The message from a single graph of data from the first large-scale hydraulic fracking surface test is simple: far less proppant flows from the first clusters passed in a phase than from the last.
The likely explanation, emerging from a surface test created by GEODynamics, is that the momentum of these relatively large grains of sand prevents them from making this turn early, leaving plenty of sand for later stages where the slowed flow eases the turn.
“The current is going at 45 miles an hour; Sand particles have to rotate three-eighths of an inch,” said Jack Kolle, senior technical adviser at a sister company, Oil States Energy Services. He made the comments during a presentation on modeling fracking test data to create an engineering model of proppant flows at the recent SPE Hydraulic Fracturing Technology Conference and Exhibition (SPE 209178).
When he first heard about the test results, Kolle thought he could create a model using basic fluid mechanics concepts. However, after beginning work on a model, he found that the proppant flow was more complex than expected. “It quickly became clear that we couldn’t explain it without CFD modelling,” he said.
He was referring to Computational Flow Dynamics (CFD), which requires massive amounts of computational power to model complex flows such as airflow around an airplane wing. Historically it has been used in studies that concluded that the fast-moving flow of water and sand during fracturing resulted in an uneven distribution of water and sand.
The model he created based on data from GEODynamic’s unique surface test setup and subsurface fracture analysis grew into the company’s fracture flow consultant program, StageCoach.
Based on a quick look at four graphs in a paper on the tests, it appears that larger grain proppant is far more likely to slip past early clusters than smaller grains, which tend to be evenly distributed among clusters. And fracturing designs that distribute the slurry more evenly across the clusters can further flatten the distribution (SPE 209141).
The results favor some established trends. The industry has adopted 100 mesh proppants for fracturing and limited entry designs, which provide varying degrees of more even distribution.
What constitutes restricted access has evolved. A 2019 paper on the first two rounds of testing preceded current stage designs that used clusters with only one perforation per cluster, often at the top of the hole.
Test work confirmed the rule of thumb that shooting perforations on the bottom of the case is a bad idea. The rationale was that a perf gun lying on the bottom of the case and firing at close range would create a larger hole than shots from afar. As fracturing begins, the bottom holes absorb much more fluid and proppant, resulting in rapid wear that is amplified by 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 tank below. During the process, they found that it drained the larger proppant slurry so effectively in the heel-side clusters that there was little sand left in the toe-side clusters by the end of the stage.
Kolle said that if there are perforations at the top of the pipe, gravity could limit the volume of sand going into those holes. Drainage could be optimized if the perforations were in the center, slightly below the 3 or 9 o’clock position.
The paper can be read as a series of fluid and sand flow thoughts based on the proppant transport surface tests and well fracture analyzes of the oil companies that have partnered with GEODynamics.
The method used for the analysis – Eulerian Multiphase Computational Fluid Dynamics (EMP-CFD) – was chosen because it can account for the differences in the flow of sand relative to water.
- It is observed that proppant placement can be very uneven within each stage.
- Uneven proppant flow in casing can be important, as can formation variability and stress shadowing.
- Fine sand is distributed relatively evenly along the length of a perforated heel, while coarser sand tends to slip past the heel perforations and concentrate on the bottom toward the toe of the heel.
- At high axial flow velocities, the slurry exiting the perforation is drawn from a relatively small, semi-circular area of the flow – the intake area.
- The acquisition area is proportional to the ratio of the flow through the perforation to the total flow in the shell.
- Sand particles are observed to settle to the bottom of the pipe during water-mud flow at speeds comparable to those used for proppant placement
- Modeling turbulent multiphase particle flows in viscoelastic fluids, such as those containing concentrations of drag reducers, exceeds the capabilities of current multiphase CFD codes.
- For particles that overcome the turn into a perforation, the inertial forces are orders of magnitude greater than they are for gravitational settling. Therefore, friction reducing (FR) polymers can reduce the sliding of proppants past perforations, but to a lesser extent than reducing gravity settling. The optimal selection of FR loading for consistent proppant placement remains unanswered and will only be resolved by further testing up to the benchmark of proppant transport surface testing.
For further reading
SPE 209178 Modeling of proppant transport in casings and perforations based on proppant transport surface tests by Jack Kolle, Oil States Energy Services; Alan Mueller, ACMS; and Steve Baumgartner and David Cuthill, GEODynamics.
Execution of SPE 209141 and learnings from the first two surface tests to replicate unconventional fracturing and proppant transport by Phil Snider and Steve Baumgartner, GEODynamics; Mike Mayerhofer, Liberty Oilfield Services; and Matt Woltz, PDC Energy.