The message from the data from a single graph of the first full-scale hydraulic fracturing surface test is simple: far less proppant flows from the earlier batches passed through a stage than from the later ones.

The likely explanation from a surface test created by GEODynamics is that the momentum of these relatively large sand grains prevents them from making this turn early, leaving plenty of sand for later stages where the slower flow will make the turn easier. .

“The flow goes at 45 miles per hour; the sand particles have to spin in three-eighths of an inch,” said Jack Kolle, senior technical advisor for sister company Oil States Energy Services. He made the comment during a presentation on modeling fracturing test data to create an engineering model of proppant flows at the recent SPE Hydraulic Fracturing Technology Conference and Exhibition (EPS 209178).

When he first heard about the test results, Kolle thought he could create a model using the basic concepts of fluid mechanics. After starting to work on a model, however, he realized that the proppant flow was more complex than expected. “It quickly became clear that we couldn’t explain it without CFD modeling,” he said.

He was referring to computational flow dynamics (CFD) which requires huge amounts of computing power to model complex flows such as the airflow around an airplane wing. In the past, it has been used in studies concluding that the rapid flow of water and sand during fracturing results in uneven distribution of water and sand.

The model he created based on data from GEODynamic’s unique surface test setup and subsurface fracturing analysis evolved into the company’s fracturing flow advisory program, StageCoach.

Based on a quick review of four graphs in a testing article, it appears that the coarse-grained proppant is much more likely to pass the early clusters than the smaller-grained ones, which tend to be evenly distributed among the clusters. And fracturing designs that more evenly distribute mud between clusters can further flatten the distribution (EPS 209141).

The results support some established trends. The industry has adopted 100 mesh proppant for fracturing and limited access designs which, to varying degrees, provide more even distribution.

What constitutes limited entry has evolved. A 2019 article on the first two rounds of testing predated current stage designs using clusters with a single perforation per cluster, often at the top of the hole.

Test work confirmed the rule of thumb that shooting holes in the bottom of the case is a bad idea. The idea was that a perf gun laying at the bottom of the casing and firing at close range would create a bigger hole than shots fired from a distance. As fracturing begins, the bottom holes absorb much more fluid and proppant, causing rapid wear that is amplified by the force of gravity.

It wasn’t something they were trying to test. They shot down to make sure all the liquid and sand would drain into a reservoir below. During the process, they found that it so effectively drained the larger proppant sludge in the heel side clusters that little sand was left at the end of the step in the toe side clusters.

Kolle said when it comes to the punctures at the top of the pipe, gravity could limit the volume of sand entering those holes. Output could be optimized if the perforations were in the middle, a little below the three or nine o’clock positions.

The article can be read as a series of thoughts on fluid and sand flow based on proppant transport surface testing and downhole fracturing analysis by oil companies who have partnered with GEODynamics.

The method used for the analysis, Eulerian Multiphase Computational Fluid Dynamics (EMP-CFD), was chosen because it is able to take into account the flow differences of sand versus water.

• It is observed that the placement of the proppant in each step can be very non-uniform.
• Non-uniform flow of proppant in the casing can be as important as formation variability and stress shading.
• Fine sand is distributed relatively evenly along the length of a perforated finish while coarser sand tends to slide past the heel perforations and concentrate on the bottom towards the toe of the finish.
• At high axial flow velocities, the mud exiting the perforation is sucked in from a relatively small semicircular region of the flow – the ingestion zone.
• The ingestion area is proportional to the ratio of the flow through the perforation to the total flow in the casing.
• The sand particles are observed to settle towards the bottom of the tube during the flow of the water slurry at velocities comparable to those used for proppant placement
• Modeling turbulent multiphase flows of particles in viscoelastic fluids, such as those containing concentrations of friction reducer, is beyond the capabilities of current multiphase CFD codes.
• For particles negotiating the bend in a perforation, the inertial forces are orders of magnitude greater than they are for gravitational sedimentation. Therefore, friction reducing (FR) polymers can reduce proppant slip past perforations, but to a lesser degree than reducing gravitational sedimentation. The optimal selection of FR filler for uniform propp placement remains unanswered and will only be resolved by further testing the magnitude of proppant transport surface trials.

For further reading

EPS 209178 Modeling Proppant Transport in Casing and Perforations Based on Proppant Transport Surface Tests by Jack Kolle, Petroleum States Energy Services; Alan Mueller, ACMS; and Steve Baumgartner and David Cuthill, GEODynamics.

EPS 209141 Execution and lessons learned from the first two surface tests replicating unconventional fracturing and proppant transport by Phil Snider and Steve Baumgartner, GEODynamics; Mike Mayerhofer, Liberty Oilfield Services; and Matt Woltz, PDC Energy.