Collaborative refracturing workflow aids predictable, repeatable recovery of bypassed reserves

Halliburton's ACTIVATE service allows bypassed reserves to be recovered consistently and repeatedly.

Five years ago, unconventional resource exploration and development in North America were driven by high service intensity, horizontal drilling and completions, and high oil prices. However, given current low commodity prices, operators are seeking innovative, economical and profitable methods to increase incremental estimated ultimate recovery (EUR) and reduce the break-even cost per barrel of oil equivalent (BOE). Modern advancements in engineering technology—particularly, diversion technology, design methodologies, and fiber-optic diagnostics—have contributed to making refracturing a more predictable, repeatable practice.

Subsurface insight data indicate that legacy fractured wells still contain significant amounts of bypassed reserves. Typical fracturing challenges, such as understimulated stages, insufficient fracture initiation point count spacing, and loss of connectivity to the original propped fractures, made it very difficult to exceed recovery factors of 4% to 8% in several liquids-rich basins. With most of the reservoir unexploited, a large opportunity exists to refracture thousands of wells over the next several years. Refracturing is now ushering in a new era of unconventional field development, and providing a method for securing reserves economically through incremental recoveries from existing drilled laterals. It also protects new well performance when integrated as part of infill development schedules.

COLLABORATIVE WORKFLOW

As a means of predictively recovering bypassed reserves from unconventional reservoirs, Halliburton has introduced ACTIVATE refracturing service, a collaborative workflow that leverages subsurface insight expertise, and breakthrough stimulation diversion processes and technology. The service has been performed repeatedly at approximately one-third the cost of a new development lateral. Predictable refractured wells can enable operators to build a balanced portfolio of new wells, infills and refractures, while increasing recovery factors in the entire field by up to 25%, and lowering costs per BOE. The refracturing service involves four steps:

1. Screen the best candidate wells, based on reservoir and completion quality.
2. Design tailored refracturing treatments to reconnect existing fractures, and induce new fractures through existing and new perforations.
3. Execute the refracturing treatment for full lateral coverage.
4. Diagnose the refracturing impact to bypassed reserves and optimize the refracturing design for future wells or pads.

Step 1: Candidate selection. Even with advancements in fracturing diversion technology and proppant schedule design, the importance of candidate selection should not be overlooked. The combination of successful candidate screening, ranking, and pilot well identification, with tailored refracturing processes and designs, is important for narrowing the window of uncertainty associated with refracturing.

Working quickly and transparently, Halliburton’s local technology teams collaborate with operators to select candidate wells with the best reservoir, wellbore and completion quality. The goal of the screening process is not to identify an appropriate wellbore for refracturing; rather, it is to rapidly categorize and accurately high-grade and force-rank reservoir and well properties, to identify a series of refracturing candidate wells or pads. 

The primary candidate selection drivers for pilot refracturing treatments have been completion design improvement and a rigorous candidate selection process, relying on a learning curve established across several unconventional plays. In many fields, there is a significant inventory of wells that is understimulated in terms of fluid volumes, proppant amounts and fracture spacing, compared to current completion practices.

Many operators observe that using more proppant and narrower stage spacing increases recovery. In some cases, more recent stimulation and completion efforts achieve twice the expected recovery of older completion methods, suggesting that a considerable amount of bypassed reserves remain in existing drilled laterals. Additionally, permanently deployed fiber-optic monitoring, with distributed acoustic sensing/distributed temperature sensing (DAS/DTS), has presented evidence that during stimulation treatments, a significant number of perforation clusters in each stage do not receive the appropriate allocations of fracturing rate and fluid volumes, thereby increasing the effective fracture spacing along laterals. An opportunity is, therefore, present to stimulate
bypassed reserves.

Several factors impact bypassed reserves:

• The “learning curve” design progression over historical time has left many laterals understimulated.
• Poor fluid exit-point efficiency and distribution of proppant along laterals; clusters that either never took fluid at all, or that screened-out before pumping the designed volumes of fluid and proppant.
• Interaction of development well stimulation treatments with the drained reservoir volume associated with delineation wellbores (also called “well bashing”), which can decrease the overall, stimulated reservoir volume and leave far-field bypassed reserves.
• Production damage mechanisms, resulting in loss of fracture conductivity.
• High draw-down rates associated with poor choke management practices.

A sequential filtering process is used to group the candidates. Reservoir quality is a crucial, first-pass filtering regime, which uses both quantitative and deterministic screening processes. A matrix of decline-rate analyses, production analytics, and original completion and stimulation data, are used to force-rank properties. The result of this streamlined process is that it force-fits root cause(s) of initial underperformance into a limited number of industry-accepted groups, such that:

• Potential solutions and improvements directly address the most likely root cause(s) of the original well’s underperformance.
• Windows of uncertainty are narrowed by filtering out candidates with low-level contributing causes, and focus is placed on root causes with the highest probable impact on recovery factors and acceleration of reserve recovery.

Extrinsic factors are also considered. For example, a refracturing treatment might be performed to pressure up depleted fracture networks associated with an existing parent delineation well before new infill or development well completion(s). This would help prevent asymmetric fracturing into the lower-pressure, lower-stress environment of the parent well, which can result in a downgrading of expected infill recovery.

Rigorous fracture and reservoir modeling of the top pilot candidate(s), involving production history matching, forward modeling and fiscal forecasting, can be performed to quantify the risk window and examine the potential production improvement profile. Finally, the mechanical integrity of the selected wells is evaluated in detail, to help ensure that they can withstand the pressures to which they would be subjected during the refracturing treatment.

Step 2: Design. The main goal of a refracturing treatment is to promote conductive contact with productive reserves that might have been bypassed during the original completion. Pre-existing fractures can be reconnected near the wellbore, or new perforation clusters can access reserves between the original clusters along the lateral.

Often, the set of existing fluid exit points along the lateral is associated with volumes of higher-pressure depletion occurring along the lateral, called pressure sink zones. Severely depleted reservoir volumes can actually present a challenge, with respect to contacting new zones with higher reserve recovery potential. Halliburton’s proprietary process, called pressure-sink mitigation (PSM), was developed to help stem losses to these zones, enabling full lateral connections to be made in the higher-pressure rock volumes and unlocking significantly more bypassed reserves than legacy refracturing methods.

A formation evaluation technique is then used to help maximize contact with previously unstimulated areas of the formation, and pinpoint the best locations to place new perforations. The technique also is able to efficiently grade the reservoir and natural fracturing attributes along the lateral and provide information about the stress contrast between existing and new clusters. This process helps tailor refracturing treatment schedules to each of the identified stimulation targets. Specific refracturing objectives along a given lateral are identified with a combination of technologies, including (but not limited to) the following:

• Near-wellbore diagnostics
• Production logging, conventional or coiled-tubing production profiling
• Basin knowledge and a vast library of previous fracturing diagnostics for specific fields and basins
• Three-dimensional seismic survey and/or microseismic response.

This service provides valuable subsurface insight that is essential for tailoring the refracturing treatment volumes and schedules, to achieve the design goals of maximizing the return on investment (ROI) and stimulation of remaining reserves for each pilot. Forward fracture and reservoir modeling with CYPHER seismic-to-stimulation service can be used to understand how the reservoir might react when subjected to a refracturing treatment, and to quantify the expected incremental EUR potential.

Step 3: Execution. After the stimulation targets have been generated, and the restimulation is designed, the treatment program is performed to promote adequate coverage of the lateral. The broad-scope challenge is to manage all significant variability in stresses, and preference of fluid-flow paths in the well, to deliver the correct proppant schedules to the target zones, based on their bypassed reserve potential. Several new strategies have been developed to achieve this objective, while helping to mitigate the overall treatment risk.

The goal is to integrate technical solutions into the design that address several challenges: maintain the connection to stimulation fluid exit points along the full lateral during the treatment; control leak-off; and facilitate conductive fracture placements. Efforts to achieve these have led to a “factory-friendly” solution that is predictable and repeatable across a wide set of candidates.

Historical refracturing treatment diagnostics have helped identify root and contributing causes of technical challenges related to refracturing underperformance:

• High pressure and stress contrast between unstimulated clusters and pre-existing fractures
• Excessive leak-off, causing proppant fill effects
• Poor diversion
• Optimized timing of proppant and pumping schedules, as stress conditions and fracturing locations change.

If any of these challenges are not addressed, the less-than optimal coverage patterns, mentioned previously, can occur.

Wellbore preparation. Preparing the wellbore before performing the refracturing treatment is essential for success. If a too-large fracturing location is created in the heel, for example, it can be difficult to achieve effective diversion and avoid the heel-dominant pattern. The PSM fracture-diagnostic diverter sequence controls leak-off to pressure-sink zones, enabling connections farther down the lateral.

By initiating diversion and/or leak-off control before the main refracturing cycles, the non-planar dominance can be mitigated, to help improve the accuracy of each proppant cycle placed, beginning with the initial one, while also reducing proppant fill effects in the wellbore, resulting from excessive leak-off into the existing fractures. Therefore, formation preparation may be the most important factor for ensuring positive treatment outcomes by mitigating proppant fill and achieving adequate lateral coverage of the higher-stress new zones. If PSM is not performed upfront, the initial fracturing cycles can be placed over a large flow area (i.e., a low rate per cluster), which typically results in poor diversion and less-than-optimal stimulation.

On the other hand, if too much initial diversion is performed, some potential to re-energize and establish connection to existing productive fractures is lost. Therefore, determining the sweet spot for sealing depleted pressure sinks; controlling leak-off; building pressure support; and establishing a dominant fluid exit location on the initial cycles are the main technical challenges. Once a primary fracturing location is achieved, the diversion sealing performance is high, and specific fluid sequences, process controls and appropriate cycle intensity are used to achieve incremental coverage across all desired zones.

Cycle intensity and progressive proppant placement. Because not all exit points in the well contain the same bypassed reserve potential or require the same degree of stimulation, pumping equally-sized stages of proppant, which is typical of new well stimulation, is inefficient. For refracturing treatments, a new approach has been developed to more appropriately proportion stimulation materials, based on the bypassed reserve potential of pre-existing zones and new zones contacted in the well. It is important to note that the relatively higher pressure and stress reservoir volumes will likely contain the highest bypassed reserve potential, so contact with them becomes a priority.

By progressively tuning the proppant schedules, volume fractions, and total materials pumped over time, the most productive fractures can be placed consecutively during each cycle, and in the most economic manner. The progressive-proppant-per-cycle approach typically divides the total planned proppant amount into the following sequential schedules:

• Short cycles, which target repressurization of pre-existing propped fractures.
• Intermediate cycles, which target existing understimulated clusters.
• Long cycles, which target new zones.

Ultra-dynamic diversion. AccessFrac diversion spacers, containing a blend of degradable materials, are pumped on demand to isolate fractures in the well. Each spacer uses optimized process controls, along with precisely designed material volumes, to temporarily seal off dominant fracturing locations (i.e., path of least resistance), providing lasting pressure support for diverting the treatment, to place new fractures in unstimulated areas that require higher pressures to break down and accept fluid flow. The materials degrade completely with time and temperature, and leave no residual formation damage.

Diversion treatments of varying size are performed, based on formation response, initial baseline conditions, original fracture and well spacing, treatment goals for infill protection, and real-time well interference monitoring feedback. The diversion element incorporates process controls, beyond using the diverter alone, to precisely focus each cycle in productive rock. The goal is to place conductive fractures in the reservoir with appropriate cycles of fluid and proppant to fully stimulate the zone of interest, and help prevent bypassing reserves.

Design tools and contingency planning allow the treatment to be adapted in real time to changing reservoir conditions, improving service delivery and helping to mitigate risk.

Fig. 1. Pilot Well 1’s full lateral-stimulation service treatment plot, showing 6.8 million lb of total proppant placed over 20 proppant cycles, each separated by an ultradynamic diversion spacer or isolation.

With intense quality control of the placement of each sequential proppant cycle and improved diversion performance, an important parameter for a successful refracturing treatment then becomes the required cycle intensity to contact all of the desired reservoir volumes. Using these solutions together allows the industry to take advantage of substantial improvements in refracturing practices.

Step 4: Diagnostics are performed to measure impact to bypassed reserves and to capture learnings to help improve refracturing designs for future applications, which will enable expansion of the number of refracturing candidates for larger field development campaigns. FiberCoil can be used after a refracturing treatment to evaluate the post-treatment production profiles of existing and new zones along the lateral, which can then be compared with the pre-job results to accurately quantify success. Additionally, it can provide valuable information before, during and after the refracturing treatment, such as depleted zone characterization and PSM guidance. It also can be used as part of the well-interference monitoring service to increase real-time decision-making capabilities and support designs for pad refracturing treatments.

Fig. 2. Pilot Well 2’s full lateral-stimulation service treatment plot, showing 6.4 million lb of total proppant placed over 26 progressive proppant cycles, each separated by an ultradynamic diversion spacer or isolation.

After the refracture is completed, further subsurface insight is obtained to measure the performance of the stimulation using integrated sensor diagnostics (ISD). Downhole microseismic monitoring (MSM) and near-wellbore fiber optic monitoring provide information in real time, to evaluate fluid distribution in or out of each cluster, and help optimize refracturing treatment cycles and diversion planning. Results from the sensor data are compiled to quantify all aspects of the stimulation treatment, and draw logical conclusions with respect to optimizing well and fracture spacing.

EAGLE FORD CASE STUDY

A series of pilot refracturing treatments was conducted across a broad set of Eagle Ford horizontal wells. The two case study wells had differing attributes and were analyzed using publicly available production data to evaluate the results of tailored refracturing treatments. The primary candidate selection driver for the pilot refracturing treatments was design improvement progression across the field over historical time. It left the wells understimulated in terms of proppant volume and effective propped fracture spacing, compared to modern completion practices.

Fig. 3. Improvement in production, post-refracturing, for Pilot Wells 1 and 2.

Pilot Well 1. Halliburton pumped 6.8 million lb of proppant during a 20-cycle AccessFrac stimulation service treatment across the existing perforations, Fig. 1. The 30-day average output after the refracturing treatment was more than 500 bopd, which was 69% of the original 30-day initial production (IP) for the well. The refracturing treatment provided an incremental EUR estimate of 560,826 BOE, or a 126% increase compared to the original projection without the refracturing treatment.

Pilot Well 2. Halliburton pumped 6.4 million lb of proppant over a 26-cycle AccessFrac service treatment, with a significant number of new perforation clusters added along the lateral, Fig. 2. The 30-day average output after the refracturing treatment was more than 500 bopd, which was 96% of the original 30-day IP for the well. The refracturing treatment provided an incremental EUR estimate of 421,158 BOE, or a 116% increase compared to the original projection without the refracturing treatment.

CONCLUSION

Each refracturing treatment requires customization tailored to accessing the remaining bypassed reserve potential, to achieve successful incremental recovery, economic performance and protection of offsets. Results from pilot wells using the ACTIVATE service indicate that bypassed reserves can be consistently and repeatedly recovered across a broad set of candidates at a lower cost per BOE compared to the development costs for new drills. 

Leveraging these learnings, operators now can create a balanced portfolio by integrating refracturing treatment applications with their new wells and infills. This larger-scale refracturing, field development strategy can enable operators to incrementally increase reserves and reduce costs per BOE for their specific acreage. By integrating refracturing treatments with delineation and development wells, operators can increase recovery factors for the
entire field.