Frac packs provide vertical connection
Arthur Barreto Coutinho Neto
Baker Oil Tools
Advancement of open-hole horizontal and frac pack completion technology continues to expand the envelope for developing deep and ultra-deepwater reserves. The application of these technologies differs by global region and is driven by varying reservoir characteristics.
To deliver fit-for-purpose solutions that address the technical and economic challenges in deepwater, it is necessary for service companies to be an integral component of the field planning and development process. One such example is the successful completion of over 65 subsea wells integrating multilateral, open-hole, frac pack, and multi-zone systems in deepwater Brazil where Baker Oil Tools and Petrobras have worked for over five years pushing technical horizons.
In the Gulf of Mexico, frac packs continue to be the primary completion type to minimize risk and maximize production rate. The requirement to deliver high-rate pumping and large volumes of proppant in high permeability reservoirs continues to provide the staging grounds for fast-paced development of tools, vessels, and completion techniques.
In the Campos basin, well completions have successfully incorporated the enhanced open-hole gravel pack systems.
In the Campos basin, well completions have successfully incorporated the enhanced open-hole gravel pack systems.
Open-hole technology has evolved to provide gravel packing of multi-zone open-hole completions with positive zonal isolation and a reservoir barrier for well suspension or fluid loss control all in a single trip.
Single trip systems continue to advance for both frac pack and open-hole systems to minimize rig time and overall operational risk. This technology has been used for completions in water depths exceeding 4,300 ft and open-hole laterals exceeding 7,000 ft resulting in lower development costs/boe.
In the Campos basin, well completions have successfully incorporated the enhanced open-hole gravel pack systems. A number of challenges have been successfully addressed, including:
- Extending horizontal gravel packs into ultra-deepwater environments from dynamically positioned rigs and ships
- Providing better-than-expected productivities while avoiding the use of acid in producers
- Successfully gravel packing single and multi-zone production and injection wells
- Providing service tools with the ability to deliver acid when pulling the service tool out of the gravel pack assembly for filter cake cleanup and maximized injectivity.
Similarly, successful tip screen-out frac pack completions verify the need to select equipment based on the ability to handle the frac operation as a process of closely interrelated activities that impact the other. They include frac fluid selection, pre-frac design, mini frac and data analysis, frac redesign, and implementation.
Both the open-hole and frac pack completion systems share common components inclusive of a weight-down feature, Smart Collet, and a live annulus feature, FAS tool.
To address the design and operational challenges, Baker Oil Tools has developed common system platforms to address the specific requirements for both open-hole and frac pack completions, the CS-300 and CK-FRAQ. Both the open-hole and frac pack completion systems share common components inclusive of a weight down feature, Smart Collet, and a live annulus feature, FAS Tool.
The Smart Collet allows weight to be slacked off in all tool positions to prevent tool movement when working from floating vessels and when pumping high-pressure frac packs. The FAS Tool serves to maintain hydrostatic pressure on the formation for hole stability in open-hole applications and provides a live annulus for frac pack design and evaluation. The FAS Tool is actuated after sand-out is achieved to reverse out excess gravel or proppant.
To assess risk in the design phase and real time, software programs have advanced to complement each specific completion system. XOT Paq, XOT Live, and XOT Move software programs provide design engineers with critical information such as erosional wear, pump rate, proppant loading, fracture gradient limitations, torque and drag, expected hook load weights, and tubing movement during all aspects of the pumping operation.
The decision to complete a well with a horizontal gravel pack or a frac pack is based on reservoir characteristics and well objectives. Horizontal wells are typically used in reservoirs with good vertical permeability where the horizontal section provides access to multiple reservoir lobes or extended reservoir exposure while minimizing potential for water coning. With the ability to provide zonal isolation, extended laterals can lead to increased recovery and an optimized economic well life via intersection of multiple lobes within the same reservoir or multiple productive intervals.
Typically, frac packs are applied to formations where vertical permeability limits the application of horizontal wells, or wellbore construction places multiple targets behind casing. Frac packs provide vertical connection and bypass near-wellbore damage, enhancing the connection to the reservoir to accelerate reserve depletion.
In deepwater, the wellbore intersection of multiple reservoirs and exposure to intervals exceeding several hundred feet of highly permeable rock has driven the design of single trip multi-zone frac systems. This allows the operator to design and complete in one trip specific fracture treatments for selected intervals within the same reservoir and/or multiple sands within the same wellbore. The overall benefit is significant reduction in rig time, zonal productivity, and operational risk reduction due to the reduction in trips required for multi-zone completion installation.
In the Campos basin, Petrobras uses a combination of premium completion tools, premium screens, and gravel packing as the preferred completion type for both the producers and injectors in unconsolidated formations. Additionally, wellbore conditioning after drilling and prior to displacing the hole to brine is enhanced to thoroughly remove the cuttings from the open-hole and riser to prepare the well to receive the gravel pack.
The objective is to provide well-life sand-free production and prevent corrosion when producing fluids or injecting water. Sand screens for this high-risk environment are constructed with a wire cloth membrane filtration layer to provide both sand retention and plugging resistance carried in a rugged protective shroud.
Several advantages are obtained from gravel packing a horizontal well with this process. Gravel packing in brine produces a tighter pack, avoiding "holidays" or void areas in the pack, and maximizes the retained permeability of the gravel by avoiding damaging gels. Using the correct process and tool assemblies, extremely long horizontal wells can be successfully gravel packed with these techniques. Globally, open-hole lateral sections exceeding 7,000 ft (2,133 m) have been successfully gravel packed.
Critical to the success of the deepwater open-hole horizontal gravel pack process employed by Petrobras is the ability of the gravel pack tool system to maintain completion fluid hydrostatic overbalance pressure during all phases of the operation and during all tool movements to hold the filter cake in place.
The CS-300 open-hole horizontal gravel pack system was developed to addresses the critical aspects required to achieve this process. The open-hole gravel pack packers have differential pressure ratings to address both gravel pack and production modes. The standard open-hole gravel pack packers have 6,000-7,500 psi differential pressure ratings in the production mode and 10,000 psi rating in the gravel pack service positions.
The system also incorporates a rotational locking device that permits the screens to be rotated in the open hole when necessary. The CS-300 uses a mini-bypass flow path to maintain designed hydrostatic overbalance pressure across the gravel pack packer assembly during all operational movements.
To address drag and torque issues in deviated wellbores, a soft hydraulic release mechanism is incorporated into the crossover service tool and is activated by applied tubing pressure with little to no set-down weight.
The FAS Tool is part of the CS-300 system crossover service tool. It permits the tool operator to selectively activate, i.e., close flapper valve to control fluid loss from the annulus down the washpipe.
Another feature of the gravel pack system is the Smart Collet multi-acting reposition tool. A device that positively locates gravel pack crossover tools downhole in different positions during the gravel pack operation is necessary to ensure that seals are kept in a static position.
The initial horizontal gravel packs in the Campos basin achieved productivity index ranges from a PI of 36 b/d/psi) to 80 b/d/psi with reported production rates of 21,576 b/d during initial production. Recent open-hole gravel pack completions have achieved productivity indexes in excess of 300 b/d/psi.
Deepwater frac packing
Deepwater frac packs provide additional challenges beyond gravel packing to the completion engineer because of the higher rig costs and larger work string volumes required, coupled with the challenges of fluid cool-down and the associated impact on predicting fluid performance and the work string length changes.
Tip screen out (TSO) fracture treatments are applied to deepwater completions for several reasons: to bypass near-well bore damage; to provide vertical connection of laminated sands, and to stimulate low-permeability reservoirs.
In high-permeability reservoirs, short fracture lengths of several feet, i.e., 20-40 ft, bypass near-well-bore damage caused by drilling mud, perforating debris, fluid loss pills, and completion fluid losses. TSO fracture treatments may be used to create longer fracture lengths of 100 to 200 ft to stimulate lower-permeability reservoir or to create higher fracture heights.
Improving the process
When completing successful frac packs in soft rock formations, achieving a TSO is critical to developing a conductive frac back to the producing wellbore. In review of four unsuccessful attempts by an operating company to obtain a TSO frac pack, evaluation revealed that basic frac pack principles had not been applied to the deepwater well, resulting in failure to obtain a TSO event. An intensive review process consisted of examining previous treatments, evaluating logs, studying laboratory reports, and interviewing operations engineers.
A number of frac pack process improvements emerged from this review. Areas for improving frac pack placement centered around recognizing the low fracture rate of the formation, using spotting fluids, and using shut-downs to let the formation close before proceeding to the next treatment phase.
The system selected to improve the frac packing process was the Baker CK-FRAQ system equipped with Smart collet and FAS Tool as previously described. The system provides three positions:
- Spotting-reverse position
- Circulating position with weight down on the crossover-service tool
- Squeeze position with weight down on the crossover-setting service tool.
A deepwater gas well was selected to implement the revised pumping procedures. Conditions were as follows:
Using a frac pack tool with a weight-down capability in circulating and squeeze positions enabled using the casing-workstring annulus as a dead string to monitor real-time formation pressure response via the washpipe. This feature allowed the frac team to identify the TSO event and decide to extend the proppant stages. The treatment obtained a TSO and built 600 psi net pressure gain after the TSO event, not accomplished on four previous attempted frac packs. Production results were less than expected, attributed to a low 3-md permeability and a positive system skin, producing less-than-expected gas rate with some water. The system had a skin of + 8 to + 10. The skin was probably related to the small 50-70 mesh ceramic proppant used. Later treatments used 30-60 mesh ceramic proppant and responded better. Most frac packs use 20-40 mesh proppants for enhanced conductivity.
The latest technical advancements, design software, gravel and frac pack packer design, and process improvements, have enabled the application of well life sand control methods for asset optimization and operational risk minimization in deep and ultra-deepwater.