Recent advancements in diversion technology and fiber-optic diagnostics have enabled refracturing to become a more predictable and repeatable practice. Halliburton uses a diversion technology that bridges off flow at the fracture face. New completions also are benefiting from this technology in both cemented and uncemented wells, where intra-stage diversion is used to create more transverse fractures per stage than a conventional design would.
Modern refracturing process
Halliburton recently launched a refracturing service that combines subsurface insight with diversion technology and sensor diagnostics. A four-step process is being adopted by operators in multiwell pilot programs. The process involves:
1. Screening the best candidate wells based on both reservoir and completion quality. The company’s local technology teams collaborate with operators to quickly and transparently select candidate wells.
2. Designing the optimal refracture treatment to create new fractures and connect existing ones. Each treatment is designed with respect to the initial completion quality. Three-dimensional discrete fracture reservoir modeling can be used to history-match and predict production.
3. Executing the refracturing treatment to ensure full coverage of the pay zone. Simply bullheading a treatment into a well does not provide control of fracture placement and is one reason why some refracks are not fiscally successful. Older wells typically have portions of the reservoir (called pressure sink zones) with higher levels of depletion than the target intervals. The company employs a process called pressure sink mitigation, which forces fluid away from the old fractures to create a more effective distribution of new conductive fractures.
4. Diagnosing the refracturing efficacy to optimize the refrack design for future wells. A diagnostic plan incorporates basic data collection for every well to accelerate the learning curve.
Each unconventional play has unique motives for refracturing. M. Vincent presented a field study of more than 140 wells in 60 different formations; suggesting there have been successful refracks in every reservoir type, both gas- and oil-bearing formations, including sandstone, shale, limestone, diatomite, conglomerates and coal. This exemplifies the opportunity for operators to rejuvenate production in a variety of assets. Vincent also concluded there have been uneconomic refracks in almost every reservoir type as well. The four-step process Halliburton’s clients are adopting helps to narrow the uncertainty and minimize that risk.
The Bakken Formation is an example of an unconventional play with significant opportunity. The average stage count in horizontal wells has more than doubled while the average mass of proppant pumped has increased by 168% since 2008. The progressive change in completions has coincided with a 35% increase in average well productivity in the first year.
Properly engineered refracturing applications have potential for operators who have vintage completions, wells with low completion efficiency, wells that experienced problems during the initial drilling and completion or wells which never had multistage fracturing.
There are varying perceptions about what is technically considered a refracture operation as opposed to a recompletion. By definition, a refracture is a secondary fracture treatment in the same approximate reservoir volume as the initial fracture treatment, typically after an extended period of production. A recompletion is a more holistic description that may include adding pay intervals or installing new tubular in addition to the refrack.
Further sub-categorization is possible:
- A refracture is designed to access underproduced or noncontributing portions of the reservoir. It can be completed with or without a mechanical isolation;
- A reentry is an operation that installs a liner in an existing openhole lateral followed by multistage refracturing; and
- A remediation is a scenario where the primary completion may have experienced a design or mechanical failure. Often, the affected portion of a lateral can be reperforated and refractured to access additional reservoir previously bypassed.
Refracturing may simply accelerate the rate of reserves recovery (possibly a fiscal failure at low-interest rates), or it can decrease the decline rate and subsequently increase the recovery factor. Three cases studies from the Bakken Formation exemplify each recompletion sub-category described previously. The production profile has been improved with a shallower decline rate, resulting in incremental EURs. For each of these examples, the operator’s original five-year finding and development cost was $20/boe.
A well that was refractured using chemical diverters had an increase in EUR is shown in Figure 1. The EUR increased by 64%. At the date of the refrack there were six wells in the same section with a cumulative production of more than 1 MMboe already extracted.
Figure 2 shows a well that was completed with a 2,591-m (8,500-ft) openhole lateral. Three years later it was reentered to wash and ream the openhole section and install a liner. A multistage refrack treatment was pumped, resulting in a 158% increase in EUR. The capital input is higher in a reentry operation yet still attractive in comparison to the operator’s average finding and development cost.
Another well was recompleted with new perforations added and refractured using chemical diverters had an EUR increase of 397 Mboe, or 76%. This yielded an estimated cost of $5/boe.
These case histories provide evidence that refracturing has a large potential for operating companies to add reserves in existing fields at an exceptionally low unit cost.