Good formation access is the key to expanding stimulated reservoir volume and increasing production from hydraulic fractured, unconventional wells.
In Europe, using long perforated intervals for these treatments in tight-gas operations in wells has led to perforation “screenouts.” Higher fluid viscosities in combination with larger pad volumes were often used to avoid these screenouts, as they were thought to be related to the creation of multiple fractures with less fracture width development. Dr. Stephen Holditch, a petroleum engineering consultant, has addressed perforation issues in his 20 golden rules for fracturing tight-gas wells. Rule 18 states: “No well is straight and no well is vertical. As such, the chances that multiple fractures will occur increase with the length of the perforated interval. A perforation interval of 20 feet or less, located in the most porous and permeable zone, should be sufficient for most cases. A properly packed fracture will connect the entire pay interval to the wellbore, assuming the treatment is pumped correctly and is not over-flushed.” The 20-ft perforated length focuses the energy to create one single frac that has enough width to accept the designed size and quantity of proppant.
Horizontal drilling, which became common in Europe three decades ago, uses multistage frac treatments. In the 1980s and 1990s, deep, horizontal tight-gas wells were stimulated based on a perforated length of four times the casing/liner diameter in order to create single dominant fractures. Meanwhile, in the U.S., openhole horizontal wells were being treated at rates exceeding 300 bpm to achieve stimulation of the whole lateral length. Jim Surjaatmadja, a Technology Fellow with the Halliburton Technology Center in Duncan, Oklahoma, introduced SurgiFrac® Service, a technique, where dynamic diversion does not require the use of plugs in between treatment stages – meaning that high rates are not required to create a multitude of fracs in openhole scenarios.
In unconventional wells requiring horizontal drilling and fracturing, maximum reservoir contact is used to overcome the hydrocarbon flow restriction associated with very-low-permeability rock. The plug-and-perf process is the most-used completion option, as is the use of single or multiple operated sleeves. Another option, also based on Jim’s work, involves pinpoint stimulation – hydrajetting and stimulating one single frac per each pumping operation. After treatments, a wellbore is left with many holes open for flow. Some completions, especially near water-bearing zones, allow for mechanically closing sleeves to isolate certain sections of the horizontal wellbores.
To determine if not all perforations are producing, operators can use production logging tools, microseismic, and fiber-optic cables (FiberCoil™ tubing) for distributed temperature sensing (DTS) and distributed acoustic sensing (DAS). Causes for nonproducing perforations may include differences in entrance hole diameters (EHDs), perforations in the wrong areas, and stress variations that are impeding perforations from contributing to flow. Thus, to help operators reduce EHD issues, MaxForce® Perforating Shaped Charges are designed to maximize hole-size performance while maintaining entry-hole consistency in the casing. Additionally, FracInsight® Analysis helps operators solve geosteering complexities, optimize treatments, and select the best zones for perforating the target zones.
Refracturing horizontal wells can also be considered for increasing the number of producing perforations and expanding the stimulated reservoir volume. AccessFrac® stimulation service is used to increase the number of effectively stimulated fractures for maximized cluster efficiency and well performance. Additional perforations can be shot in areas that were bypassed before. Diversion is required to plug off depleted sections of the horizontal wellbore and to contact more perforations and more reservoir areas. The diverting materials used for both versions are biodegradable particulates that self-degrade in water.
By using the latest developments in perforating and sleeve technology, in combination with the right diverter material, better access to the formation can be obtained, leading to increased production and lower BOE costs in unconventional wells.
SPE 154397 Overdisplacing Propped Fracture Treatments-Good Practice or Asking for Trouble? Klaas A.W. Van Gijtenbeek, Halliburton; Josef R. Shaoul, Hans J. De Pater, Fenix Consulting Delft
SPE 174757 Optimized Fracture Stage and Perforation Placement in Horizontal Wells using a New Calibrated Pulsed-Neutron Log Workflow – Erek Hutto, Dan Buller, Bill Johnson, Chris Mcilroy, Halliburton; Jeff Dahl, James Samaripa, John Spaid, Devon Energy)
“Methods Improve Stimulation Efficiency of Perforation Clusters in Completions” by Stephen Ingram, Matthew Lahman, and Stephen Persac. O414-0032- Journal of Petroleum Technology (JPT), SPE Journal Paper-2014
SPE 174173 Operator Uses Advanced Perforation Flow Laboratory to Support HMX Perforating by Coiled Tubing in HPHT Field – D. Haggerty, Halliburton; S. Christie, DONG Energy