Sorting Out Where the Sand Flows During Fracturing

Abstract

The message from a single chart’s data from the first full-scale hydraulic fracturing surface test is simple: Far less proppant flows out of the first clusters passed in a stage than the last ones. The likely explanation drawn from a surface test created by GEODynamics is that the momentum of those relatively large sand grains prevents them from making that turn early on, leaving a lot of sand for later stages where the slowing flow will make the turn easier. “The flow is going 45 miles an hour; sand particles have to turn in three-eighths of an inch,” said Jack Kolle, senior technical advisor at a sister company, Oil States Energy Services. He made the comment during a presentation about modeling fracturing test data to create an engineering model of proppant flows during 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 by using basic fluid mechanics concepts. After he started working on a model, though, he realized that proppant flow was more complex than expected. “It quickly became apparent we could not explain it without CFD modeling,” he said. He was referring to computational flow dynamics (CFD) which requires massive amounts of computing power to model complex flows such as the flow of air around an aircraft wing. In the past it has been used in studies that concluded that the fast-moving flow of water and sand during fracturing resulted in uneven distribution of water and sand. The model he created based on data from GEODynamic’s unique surface testing setup and subsurface fracturing analysis evolved into the company’s fracturing-flow advisor program, StageCoach. Based on a quick look at four charts in a paper about the testing, it appears that larger-grained proppant is far more likely to slip past early clusters than smaller grains, which tend to be distributed evenly among clusters. And fracturing designs that more evenly distribute the slurry among the clusters can further flatten the distribution (SPE 209141). The results favor some established trends. The industry has embraced 100 mesh proppant for fracturing and limited-entry designs which, to varying degrees, ensure more-even distribution. What constitutes limited entry has evolved. A 2019 paper on the first two rounds of testing preceded current stage designs using clusters with only one perforation per cluster, often at the top of the hole. The test work supported the rule of thumb that shooting perforations at the bottom of the casing is a bad idea. The thinking has been that a perf gun lying on the bottom of the casing and shooting at point-blank range will create a larger hole than shots made from a distance. When fracturing begins, bottom holes will take in far more of the fluid and proppant, causing rapid wear which is magnified by the force of gravity.