After Years of Ever-Longer Fracturing Stages, Some Engineers Say Shorter Can Be BetterStephen Rassenfoss
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The trend in fracturing designs has been longer stages with more perforation clusters, which save time and money.
Based on papers at this year’s SPE Hydraulic Fracturing Technology Conference and Exhibition, the thinking that has sharply reduced the number of perforations per cluster and improved the effectiveness of fracturing is becoming the industry norm.
But there are a couple of companies questioning the consensus by fracturing wells with shorter stages and fewer clusters to see if they deliver more oil and gas production.
ConocoPhillips has been considering “going backward to fewer clusters per stage,” said Dave Cramer, a senior engineering fellow at the company.
“In some areas, we are testing out fewer clusters per stage based on fiber-based observations in offset wells that (indicate) far-field treatment uniformity is improved as a result,” he said, adding “another advantage of reducing stage length is that injection rate into the hydraulic fractures is increased, which leads to increased fracture width and improved proppant transport.”
The offset-well observations mentioned were in a recent paper by Devon Energy that reported on a test which concluded that stages with fewer clusters were more efficiently fractured than longer stages with more of them (SPE 212340).
The 36-page paper was based on extensive testing at Phase 3 of the Hydraulic Fracturing Test Site 1 in the Eagle Ford, where a well fractured using a wide range of clusters per stage was used to find a way to most effectively refracture productive rock missed by an old fracture design.
The data were shared with companies backing the private-public partnership with the US Department of Energy, which included ConocoPhillips.
The paper’s authors wrote, “Generally, cluster efficiencies are higher for stage designs with fewer clusters.”
Cluster efficiency depends on the entry holes in a cluster getting enough fluid at a high enough rate to create a productive fracture. Fracture length is the result of other choices, including the pump rate, the entry-hole numbers, diameter, and placement.
The thinking behind these designs—as with most fracturing nowadays—is based on the limited-entry method. This technique ensures the pumping rate and fluid volumes are enough to properly stimulate all the perforations, which are sized and placed to ensure all the entry holes have a chance of developing a fracture.
The industry’s focus on equalizing treatment goes back to early studies using fiber optics that revealed that the first clusters passed nearest the heel side of the lateral were taking in the lion’s share of the fluid and developing dominant fractures, leaving many later clusters under stimulated.
Based on recent papers from Devon and Hess, limited-entry ensures most clusters are stimulated, but dominant clusters are still getting more than their share because the high flow rates that favor them with more fluid also ensure they erode faster, allowing them to take in more fluid.