The test chamber of the PTMS includes interacting projections extending from the inner surface of a cup receiving the fluid, and from an axial support extending into the fluid in the cup. (Graphics courtesy of Halliburton)

A patented viscometer, known as the Mimic proppant transport measurement system (PTMS), was designed to work within a conventional Fann Model 50-type viscometer, using the Fann 50 viscometer drive mechanism and measurement capabilities. The PTMS has been used to measure typical fracturing fluids containing realistic concentrations of proppants, under conditions representative of actual fracturing treatments. The measurements show regions of elastic transport typical of viscoelastic fracturing fluids where proppant is transported efficiently, usually followed by regions of purely viscous transport where proppant slowly settles.

There are two key areas that Halliburton’s PTMS test data and the industry standard neat-gel viscosity data diverge:

  • Proppant is present in PTMS-tested fluids. The proppant may have both physical and chemical influence on fluid properties that are not observed in neat-gel viscosity tests.
  • Conventional viscometers measure only viscous properties. The PTMS device measures both viscous and elastic properties. Elastic transport cannot be accurately predicted by measuring only the viscous component of a viscoelastic fluid.

How it works

The device and method for testing a viscosified fluid containing particulates indicate when the particulate is in suspension within the fluid and when it is not. The device stirs the fluid and particulate at a constant rate, during which time the viscosity of the fluid causes the particulate to remain suspended in the fluid, then the proppant begins to settle. Eventually all the particulate settles out of suspension in the fluid. The device generates a signal to indicate the change in particle-carrying capability of the fluid. Other characteristics, including crosslink break time, can also be determined.

The test chamber includes interacting projections extending from the inner surface of a cup (Figure 1) receiving the fluid, and from an axial support extending into the fluid in the cup. The PTMS is based on a Fann Model 50-type viscometer with modified bob and cup. The PTMS has an inner sensing surface composed of flags, in place of the concentric cylinder design of the Model 50 viscometer. To aid mixing within the cup, protrusions are included in the inner walls of the cup. Proper clearances were provided between the flags and the wall extensions to help prevent proppant from sticking between the surfaces. As the flags on the moving cup sweep by the static flags on the central sensing shaft, a cyclic torque is measured. The oscillating force contains both viscous and elastic components.

The proppant viscometer has complicated geometric surfaces, making calculation of shear rate difficult. However, laboratory personnel approximated volume-average shear rate by studying known fluids. The resistance to movement of a slurry increases as proppant concentration increases. Placement of the mixing and sensing surfaces toward the bottom of the cup allows greater sensitivity to concentrating of proppant as it settles. An increase in torque indicates that proppant is settling.

In contrast, a conventional viscometer consists of two concentric cylinders having tight clearance between them. To test fluid viscosity, one cylinder is turned while the other remains stationary. Test fluid, containing no particulates, is flowed through the annular space between the cylinders while one cylinder is rotated. Torque required to rotate the cylinder against the resistance of the test fluid gives an indication of test-fluid viscosity; proppant-carrying capability is then estimated based on the recorded viscosity profile.

The PTMS apparatus is capable of routine measurement of gelled fracturing fluids containing a wide range of proppant particles ranging from linear to fully crosslinked to broken gels. By measuring various types of fracturing fluids, it is possible to classify the transport properties of each fluid type. By adding breaker chemicals to fluid, it is possible to determine the length of time a fluid maintains its transport capability, or whether the proppant will settle quickly.

Conventional viscometer approach

Where viscous drag dominates, as in the classical case of a two-winged vertical fracture with parallel-plate geometry, the challenge in proppant transport is to ensure that vertical settling time is much greater than horizontal travel time. Sufficient vertical settling time allows the particle to reach a maximum horizontal distance, thus avoiding a duning effect.

The Fann Model 50 viscometer was designed for characterizing fracturing gels under simulated downhole temperature-time conditions. However, the Model 50 viscometer and most other bench-top viscometers/rheometers are not adequately equipped to handle proppant-laden fluids. In the case of concentric cylinders, the centrifugal forces tend to stratify the particles, thus resulting in nonrelevant data. In cone-plate and plate-plate viscometers, the small gaps necessary to provide torque sensitivities result in “particle jamming.” Additionally, the large density differences between most proppants and conventional fracturing fluids result in settling during testing, thus producing unreliable results (SPE paper 95287, “Measurement of Proppant Transport of Frac Fluids, P.C. Harris, R.G. Morgan and S.J. Heath).

Figure 2 shows settling stages witnessed in laboratory experiments. Illustrations of four stages of settling are correlated to the graph showing proppant settling vs time.

Application

In a recent field application of PTMS technology, fracturing operations were interrupted by an early screenout. Proppant-transport testing was being conducted by conventional means, using concentric-cylinder viscometers to evaluate frac fluids. The frac fluid was a borate-crosslinked guar transporting ceramic proppant into a 200° F (93º C) formation. Ceramic proppant was used based on the formations closure stress.

The operator elected to use PTMS technology to test proppant-laden frac fluid. Test results led frac designers to alter the fluid pH and crosslinker concentration. Fracture operations proceeded without further incident.

In an after-action experiment, the frac fluid used in the treatment, but without proppant in it, was tested in the conventional manner in a concentric-cylinder viscometer. Test results showed that the fluid, which had just been used in a successful frac treatment, would not provide transport according to the operator’s “rule of thumb” (arbitrary viscosity requirements) for frac fluids applied in his field.

Conclusions

This example, along with other testing performed using the PTMS, indicate that the standard used today for proppant transport measurement may not accurately measure the ability of a fluid to transport proppant. No longer can we assume that that relationship between proppant transport ability and gel loading is essentially linear. Continued effort is required to develop a new proppant transport index for many fluid systems and PTMS will contribute significantly to improving the industry’s understanding.