No abstract

No abstract

No abstract

Borate-crosslinked guar gels were prepared and characterized to understand their capability to suspend and transport sand particles through a fracture created in a petroleum reservoir. In this study the crosslinked gels were formulated by varying the borate crosslinker concentrations that were selected such that the gels satisfied the minimum viscosity criteria (100 cP at 100/s) used to evaluate crosslinked gels for their suspension capabilities. However, some of these gels did not exhibit satisfactory particles transport through a slot that models a parallel fracture. These gels were then characterized using oscillatory measurements and atomic force microscopy (AFM) to understand the influence of the microscopic behavior of the crosslinked gels on their macroscopic performance in the slot. The results showed that the suspension transport characteristics of these gels could be described through crosslinked networks formed across the guar polymer. The AFM images and rheological measurements of these gels suggest that the elastic modulus provides more useful information than the viscosity about the crosslinked gel structure and their capability to suspend sand particles.

This paper evaluates the maximum flow rate before the sand production initiates while flowing back a reservoir after the hydraulic fracturing treatment. The experiments are performed in a high pressure slot which simulates a fracture. The slot is a parallel plate device, which is 7 ft high and 9 1/3 ft long with provisions for varying fracture gap width. The slot is filled with proppant to simulate a propped fracture, then, the closure pressure is applied with 12 actuators to simulate confining pressure on the proppant pack. Water is pumped into the slot to simulate flowing back of the fracturing fluid. The water flow rate is varied till the proppant pack destabilizes, and the sand production begins. The sand distribution is observed in the fracture with a vision system. The experimental results show that the critical water flowback rate decreases as the closure pressure increases, or when the fracture gap width increases. However, the sand-free maximum water flow rate increases as the sand size increases for a given fracture gap width. Lastly, the cumulative sand production decreases as the closure stress decreases at a particular flow rate. The visual observations of the proppant flowback phenomena show that the sand production initiates close to the perforation. After the initiation of the sand flow, continued pumping of the water results in sand production through a channel formed in the proppant pack.

Accurate rheological characterization of hydraulic fracturing fluids in a laboratory is extremely important prior to their use in the field. A borate-crosslinked Guar gel rheology study was performed to compare and evaluate laboratory measurements with a field-scale characterization of the gel. Field-scale fracturing operation was simulated by slot measurements at the Fracturing Fluid Characterization Facility (FFCF) of the University of Oklahoma and laboratory-scale simulations were obtained from viscometer measurements at three service company test facilities. These companies volunteered to participate in a joint effort to understand borate-crosslinked gel behavior. Both the FFCF and laboratories used identical chemicals, water, and fluid formulation procedure for the present study. The results show that slot yields reproducible borate-crosslinked Guar gel rheology data under various conditions. The comparison of results show that the slot and laboratory measurements yield different viscosities. Moreover, the laboratory viscosities show disagreement among themselves. The results suggest that the laboratory measurements must consider shear preconditioning in their rheological characterization. Furthermore, a standardized laboratory borate-crosslinked gel preparation and evaluation procedure can provide reproducible data from laboratory measurements. Introduction The hydraulic fracturing technique is widely used in the Petroleum Industry to stimulate production from a reservoir. This technique enhances the production of oil and gas and improves the economics of the formation. The reservoir is hydraulically fractured with a specially formulated fluid system. Several case histories have shown that an effective fluid system improves fracture treatment results. The ideal characteristics of a fracturing fluid are detailed in Recommended Practices prepared by the API subcommittee on Fracturing Fluid Rheology. The most important property of the fluid is its viscosity. The fluid viscosity must be sufficient to produce a wide fracture, eliminate premature proppant screenout and carry proppant deep into the formation. About 75% of fracture treatments performed today are with borate-crosslinked fluids. Borate-crosslinked fluids exhibit a non-Newtonian rheological behavior. Furthermore, their viscous properties are influenced by steady shear during flow in surface equipment, wellbore and fracture. These characteristics of borate-crosslinked fluids necessitate a study of their rheology in the laboratory. The laboratory testing of these fluids is extremely important prior to their use in an actual fracturing treatment. API recommends a standard testing procedure for measuring the viscous properties of crosslinked water-based fracturing fluids. Some researchers have suggested dynamic oscillatory measurements as a useful tool to measure the rheological properties of crosslinked fluids. The main objective of these measurements is to provide reproducible rheological data for borate-crosslinked fluids. However, the crosslinked fluid may exhibit ideal fluid properties under laboratory conditions, under actual field conditions its behavior may be completely different. Hence the borate-crosslinked fluid rheology must be determined under conditions that closely resemble actual field and downhole conditions. The FFCF was established to study fluid behavior under representative surface and downhole conditions. Fluids are prepared and handled at the facility using field-scale mixing and pumping equipment. The fluids are pumped through coiled tubing lengths of up to 5000 ft, and through a formation simulator/heat exchanger to provide shear and thermal preconditioning that are representative of field conditions. P. 503^

Summary Successful design and completion of hydraulic fracturing treatments depend on accurate estimation of fracturing fluid viscosity. This requires better knowledge of the fluid viscosity under in situ conditions. The viscosity estimation is even more important for borate-crosslinked guar gel because the gel behavior depends on temperature, wellbore shear pre-conditioning, fracture shear rate, and fluid pH. These factors require that the fluid characterization be performed under representative field conditions. Borate-crosslinked gel rheology is currently characterized in the laboratory and there is no theoretical correlation available to describe the gel rheology as a function of its dependent properties. The present study describes a correlation that is developed from data gathered by utilizing a field-scale fracture simulator and is based on the method of reduced variables. The study also extends the method to include the effect of shear history on the rheology of guar crosslinked with borate ions. The correlation relates the apparent viscosity of the crosslinked guar to the fluid temperature, shear history, and shear rate. It is presented in a simple equation, which can be incorporated into fracturing software. The present study also provides master curves of the reduced apparent viscosity-shear rate for the borate-crosslinked guar at three pH values. Introduction Estimation of fluid viscosity is crucial for the success of a hydraulic fracturing treatment. A successful treatment means a wide fracture and proper distribution of proppant in complete length of the fracture; it can be realized with a high-viscosity fracturing fluid. This fact is supported by several case histories which have shown that the viscous fluid system improves the treatment results.1 Prior to a treatment, knowledge of the viscosity is important for proper fracture design. Moreover, the information on the viscosity is necessary for estimation of friction pressure losses down tubular and for prediction of proppant transport in the fracture. Also, reliable predictive viscosity correlations can be integrated into hydraulic fracturing computer simulators. Furthermore, the usefulness of correlations is considerably increased when it is developed from the results of experiments performed using a field-scale fracture simulator. The Fracturing Fluid Characterization Facility (FFCF) has been established to study fluid behavior under representative surface and downhole conditions. The facility characterizes the fluid using field-scale mixing and pumping equipment. The fluid viscosity is measured in a parallel-plate-type device, which has provisions to dynamically vary the fracture gap width. Prior to the viscosity measurements, the fluid is subjected to shear pre-conditioning through various lengths of coiled tubing. This pre-conditioning provides a shear history simulation for the fluid and brings the viscosity measurements under conditions that are closer to the fluid flow in an actual downhole situation. The Importance of Shear History. Shear pre-conditioning of a fluid is important in hydraulic fracturing. This importance arises from the highly sensitive characteristics of crosslinked-based fracturing fluids. Several researchers have shown the effect of pre-shearing on the viscosity of guar crosslinked with different crosslinkers like titanium, zirconium, and borate.2–5 The shear history effects are also well known for other fluid systems;6–10 these effects, however, are not included into the viscosity models used to describe the various systems. The present work is part of a study conducted to include the effect of shear history on the rheological properties of fracturing fluids. The results from experiments performed on the rheology of borate-crosslinked guar-based fracturing fluids were presented earlier.5 In the present paper, these results are utilized to develop an empirical relationship between the fluid viscosity and its controlling parameters. The empirical relationship is based on the method of reduced variables.11 This method is widely used to correlate the rheology of a polymer at different temperatures,12,13 and is also known as the time-temperature superposition.14 The method of reduced variables has been used earlier by other researchers to relate the viscoelastic properties of guar and hydroxypropyl guar (HPG) crosslinked with borate ions.15,16 These studies, however, did not consider the effect of shear history in their measurements. In the present work, the effect of shear history is included in the method of reduced variables and a correlation is developed to relate the nominal shear rate with the borate-crosslinked guar viscosity measured at different temperatures and shear histories. The present study also provides viscosity-shear rate master curves at different fluid pH values. Experimental Procedure A schematic of the experimental setup is shown in Fig. 1. A brief description of the setup is given below; a detailed description of the instruments used in the setup is presented elsewhere.17 A batch of 100 bbl of 35 lbm/Mgal guar polymer solution was prepared in mixing and storage tanks. Upon hydration, the fluid pH was adjusted to a desired level. The adjustment in pH was necessary to initiate a reaction between the borax crosslinker and the polymer. The guar solution was fed to a triplex pump by a centrifugal pump, which was also used to mix the crosslinking agent into the base polymer. The fluid was then pumped through different lengths of coiled tubing. These lengths were varied from 0 to 5,000 ft. The tubings have an internal diameter of 1.188 in. A constant flow rate of 60 gal/min was maintained through the coiled tubing and the pressure drop was measured. A single flow rate was used to maintain uniform shearing prior to characterizing the fluid in the slot flow apparatus. 18,19 The flow through the coiled tubing helped to simulate pre-conditioning of the fluid, and to introduce the effect of shear history into the rheological properties. For the case where fluid viscosity was measured without shearing in coiled tubing, the measurement was termed as "no pre-shear" characterization.

No abstract