Expandable sand screens are a cost-effective sand control solution for gas storage wells

Types of underground natural gas storage facilities. Source: PB-KBB/EIA.

In the Northern Hemisphere, natural gas production typically exceeds consumption from April through October, and gas is placed in storage. When the demand for natural gas exceeds production, typically from November through March, it is withdrawn from storage. Each winter, about 20% of all gas consumed is derived from underground gas storage (UGS) wells. This enables operators to efficiently regulate the flow of gas to customers, even if production is disrupted temporarily. UGS wells also help interstate pipeline companies to balance system supply on their long-haul transmission lines.

Withdrawal strategies. There are two primary withdrawal strategies for natural gas. The first one is focused on the annual summer and winter cycling scenario, where there is typically one production cycle in winter and one injection cycle in summer. This method involves relatively few injections/withdrawals, but these operations produce large pressure variations. The second strategy is peak shaving, which allows operators to satisfy peak demand that is not serviced by existing infrastructure. This method involves many small pressure variations each year.

Formation damage. There are several geological structures that are well-suited to accommodate UGS wells, including natural or man-made salt caverns, depleted oil and gas reservoirs, and aquifers. However, the consistent injection and withdrawal can have negative effects on storage formations. Over time, the cyclic production and injection reduces reservoir pressure and increases stress on the formation, which can lead to borehole collapse. The formation sand, and solids associated with these failures, can be produced through the wellbore and erode surface facilities, impair gas-rate productivity, and lead to the loss of the storage well. Operators can manage produced solids by capturing them at the surface or retaining them downhole.

In high-flow rate gas wells, retaining produced solids downhole is the preferred method. There are three primary methods for controlling produced solids in UGS wells. These include cased-hole/open-hole gravel packing, and expandable sand screens.

SOLIDS CONTROL: TECHNOLOGY OVERVIEW

In almost every case, techniques for controlling downhole sand production involve the installation of a pipe-based filter that is either gravel-packed or left as a standalone screen. Gravel-packing techniques have evolved over the last 70 years, but they still involve complex fluid- and gravel-pumping operations. Standalone screens, while simple to install and operate, often do not adequately support the formation, which can undermine their ability to contain sand production reliably. Expandable sand screens were developed to overcome the shortcomings of gravel packing and standalone screens in certain applications, while also providing some unique benefits.

EXPANDABLE RESERVOIR COMPLETION SYSTEM

Fig. 1. The sand screen is composed of a multitude of layered systems and technologies, to ensure longevity and maximize performance.

To reduce costs and increase downhole solids retention efficiency, Weatherford offers a reservoir completion system that features expandable sand screen technology (ESS). The system can be configured with swell, or cup-sealed packers for zonal isolation, in addition to an appropriate length of blank pipe. Components are modular and can be customized to conform to specific reservoir characteristics. A proprietary single-trip hanger allows for deployment and subsequent expansion of the screen technology. The expandable slotted base pipe is covered longitudinally with four overlapping sheets of filter media, which is composed of woven wire mesh, and placed between the base pipe and protective outer shroud, Fig 1.

The screens are composed of various compounded alloys to achieve the desired metallurgical properties to maximize longevity and performance under different downhole conditions. They are available in four mesh sizes, ranging from 120 to 270 microns. Filter tests have shown that careful selection of aperture size for a given sand mixture minimizes plugging and controls sand production by filtering out all but the smallest sand grains.

Fig. 2. The ESS is flexible and readily expands to fully comply with irregular and non-uniform wellbores.

ESS adjusts to non-uniform boreholes. One of the primary advantages of ESS technology is that the screens expand compliantly against the entire diameter of the borehole and adjust to variations in hole shape, even in non-uniform wellbores, Fig. 2. To initiate expansion, a compliant expansion tool is driven through the unexpanded base pipe. Screen expansion is achieved by applying a combination of weight-on-bit, backpressure and downward pipe movement. The inside diameter (ID) of the base pipe expands slightly more than the outside diameter (OD) of the expansion tool.

During expansion, the base-pipe slots open to maximize the inflow area. Overlapped sheets of filter media unravel to form a single layer of screen to provide sand control. In spite of expansion in the base pipe, the aperture size of the filter media is not altered.

Operational advantages. The benefits of using expandable screen technology are well-documented, especially in long, single-zone reservoir sections. The elimination of the annular gap by compliant expansion prevents crossflow of solids and particle migration, which can lead to filter plugging, localized flow concentrations, and consequently erosion of the screen. The system’s large, exposed filter surface also reduces the volume of debris and the risk of damage, thereby maximizing production and injection rates.

Additionally, the large ID of the screen facilitates completion optimization and allows for the possibility of installing an intelligent inner string, as well as providing ample space for any subsequent remedial interventions. The ID also reduces friction-induced pressure losses and promotes a more uniform inflow profile during production, and outflow profile during injection operations.

Single-trip installation. Because the screen can be installed in a single trip, ESS technology enhances operational simplicity, minimizing installation time and associated costs. Expandable screen completions do not require displacing the wellbore to brine, which results in significant cost-savings. Finally, rig pumps can be used to create the necessary backpressure for tool expansion, eliminating the need to transport pumping equipment to the location.

Reliable-repeatable. Expandable sand screens have been applied successfully with minimal interventions in a variety of harsh downhole applications, including wells flowing at 30,000 bopd, in gas wells with production up to 370 MMcfd, and in water-injectors with input of 65,000 bpd. ESS technology has proved reliable in deepwater operations, HT wells, and in mature regions where they have been in service for more than 15 years. To date, the service provider has deployed ESS technology globally in more than 770 wells with approximately 450,000 ft of screens installed.

UNDERGROUND GAS STORAGE WELLS

Early UGS wells typically used completion systems that incorporated traditional cased-hole and open-hole gravel packs. However, when injection wells were completed using these techniques, the gravel pack and the near-wellbore formation would become fluidized. When operators switched from an injection cycle to a production cycle, the gravel pack returned to its previous packed condition, but formation fines remained trapped within the gravel and resulted in mechanical skin that reduced permeability in the near-wellbore area and risked plugging the formation matrix. With each subsequent injection, the migration of fines into the pack increased.

Also, UGS reservoirs are often depleted and highly overbalanced during drilling, which can result in a narrow window between the hydrostatic pressure and fracture pressure of the rock. When installing completions in depleted reservoirs, low fracture-gradients can exclude a gravel packing option, because overbalanced pressure can damage the formation, induce losses, and result in premature bridging.

Expandable screen solution. To solve the operational issues, in addition to reducing installation and intervention costs, operators began experiments with expandable sand screens in UGS wells.

Fig. 3. Produced formation fines pass through the ESS weave, mitigating the mechanical skin issue prevalent with gravel-pack completions.

ESS technology avoids the produced fines issue by enabling the near-wellbore formation to become fluidized during the injection cycle and allowing the formation to pack around the expandable screen, once the injection cycle ends. The fines are able to pass through the ESS weave and do not induce the mechanical skin common with gravel-pack completions, Fig. 3. This enables the operator to maintain sand control capabilities over time.

Also, ESS technology can be installed efficiently in wells with low fracture-gradient and pore pressures. A primary objective in UGS operations is to achieve sustained sand control, maintain consistent injection and depletion rates with fewer wells, and reduce the time required for injection and withdrawal. The large, exposed filter surface provided by ESS technology enables operators to inject and withdraw gas at higher and more consistent rates than conventional sand-control techniques allow. ESS technology is proven to maintain injectivity and depletion rates over time in UGS wells.

Finally, many UGS wellsites are remote and not amenable to sourcing and delivering of the pumping equipment required for gravel-packing operations. Gravel-pack installations are also inherently more complex. In contrast, ESS operations consume a small footprint at the wellsite and can be installed quickly and easily.

North American case study. In an onshore U.S. application, an operator sought a reliable sand control technology to cycle gas injection and withdrawal at higher rates than allowed by conventional sand control techniques. Another major objective was to reduce risks associated with underreaming wells for traditional open-hole gravel packs.

To date, five ESS systems have been installed in re-entry UGS wells for the client. Three of the installations took place in 2006 and are still outperforming wells using conventional sand control techniques. In response, the operator requested two more ESS systems in 2015. The company completed four 6 1/8-in. open-hole wells with 4 1/2-in. ESS joints, and one 8 1/2-in. open-hole well with 7-in. ESS joints. The five open-hole wells included trajectories with deviations ranging from 29° to 85°. The average hole length is 388 ft, with an average well depth of 8,360 ft.

Since installation, pressure cycles in the UGS reservoirs have ranged from 1,200 psi to 3,600 psi without any compromise in sand control capabilities. Additionally, the UGS wells have undergone multiple injection and production cycles with zero sand production. Gas production rates have reached 35 MMcfd.

Without operational complexities or the need to underream the wells, the operator has significantly improved production, saved time and reduced costs while keeping the five wells free of sand and minimizing completion skin.

Austria case study. An operator sought to control sand production in an onshore UGS well in Schoenkirchen field, Austria. They also wanted to maximize well productivity and operational efficiency. The service provider considered various completion options, including standalone screens and gravel packing. However, the engineering team selected ESS technology for its capability to expand compliantly, remove the annulus and stabilize the formation face. Additional benefits included a fullbore ID, lower deployment risks, and optimized displacement and cleanup of drilling mud.

The well’s hole size is 8 3/8 in. with an 87° inclination that required 5 1/2-in. ESS joints to a TD of 4,970 ft. A 9 5/8 × 7 5/8-in. EXR single-trip hanger was set at 4,518 ft and spaced with 230 ft of 7 5/8-in. blank pipe for a total length of 163 ft. A post-installation log confirmed the compliant expansion of the ESS system against the wellbore. The single-trip installation and expansion of the ESS completion reduced the rig time required, if a gravel pack was deployed.

The increased ID enabled the well to produce at a significantly higher flowrate compared to an offset well in the same field with an open-hole gravel-pack completion. During the winter period, natural gas flowed from the well at 32 MMcfd, a new field production record.

ESS EFFICIENTLY MAINTAINS UGS INFRACTURE

UGS wells are a critical link in the natural gas supply chain, and often require remediation to mitigate sand production and the associated risks to equipment, gas-rate productivity and well integrity. Compared to conventional cased-hole/open-hole gravel packing and standalone screens, expandable sand screens have proven to be more efficient and less costly in remediating UGS wells. They are also highly effective in controlling sand production over long periods of time without intervention. The ESS system provides a simple, yet comprehensive solution for eliminating annular gaps, preventing the migration of sand, and maximizing injection and withdrawal rates in underground storage wells.