TX 75083-3836, U.S.A., fax 01-972-952-9435. AbstractAs coiled tubing is presently utilized for a multitude of projects in the field, the necessity of estimating the friction pressure of the working fluids in the reeled portion of the coiled tubing string becomes extremely important. Past authors have shown the significant difference in friction pressure between straightened and reeled coiled tubing.This work discusses the experimental investigation of five different polymeric solutions and one drilling mud in three different coiled tubing sizes. The analysis of the frictional pressure loss data includes the effect of fluid rheology, coiled tubing curvature, and tubing diameter.The intent of the analysis is to arrive at a correlation describing frictional pressure loss behavior of polymer solutions and drilling muds in reeled as well as straightened tubing.The experimental setup includes 3000 ft of 2 3/8 in. reeled tubing, 5000 ft of 1 1/2 in. reeled tubing, and 1500 ft of 1 in. reeled tubing. Straightened sections of tubing are also utilized in the setup for comparison and modelling of data.The experimental results are compared with correlations developed by earlier investigators including Mashelkar, McCann, Ito, and Shah. The analysis of the data led to the development of a new model for polymer solutions and a model for drilling muds. The model for polymer solutions is recommended for power law type, pseudoplastic fluids with a power law index less than one, while the model developed for drilling mud was developed for only one concentration of mud. However, the drilling mud correlation suggests the form that other concentrations of drilling mud may follow.
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^
Even though pressure drop across perforations for clean fracturing fluids can generally be accurately predicted, it is not well understood for fracturing slurries. In this paper, two wellbore models-one transparent and one high pressure-were used to study the perforation friction pressure behavior of sand laden fluids. The transparent model constructed with cast acrylic allowed visual observation of fluid exchange in the "rat-hole" and flow patterns of the slurries in the wellbore and through the perforations. Critical velocity at which sand begins to screenout at the perforations was also determined. Tests were performed in the high pressure model varying gel concentration, sand concentration, proppant size, and perforation diameter to gather pressure drop data. The effect of the ratio of perforation diameter to the average proppant size on the sand screenout tendency at the perforation was also investigated.A correlation to predict the change of perforation coefficient due to proppant erosion was developed from the laboratory data.This paper presents a field procedure to better estimate the change of perforation coefficient during proppant stages for calculating the change of perforation friction.References and illustrations at end of paper 479 Incorporating this change of perforation pressure drop during proppant stages in the real-time bottomhole treating pressure calculation will enhance interpretation of the treatment analysis.
Borate-crosslinked fluids have increased in usage for fracturing treatments in recent years because of their low cost and low damage potential to formations. Until recently, most applications have been in shallow wells below 200°F because of high friction pressures and relatively low thermal stability. Recent advances in borate fluid chemistry have resulted in new techniques to minimize pumping pressure through the use of chemically delayed borate crosslinking agent.Ability to minimize friction pressure with delayed borate crosslinker has reduced treatment cost and extended the application of borate-crosslinked fluids in deeper wells with higher bottomhole temperature (to 300°F). Laboratory and field data have been gathered to investigate the effects of fluid parameters on the friction behavior of the delayed borate-crosslinked fluid.The laboratory experiments were performed with a recirculation flow loop to simulate field conditions, The field test was performed in a vertical wellbore. Measurements were made with three bottomhole memory gauges installed at various depths.References and illustrations at end of paper This paper presents friction pressure correlations derived from laboratory and field data for calculating realtime bottomhole treating pressure on-site to enhance the fracturing treatment analysis. Case histories are used to demonstrate the accuracy in friction pressure prediction for delayed borate-crosslinked fluids using the correlations presented in this paper. A method is also presented for field application of the friction pressure correlations provided in this paper.
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