A Device and Method of Determining the Rheological Quality of Gelled Fracturing Fluids

Abstract

A method of determining the quality of cross-linked hydraulic fracturing gels in the field was developed at the Alberta Research Council. The method is based on the measurement of the pressure required to push the gel through an orifice. The measurement can be repeated several times in order to quantify transient changes in the rheology of the gel. The quality of the gel prepared in the field is quantified by plotting the extrusion pressure vs. shear rate at the orifice. This quality control technique was compared, for certain types of water-based and oilbased gels, to existing techniques based on shear rheometry. The comparison showed that the water-based gels we investigated could be characterized using the gel tester, but not with the existing technology. The oil-based gels we investigated could be characterized better using the existing technology.

Introduction

Hydraulic fracturing is a common method of enhancing formation Productivity ⁽¹⁾. Polymer gels are often used as fracturing fluids in order to carry solid particles (proppant), such as sand, into fractures. The sand keeps the fractures open after the injection is stopped. In order to maximize the fracture width, the gel must be able to prevent the sand from settling within the well and transport it through the perforations. In addition, the gel must have the proper leak-off properties in order to keep the fracture open when the fluid is being injected.

According to Ely ⁽²⁾, most fracture treatments fail due to the inability of a fracturing fluid to carry proppant for the duration of the treatment at in situ conditions of temperature and shear, and/or to properly degrade back to water after the treatment.

The current field quality control practice is to: 1) measure the viscosity of the ungelled polymer solution using either a Fann-35 rheometer or, more recently, a Brookfield PVS rheometer; 2) do a lip test; and 3) measure the vortex closure time. Water-based fracturing gels are commonly used worldwide since they have the following advantages according to Ely et al. ⁽³⁾; 1) they are economical compared to oil, condensate, and methanol; 2) they yield increased hydrostatic head compared to oil, gases, and methanol; 3) they are incombustible; and 4) they are readily available. Hydrocarbon-based gels are used where the formation may be sensitive to water injection.

As described in a monograph on hydraulic fracturing ⁽⁴⁾, computerized fracturing simulators normally require the consistency and flow behaviour indices to calculate the pressure drop along injection wells and within fractures. These parameters are obtained by fitting shear stress vs. shear rate measurements to a power law. The measurements are made using shear rheometers, such as the Fann-35, Fann-50 or the Brookfield PVS rheometer. It is difficult to measure the "viscosity" of certain cross-linked gels. The problem is that the gel slips on the walls of the bob and cup of the shear rheometers, as was observed by Cameron et al. ⁽⁵⁾ invisualization experiments showing the flow of coloured tracer particles within the gap.