Method for determining the rheology of a gelled fluid

#High viscosity gelled hydrocarbon fluids provide unique functional characteristics for a variety of applications. Although these fluids have a high viscosity, they are capable of breaking into thinner fluids under predetermined conditions. The oil industry uses these fluids in many day-to-day operations. Common applications include: fracturing subterranean formations penetrated by a well bore, forming gravel packs in a downhole environment, performing pigging operations in a pipeline and other applications requiring a viscous fluid. Although high in initial viscosity, these fluids are designed to subsequently breakdown into thinner fluids. The subsequent drop in fluid viscosity is necessary to permit production of hydrocarbons from the well.

#Efficient formulation of gelled or viscosified hydrocarbon liquids requires prior knowledge of the viscosity of the liquid under the intended operating conditions. Due to difficulties in accurately determining rheology under downhole conditions, most well operators estimate the concentration of viscosity breaking agents necessary to lower, i.e. break, the viscosity of the gelled fluid using tests without the presence of liquid carbon dioxide. If the operator underestimates the concentration required to lower the viscosity, then production may be permanently impaired due to an unbroken gel. Absent the presence of viscosity breaking agents, oil gels will not break on their own accord over time. Accordingly, most operators add an excess of viscosity breaker to ensure an adequate drop in viscosity. However, excess use of viscosity breaker will prematurely degrade the viscosity of the fluid. This could result in a failure of the treatment to properly place the proppant in the fracture. Additionally, excess breaker usage unnecessarily increases costs. Clearly, the ability to accurately formulate the downhole fluid will greatly improve operational efficiency and reduce costs by eliminating over use of the viscosity breaking agents and other additives. Therefore, a method for determining the rheology of the gelled or viscosified hydrocarbon liquid under operational conditions will enhance the current state of the art.

#The rotary viscometer is a tool commonly used to determine rheology of gelled hydrocarbon liquids. In general, a rotary viscometer includes a sample chamber, a rotary sleeve and a static bob coaxially positioned within the rotary sleeve. Depending upon the sample to be tested, the sample chamber may also include a heater.

#FIG. 1 is a side elevation of a bottom driven rotary viscometer suitable for performing the method of the current invention.

#FIG. 2 is a side cut-away view showing a bob positioned within a rotatable sleeve.

#FIG. 3 is a side view of a rotatable sleeve with stirring fins.

#The methods of the current invention will be readily understood by those skilled in the art by referring to the drawings. In illustrating and describing the current invention, like or identical reference numerals are used to identify common or similar elements. The figures are not necessarily to scale and certain features and views of the figures may be shown exaggerated in scale or in schematic form in the interest of clarity and conciseness. As used herein, the terms gelled and viscosified are used interchangeably and refer to the increased apparent viscosity of a liquid due to a cross-linking agent. The processes and additives for gelling or viscosifying hydrocarbon liquids are well known in the art. The term rheology refers to the mathematical description of the fluid flow behavior derived by measurements of shear stress at calculated shear rates. Finally, the term non-Newtonian fluid includes mixtures having non-Newtonian rheological characteristics.

#The current invention provides a method for measuring the rheology of gelled fluids. The method of the current invention utilizes a bottom driven rotary viscometer, preferably computer controlled, of the type shown in FIG. 1 and designated with the number 10. Rotary viscometer 10 includes a sample chamber 14, a rotatable sleeve 18 positioned within sample chamber 14 and a static bob 22 located coaxially within sleeve 18. Preferably, sleeve 18 also carries a means for mixing fluid within sample chamber 14. Examples of means for mixing the sample include helical fins 36a, as shown in FIG. 3, and spiral grooves 36b, as shown in FIG. 4. Fins 36a typically extend halfway into the annulus formed between sleeve 18 and sample chamber 14.

#As shown in FIG. 2, the annulus formed between bob 22 and sleeve 18 is designated as distance D. While D will vary depending upon the size of bob 22, D is a fixed known distance for each test. As known to those skilled in the art, the distance D is used to define the shear rate at a given rotational speed. In most rotary viscometers, shaft 24 connects static bob 22 to at least one spring, not shown, and a sensor, not shown. Each spring has a known spring constant. The sensor measures the degree of deflection experienced by bob 22. Preferably, the sensor communicates the deflection measurement to the computer or to a visual display, not shown. However, a spring/sensor configuration is only one of many means for measuring torque applied to static bob 22. Other arrangements may include electronic sensors or magnets attached to static bob 22 or shaft 24. Alternatively, the force required to rotate sleeve 18 at a constant speed may be measured and used to calculate fluid rheology. The actual mechanical means for obtaining a torque reading and/or rheology is not critical to the current invention.