By Glen Robitaille, Weatherford International Ltd
The introduction of swellable elastomer packers to the market in the early 2000s provided operators with a wider set of technology options for zonal isolation in multizone fracture stimulation. Swellable packers offer a less expensive and often more effective alternative to traditional cementing and mechanical and open-hole packer methods.
When an onshore gas well in the Sichuan province of China was badly damaged by a catastrophic earthquake, a major Chinese operator sought out only those service companies offering swellable products. The operator ultimately went with Weatherford International’s Nemisis annulus swellable packer (ASP) and was able to successfully recover the well.
As the industry’s knowledge of and familiarity with swellable elastomers increase, so will the material’s application in the oil and gas industry. Still in the early stages of development, this emerging technology has many growth opportunities.
Effective well construction requires wellbore isolation that is versatile, reliable and cost effective. This is a difficult task because every well is different and, if those variations are not addressed, well performance suffers.
Isolation performance is complicated by a diverse scope of applications, from the most common wells to complex wellbore designs in extreme environments. Issues range from short-term stimulation concerns to long-term microannulus isolation. Differences in wellbore fluids affect swelling and compatibility, thereby complicating planning. Swellable elastomers provide solutions for many of these concerns.
Swellable elastomers are polymers that swell proportionally when immersed in well fluids (water, oil or a mixture of both). The swelling process is continuous until a swell limit is reached through either spatial confinement or internal rubber stress. Swellable packers are engineered so that swelling occurs predictably and only in response to specific well conditions, well fluids and temperature conditions. This flexibility greatly enhances planning and performance.
Swellable technologies are increasingly building a reputation in the oilfield, such as seen in the uptake of swellables for horizontal multizone completions in the US Rockies region. Prior to the advancement of swellable packers, elastomers were thought to be an incompatible material for the oil and gas industry on account of their unstable properties. Once engineers recognized the value and embraced the material’s natural tendency to expand, new discoveries in oilfield application were made.
Using elastomers to produce swellable packer systems has created more technology options for zonal isolation in multizone fracture stimulations. Although the traditional options of cementing and mechanical or open-hole packers are still frequently used, swellable packers offer many advantages that can save time and reduce costs while increasing performance. When dealing with oddly shaped holes, horizontal sections, or extreme or fragile environments, swellables are often the better method.
Swellables are easier to implement than other isolation options and can be a cost-effective solution. Swellable packers are one-trip, self-setting products without any mechanical parts and do not require a specialist for operation. This simpler approach minimizes costs for personnel and reduces time required.
Swellable packers have been found to perform well in unconventional conditions, such as in irregularly shaped wellbores and extreme environmental conditions. An elastomer has the ability to conform to wellbore configurations and to continually re-seal so long as activation fluid exists; this allows these packers to adapt to oddly shaped holes.
Swellable elastomer packers also place very little stress on formations compared with traditional packers, thus reducing micro-fractures around the element. Low setting forces enable operators to increase performance in sensitive applications like shales. Compared with cementing isolation techniques, swellables can aid in reducing skin and production impairment. Because elastomers are resistant to H2S, CO2, acids, brines and chemical additives, it is less vulnerable to environmental elements.
CHINA CASE STUDY
Swellable packers were selected to repair an earthquake-damaged well in the Sichuan province of China. This onshore gas well suffered sustained damage that resulted in a 7-in., 35 lb/ft casing with 5 ¼-in. (133-mm) ID restriction. In this instance, the packer had to pass the casing restriction and still be able to seal in full-bore casing. The technical options were limited.
After considering cementing and mechanical options, the well operator sought out only those service companies offering swellable products. Swellable elastomers is the only product narrow enough to maneuver through the limitation and expand to isolate the zone. Another reason for selecting swellable packers was the presence of high levels of H2S. Oil-swellable elastomers are resistant to this corrosive chemical.
Ultimately, the operator went with Weatherford’s Nemisis ASP to successfully and cost-efficiently repair the well.
The selected swellable packer has a patented anti-extrusion backup system that allows for significantly shorter length and therefore was significantly less expensive. The backup system ensures the product can maintain higher differential pressure across the tool in overgauge holes, as well as enhance the thermal stability of the pressure seal in various well applications.
These anti-extrusion properties also result in much shorter and slimmer packers that are ideal for tight holes and short-radius wellbores. For this project, the tool consisted of just 10 ft (3 m) of packer material, saving the time and expense of an additional 56 ft (17 m) of packer materials and the cost of a workover operation.
Through the use of the swellable packer tool, the operator achieved full differential pressure, reducing drag while running in hole and the possibility of premature setting, enabling a successful acid frac job.
Weatherford developed the industry’s first swellable elastomer that responds to both oil and water in liquid, wet-gas or multiphase environments. Instead of single-fluid response of conventional swellable elastomers, it is formulated to meet specific swelling objectives and wellbore compatibility issues, as the well dynamics change over time.
Pre-installation planning data have also been developed recently to improve the operator’s ability to enhance precision and reliability at the planning stage. Weatherford’s SwellPro portable swell testing laboratory qualifies elastomer criteria, including fluid compatibility, life expectancy, bond strength, swell ratio, and elongation and shear properties. The technology has undergone rigorous testing and is ready for field trials.
Swellable elastomers have been used in the oil and gas industry for only about a decade. Although they have proven successful, room for growth remains.
Swellable elastomers can be developed further to deal with sustained casing pressure issues. The material has been used to complement cement in zonal isolation applications to prevent leaking and to re-seal areas of compromised casing/cement bonds. Bond failure is often due to microannulus during the life of the well or mud channeling during cement placement. Weatherford has combined centralizer technology with hybrid swelling elastomers to introduce this approach to well construction.
Increased exploration on applying swellable elastomers in horizontal well fracturing will bring a more cost-effective option to the market. Applying swellables can allow for longer intervals and more zones.
Opportunities also exist for adding swellables to expandables to enhance solid expandable tubular technologies in washed-out hole sections where it is currently difficult to achieve higher differential pressure holding requirements.
Swellable elastomer packers are also a key feature of inflow control device applications. The technology can cost-effectively provide the pressure isolation required for the full functionality of this open-hole completion and sand control technology.
Nemisis and SwellPro are registered trademarks of Weatherford International.