical model and the semiempirical model match experimental data with reasonable accuracy. These models can be used as a first approximation for the short-term fracture conductivity of field hydraulic fractures. We also present methodology by which engineers can develop empirical models, assess uncertainty, and test for regression-model adequacy given noisy data.

Pushing the Limit on Guar Loading: Treatment of a Green River Sandstone Formation by Use of a Novel Low-Polymer Crosslinked Fluid

Summary Guar-based fracturing fluids are the most commonly used fluids in reservoir stimulation. To provide high viscosity, borate-crosslinked gels are preferred for their ability to heal after mechanical shearing and their favorable environmental properties. More-efficient crosslinkers capable of crosslinking fluids with reduced polymer loading have always been of great interest to reduce formation and proppant-pack damage from polymer residues and to reduce overall fluid cost. Low permeability and the interwell connectivity of the Green River sandstone formation of the Uintah basin require hydraulic-fracturing treatment to produce oil economically. The most typical fracturing treatments use borate-crosslinked guar fluids to transport sand into wells with vertical depths of 3,500 to 6,800 ft and bottomhole temperatures of 115 to 160°F. Most operators in this region place emphasis on reduced polymer loadings for stimulation treatments, and, even with breakers present, broken polymer residues can remain in the formation at these reduced polymer loadings, resulting in damage and decreased production. Unlike conventional treatments that require a shut-in operation, stimulation treatments in this region are designed for immediate flowback to minimize filter-cake buildup and subsequent formation damage, which improves production significantly. Immediate flowback poses a challenge of a proper balance between polymer and breaker loading so that the fluid effectively transports up to 6 lbm/gal added of sand into the formation without screenout and breaks within the treatment time (20 to 30 minutes per stage) to minimize flowback of proppant. A recently developed novel poly-aminoboronate (PAB) crosslinker (Sun and Qu 2011; Legemah et al. 2013) was tested in the aforementioned formation. Multiple boron sites are available in the crosslinker, and it is capable of interacting with multiple polysaccharide strands to form more-complex crosslinking networks at lower polymer loadings than conventional guar fluids. The crosslinker, with up to an additional 15% reduced guar loading, is capable of matching or outperforming conventional crosslinked fluids in fresh water and, more impressively, in 7% potassium chloride. This paper will discuss the novel crosslinker developed, the laboratory testing, and the successful field application. Analysis and discussion of the chemistry, crosslinking performance, and economics will be reported.

Novel High-Efficiency Boron Crosslinkers for Low-Polymer-Loading Fracturing Fluids

Summary Boron-based crosslinkers are used commonly to increase viscosity and to improve fluid-loss control and proppant transportability of guar and its derivative fluids. Boron crosslinkers are usually preferred because of their ability to reheal after shearing and their favorable environmental properties. More-efficient crosslinkers capable of crosslinking fluids with reduced polymer loading have always been of great interest, to reduce both the formation and the proppant-pack damage from polymer residues and to reduce overall fluid cost, especially with the reduced availability and higher cost of guar. Recently, we reported in paper SPE 140817 (Sun and Qu 2011( the synthesis of polyaminoboronates, bulky compounds containing multiple boron sites and capable of interacting with multiple polysaccharide strands to form more complex crosslinking networks at lower polymer loadings than conventional guar fluids. However, to improve the marketability and acceptance of the product, a lower-cost combination of raw materials is sought. In addition, a delaying mechanism to improve the control of the fluid-viscosity buildup can make the product fit into broader applications. A readily available polyamine was used as the base scaffold, and boron was incorporated by means of intermediate borate formed in the condensation reaction between boric acid and ethylene glycol to replace volatile and highly flammable trimethylborate. New chemistry was developed to produce more-controlled crosslinking. The resulting compound exhibited desired adjustable delay characteristics. This paper shows the effect of a series of new crosslinkers in terms of delay properties. The testing results of fluids with a 20%-lower polymer loading, crosslinked with the new crosslinker, are reported and compared with those of conventional fluids. Analysis and discussion of the chemistry, crosslinker performance, and economics are presented.

Case Study of Unconventional Gas-Well Fracturing in Hungary

Summary Well Ba-E-1 was drilled in the Tompa prospect (now the Ba-IX Mining Plot) in Hungary targeting the Miocene and Cretaceous formations between 2600 and 3500 mTVD. These are tight sandstones, and the expected permeabilities were in the range of 0.001 to 0.5 md. Two hydraulic-fracture treatments were performed. The first fracture treatment was in the lower part, and the second treatment was in the upper part of the deepest interval. With no previous propped-fracturing experience in this field, the first treatment was designed as a conventional crosslinked-gel treatment to minimize the risk of a premature screenout. Following the analysis of the data from the first zone, it became clear that the average permeability was closer to the minimum expectation of 0.001 md. Because of lower-than-expected stimulation effectiveness of the first fracture, and the confirmation of the low permeability, the second fracture treatment was changed to a water-fracture design. This formation clearly falls into the unconventional category, and consequently was a good candidate for a water fracture. This paper describes the prefracture diagnostics, fracture execution, and post-fracture production evaluation of this unconventional gas well. Special emphasis is placed on the use of small-volume injection tests, or diagnostic fracture injection tests (DFITs), to obtain an estimate of the in-situ kh because it is impossible to perform prefracture welltests in such a formation. The result of the DFIT analysis is then used to constrain the postfracture well-test analysis in a numerical-simulation model that includes fracture-filtrate-cleanup modeling. Post-fracture analysis showed that the initial proppant-pack damage is high and effective fracture length is much smaller than the created length, especially with crosslinked gel. The crosslinked-gel treatment was not able to clean up effectively, and therefore showed limited stimulation effectiveness. The first-ever water fracture in a gas reservoir performed in Europe showed a more significant production improvement during the short post-fracture test. The results from this well suggest that, as in North America, water fractures appear to have better initial production than crosslinked-gel fractures because of better fracture cleanup in European reservoirs with microdarcy permeability.

Development and Field Application of a Low-pH, Efficient Fracturing Fluid for Tight Gas Fields in the Greater Green River Basin, Wyoming

Summary Hydraulic fracturing has proved to be one of the best technologies to improve productivity from tight gas wells. In such low-permeability reservoirs, careful consideration must be given to fracturing fluid selection. Some reservoirs are underpressured and require the use of energized fluids, while others are sensitive to water-based fluids because of clay swelling and migration. Proppant pack damage because of gel residue is one of the primary causes of low production rates after hydraulic fracturing treatments. To minimize the damage and maximize production, a new premium, highly efficient fracturing fluid was developed. This premium system incorporates low-polymer-loading carboxymethyl guar polymer and a zirconium-based crosslinker. An adjustable crosslink delay makes the fluid ideal for deep-well fracturing and coiled-tubing treatment as frictional pressure losses can be minimized. The system can be energized or foamed with carbon dioxide (CO2) and nitrogen (N2) or may also be used in binary foam systems. This paper will provide details on the new fracturing fluid system, in terms of proppant pack cleanup, rheological properties, and fluid loss, as well as other parameters. Various rheological evaluations using high-pressure, high-temperature rheometers as well as a foam loop, fluid leakoff testing, proppant pack conductivity, and regain permeability evaluations are presented. Field case histories will evaluate fracturing treatments using new fracturing fluid and comparable treatments using conventional fluid. Normalized production data of the treated wells of both systems are also compared.

Effects of Viscous Fingering on Fracture Conductivity

Summary Understanding the causes of damage to fracture conductivity is vital in design of fracture treatments for maximum economic value and to analyze the actual well performance. High-viscosity fluids, resulting from retention of polymer within the proppant pack during closure, play a major role in proppant-pack damage. Viscous fluids are not effectively displaced during flowback and production of hydrocarbon unless the viscosities of the phases are similar. The consequences of viscous fingering in the fracture are discussed, and a method is presented for predicting the retained permeability of proppant packs in which guar-based hydraulic fracturing fluids have been broken. Data required for the method are temperature, polymer molecular weight, and final polymer concentration.