Technology Update; Sequenced Refracturing Technology Improves Economics in Unconventional Plays
- Andrew Acock (Schlumberger) | Brian D. Clark (Schlumberger)
- Document ID
- Society of Petroleum Engineers
- Journal of Petroleum Technology
- Publication Date
- September 2015
- Document Type
- Journal Paper
- 28 - 31
- 2015. Copyright is retained by the author. This document is distributed by SPE with the permission of the author. Contact the author for permission to use material from this document.
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Under current market constraints, many operators and service companies are trying to determine how to efficiently and cost-effectively refracture declining and/or underperforming shale wells. Companies are investigating new technologies and creatively adapting existing techniques to improve refracturing operations.
Historically, hundreds or thousands of laterals in unconventional plays were stimulated with suboptimal practices or suffered damage, limiting access to significant remaining reserves.
Typically costing a fraction of the amount required to drill and complete a new well, refracturing represents an attractive opportunity for operators to increase production, ultimate recovery, and return on investment.
Problems With Refracturing
While refracturing vertical wells is not new, the industry has limited experience with refracturing horizontal wellbores and the technique is still in its infancy. Today, the common practice is to assume that if a treatment design succeeded in the original lateral, one needs to only apply essentially the same formula in refracturing the well. Instead of using mechanical bridge plugs, which are no longer a viable option, a diverting agent is pumped between each proppant fracture stage.
Diversion materials typically consist of degradable particles intended to plug existing fractures in low-pressure depleted zones and divert proppant to untreated, high-pressure sections of the lateral. Although some horizontal refracturing jobs have achieved good results with this approach, there are two serious potential technical shortcomings. One is ineffective diversion across the full length of the lateral. The other is inadequate proppant transport inside the lateral.
Conventional diverting materials have not proven reliable, often failing to isolate existing fractures. Particle sizes have not been optimized to effectively plug near-wellbore fracture openings, or the particles become strung out during pumping and thereby prevent diversion pills from achieving sufficient concentration to create impermeable barriers. As a result, portions of the subsequent proppant treatments take the path of least resistance by flowing into depleted areas and failing to attain the pressure required for fracturing any unstimulated rock.
There is another, less obvious problem with traditional refracturing of horizontal completions. As proppant-laden fracture fluids travel toward the toe of the lateral, fluid fractions enter multiple hydraulic fractures along the way and continually slow the stream.
As fluid velocity diminishes, the proppant begins to settle out and accumulate inside the casing, creating a sand dune that progressively fills the lateral. Few operators realize that hundreds or thousands of feet of casing may be clogged with sand. Inadequate proppant transport and sand settling may explain the observation that many refracturing operations preferentially stimulate the wellbore closer to the heel.
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