It is no secret that unconventional resources have come onstream in a big way due to recent advances in horizontal drilling and hydraulic fracturing. While the use of rotary steerable systems, improved bit designs, and more robust downhole tools are vastly cutting the time it takes to drill a well, fracturing techniques will most likely benefit from a wide range of innovative technology within the next five to 10 years.
The key to successful completion is in optimizing the process through proper placement and the number of fracs per well. “Most plays in the US are at more than 10 stages per well,” said Amerino Gatti, vice president, Reservoir Production Group at Schlumberger, in a presentation at Decision Strategies Inc.’s Oilfield Breakfast on Dec. 3, 2010.
In addition, initial production and economic ultimate recovery have increased year-over-year in the double digits due to the advances in the drilling and completion market. “For most authorization for expenditures, 40% to 60% are driven by drilling and completion cost,” Gatti said. By optimizing stages and increasing overall production, operators have vastly improved their investment in unconventional resources.
Not all wells are created equal. Hydraulic fracturing is predominantly carried out using a “factory approach.” By completing wells with an increasing number of stages, the question to ask is, “Is this improving production?”
Gatti addressed this question by sharing two examples taken from production logs of two Eagle Ford wells. In these wells, Gatti said, “21% of the perforation clusters are not contributing any production; furthermore, 30% to 43% from these two specific wells are contributing less than 1% of the production.”
A sampling of 150 production logs from a variety of North American unconventional plays shows that 30% of the perforation clusters as a whole did not contribute to overall production. Not surprisingly, production was shown not to be uniform across horizontal gas reservoirs. “Proper well placement and reservoir characterization are very important,” Gatti said.
Current theories contend that in any given well, some stages are contributing far more than others. Because operators prefer a factory approach to completing each well, crews lack the ability to do more analysis. More information gathered on a stage-by-stage basis could improve production on a well-to-well basis.
Loosely known as a “designer” frac, the technology exists to change techniques for every individual stage of a well. Because this comes at a premium, operator acceptance currently is low. In time, it will be possible to adjust a fracing program on the fly. Although a luxury, fracing each zone differently could be effective for large zones of a particular lithology or where natural fracture networks exist in one zone but not in another. In practice, a designer frac could consist of a slickwater frac in one zone, a hybrid frac in another zone, and a crosslink frac in yet another zone, all in the same lateral. This might be the way to go, but more production data would have to be made available to verify the benefits of designing fracs in this way.
Currently, operators tend not to implement designer fracs in lieu of the more stable, price-consistent factory approach. For the pure production play, the same recipe is used to frac every well. It is important to bear in mind, however, that producing unconventional resources is not like making steel – it is not a manufacturing process. In reality, the rocks are different.
By using a mixture of techniques within a single well, it is likely that each frac stage could begin to pull its own weight within the overall production of the play.
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