Abstract
The large increase in the use of crosslinked fracturing fluids in the past decade has led to the need for accurate evaluation of their flow properties. Traditional rotational viscometers are inadequate for studying the rheology of these crosslinked fracturing fluids, hereafter called "gels." A recently developed coiled-pipe viscometer is described, and gel response to various temperature and shear histories during preparation and testing is presented as determined by the pipe viscometer.
If gel flow properties in the fracture are to be studied accurately with any viscometer, then preparation of the gel, thermal energy input, and mechanical energy input to the test sample should be controlled to duplicate the gel of the fracture. The pipe viscometer developed has the following attributes.Gel preparation is an integral part of the viscometer. Each element of the gel is subjected to the same degree of shear while mixing.The gel develops in-line as it moves to the test section of the viscometer. There are no stagnant periods.Heat transfer occurs from the walls of the conduit to the fluid in flow as in the fracture.Parallel flow sections are incorporated for scale-up information.
Results are presented to elucidate the mechanism of gel formation and deterioration, gel stability at high temperatures, and the possible occurrence of slip flow in the fracture. The coiled-pipe viscometer shows potential to stimulate fluid preparation and flow in the fracturing process.
Introduction
Crosslinked fracturing fluids introduce complex flow behavior that has raised concerns about the efficacy of conventional study means developed for simpler non-Newtonian fluids. These particular fracturing fluids have characteristics that make their flow measurements more difficult than other non-Newtonian fluids.
A basic water-soluble polymer, hydroxypropyl guar, (a guar gum derivative) is currently in general use. The guar gum molecule is a high-molecular-weight carbohydrate polymer or polysaccharide. Propylene oxide reacted with the guar yields the hydroxypropyl polymer used in the fracturing fluids. The modified guar gums can be gelled by transition metal ions. The crosslinked fracturing fluids used in this study included such polysaccharides reacted with an organic titanate chelate as a crosslinker. A few tests were made with the borate ion as crosslinker.
In addition to pH. reactant types, and reactant concentrations, the extent of crosslinking-and thus, flow behavior-depends on shear and temperature levels during preparation for many crosslinked fracturing fluids. After the gel is formed in the initial preparation steps, its rheology depends on shear history. Then, not only is the rheology dependent on temperature in the Arrhenius sense, but thermal deterioration of the crosslinking bond significantly affects viscosity at temperatures common to the formations to be fractured.
There are other complicating factors. A static gel realizes a more rigid structure than a gel in movement. Initial energy inputs-thermal and mechanical-affect the degree of crosslinking and thus flow behavior or proppant transport capability.
Rotational viscometers, the traditional means of evaluating non-Newtonian rheology, are less effective when crosslinked gels are tested. Samples necessarily are prepared in batches and consequently this introduces a stagnant period in transfer to the viscometer-a time in which a gel structure is realized that would not occur in the fracture. There is further difficulty if some fluid remains unsheared in the bottom of the cup during the test. The batch preparation technique, common for rotational viscometer use, can affect the measured rheology. The energy input might vary, unequal shearing of portions of the batch might occur, and the shearing action might not be reproducible. Since the gel structure may degrade with shear, a series of tests on the same sample may give misleading results.
The pipe viscometer reported in this paper has advantages over other pipe viscometers when used with crosslinked fluids. It overcomes deficiencies such as:fluids in traditional pipe viscometers are batch mixed before entering the viscometer;pipe lengths may be insufficient to detect stress-induced slip flow;shear is introduced to the samples other than from pipe walls; andcapability might not exist to test such fluids at the high temperatures encountered in some reservoirs.
A pipe viscometer was designed and constructed to overcome these deficiencies and perform specifically for testing crosslinked fracturing fluids.
SPEJ
P. 575^
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