In recent years, fracturing designs tend to use longer fracturing sections and increase the number of perforation clusters with the aim of saving time and cost. However, papers presented at the 2023 SPE Hydraulic Fracturing Technology Conference and Exhibition show that the concept of reducing the number of perforations in each perforation cluster and improving the fracturing effect is gradually becoming the industry consensus.
Traditional fracturing designs believe that long fracturing sections and multiple perforation clusters can increase oil and gas production. But with in-depth research, some companies have begun to question this consensus and try to use shorter fracturing sections and fewer perforation clusters to conduct fracturing to verify whether oil and gas production can be increased.
ConocoPhillips is considering "returning to fracturing designs with fewer perforation clusters." Dave Cramer, a senior engineering researcher at the company, said that in some areas, they are testing fewer perforation clusters based on fiber optic monitoring data from adjacent wells. The data show that after reducing the number of perforation clusters, the far-field fracturing uniformity has been improved. He also added that another advantage of shortening the fracturing section is that it can increase the hydraulic fracturing injection rate, thereby increasing the fracture width and improving the proppant transportation. This exploration of shortening the fracturing section has a basis.
A paper recently published by Devon Energy concluded that short fracturing sections with fewer perforation clusters have higher fracturing efficiency than long fracturing sections with more perforation clusters. The 36-page paper is based on the large-scale fracturing test of Hydraulic Fracturing Test Site 1 (HFTS1) in the Eagle Ford hydraulic fracturing field.
A test well was used to test fracturing sections with different numbers of perforation clusters to explore the most effective repeated fracturing method and make up for the enriched oil and gas rock formations that were not fully utilized in the early fracturing design. The research data have been shared with the companies supporting this public-private partnership project, and ConocoPhillips is one of the participants. The authors of the paper said: "Overall, the fracturing section design with fewer perforation clusters has higher cluster efficiency." Perforation cluster efficiency depends on whether the liquid intake holes in the perforation cluster can obtain sufficient fluid and reach a high enough injection rate to form effective fractures, and the fracture length is affected by many factors, including the pumping rate, the number of liquid intake holes, the aperture size and its distribution. The current core concept of most fracturing designs is based on the Limited-Entry Method.
The industry's attention to balanced fracturing can be traced back to early fiber optic monitoring research. At that time, it was found that the first few perforation clusters at the heel of the horizontal section absorbed most of the fluid and formed the dominant fracture, and the subsequent perforation clusters were under insufficient pressure, resulting in uneven fracturing.
Devon Energy uses a new method developed independently to collect fracture propagation data, making its fracturing evaluation method different from traditional cognition. At the Eagle Ford test well site, Devon placed optical fibers in the observation well 225 feet away from a re-fracturing well and recorded the expansion location of each fracture through strain measurement. It also used wellbore imaging technology to observe the expansion of fractures at the liquid intake perforation during the pressure pumping process and evaluated the fracturing efficiency of the fractures by combining the two types of data.
Researchers observed that the 22-cluster fracturing section produced the most long fractures (a total of 4), but only accounted for 18% of the total number of long fractures measured in the observation well; the 7-cluster fracturing section had an average of 2.5 long fractures, accounting for 35%, performing better. Data analysis shows that the cluster efficiency of the 7-cluster fracturing section with high perforation friction pressure reaches 100%, the efficiency of the 22-cluster fracturing section is the lowest, only 78%, and the efficiency of the 12-cluster fracturing section is between the two.
In the fracturing section with a large number of perforation clusters, it was found that all the dominant fractures came from the heel of the fracturing section, indicating that the perforation clusters at the toe end were not fully reformed, which shows that the high cluster density design may lead to insufficient reform of non-dominant perforation clusters. In addition, Cramer put forward a non-intuitive view based on the proppant distribution of shorter fracturing sections. He believes that the dominant perforation cluster that absorbs the most fluid may not be able to absorb the same amount of proppant. In this case, even if the large fracture performs well in the early stage, the production capacity may decrease due to insufficient support in the long term.
Although there is currently a lack of specific data on whether shortening the fracturing section can increase sufficient production to make up for the additional fracturing cost, this issue has become crucial as the market share of listed companies in major shale blocks continues to expand. Under the trend of ultra-long fracturing sections, exploring the advantages of shortening fracturing sections is of great significance.