A New Method for Determining the Rheology of Crosslinked Fracturing Fluids using Shear History Simulation

Abstract

Abstract A new method which incorporates shear history simulation has been developed for measuring the rheological properties of crosslinked fracturing fluids. his proven method can provide the true rheology of the working fluid which enters the fracture. These data are essential when designing and conducting treatments in low-permeability formations, in order to obtain the desired fracture geometry. Recent research has shown that the rheological properties of crosslinked gels are dependent on the properties of crosslinked gels are dependent on the method of preparation and testing. A single formulation of a fluid prepared by two different methods can have resulting viscosity profiles which differ by several hundred percent. The new method of measuring rheology overcomes many of the previous discrepancies resulting from preparation and testing by incorporating a shear history into the procedure. This shear history is similar to the tubular shear history a fluid experiences before it enters a fracture. shear history apparatus is a capillary viscometer consisting of a series of tubes and additive pumps for the addition of crosslinkers. The length of the tubing as well as the pump rate can be adjusted to match laminar shear rates and shear histories encountered in fracturing treatments. The paper describes a field scale-up model which, verified the results of the laboratory tests. Correlation of field data from bottom-hole tubing pressures on several fracture treatments further pressures on several fracture treatments further substantiates the validity of the laboratory apparatus. Results are presented which show that laboratory data can now be used to predict friction pressure exerted by fracturing fluids. Perhaps more important, though, is the insight now available into the true rheology of the fluid that enters the fracture and the development of techniques for controlling this rheology. Introduction Advances in hydraulic fracturing design and treatment of deep hot wells have created the necessity for sophisticated fracturing fluids. The inherent problems with current methods of evaluating problems with current methods of evaluating crosslinked fracturing fluids have stifled significant advances in the technology of crosslinked fluids. The inability to arrive at consistent results using the API RP 39 Procedure (Modified) has promoted the development of improvements and alternatives to the above method. During a hydraulic fracturing treatment, fracturing fluid undergoes shear at the pumps, in the treating lines, in the well's tubular goods and in the fracture. A summary of the shear at these points can be termed the shear history of the fluid. Both the shear history and the time at shear can dramatically affect the viscosity of the fluid. In order to develop meaningful data, therefore, the effect of the fluid's tubular shear history on the long-term rheology of the fluid must be determined. To simulate field conditions, it is necessary to reproduce tubular shear history and fracture shear/ temperature conditions. The apparatus which is the subject of this paper incorporates these aspects conditions and eliminates two undesirable aspects of previous methods. previous methods. Unknown shear induced by the pump in a closed-loop pipe viscometer has been avoided and the zero shear period encountered by a fluid as it is transferred from a shear history device to a concentric-cylinder viscometer is eliminated. Description The method and apparatus developed offer two significant improvements in testing procedures which allow the test method to better simulate treating conditions. First, the fluid is continuously sheared at appropriate rates from the time just before crosslinker addition until the fluid evaluation is completed. Second, the shear history of the test fluid is controlled to simulate both shear rate and time at shear in the wellbore and in the fracture. The apparatus used was a capillary rheometer in conjunction with a concentric-cylinder viscometer. P. 319