The dynamics of completions are constantly evolving, driven by increasingly complex wells and the need for greater efficiency in a cost-constrained environment. Often, completion-related innovations are tied directly to the needs of particular fields and projects. In the North Sea, for example, many fields are mature or moving into mature production phases, and as such have a unique set of needs to address completion, stimulation, production and intervention. Producer wells are often drilled horizontally and require stimulation to produce effectively. Later in the well’s life, water influx or reservoir depletion require methods to shut off the water and to restimulate the well. Additionally, infill drilling programs mean that wells are often positioned near one another, leading to further complications and requiring additional technology.

National Oilwell Varco (NOV) developed the i-Valve and i-Frac systems as a flexible, reliable solution for a variety of completion challenges, including those seen in the North Sea. The i-Valve and i-Frac systems are mechanically shifted and ball-drop-activated stimulation and production sleeves, respectively. They are used in acid and proppant stimulation, can be fitted flexible ball seats with nozzles for limited-entry matrix acidizing—as is often required in infill drilling campaigns—and have cement protection features, as 80% of the wells in the North Sea are cemented. The i-Valve and i-Frac multi-open-close (MOC) sleeve has a shifting profile and an ISO 14310 V0-rated seal design that allows it to function as a standard production sleeve, with the ability to function open and closed multiple times over the life the well. This flexibility is required due to the need to manage production, close off water-producing zones and restimulate.

Case history 1
An operator in the Norwegian North Sea was drilling a horizontal producing well as part of an infill drilling campaign. The well was drilled to a total depth of 4,161 m (13,650 ft) with a 1,219-m (4,000-ft) horizontal section through two producing intervals and completed with a liner consisting of four MOC i-Frac sleeves. The well was acid-stimulated by dropping dissolving balls, and then it was put on production. As the infill drilling campaign continued, more wells were drilled in the proximity of the first well. During the drilling of an adjacent well, the first well experienced a sudden pressure buildup from 1,800 psi to 4,800 psi. This incident caused the operator to shut in the well to investigate. They first performed a wireline run, which tagged at 3,682 m (12,080 ft), just below the uppermost zone. A subsequent run with a bailer showed that there were mud, solids and formation fill in the well below the first zone.

After the well was shut in, the operator decided to work with NOV to initiate an intervention program to remove the fill and restimulate the well to remove the near-wellbore damage. The objective was to carry out the program in close cooperation between NOV and the operator, with careful monitoring of HSE practices and clearly delineated performance results for zone restimulation. The intervention program consisted of wellbore cleanup using a jetting assembly or bit/motor, a run with the i-Shift shifting tool to close the sleeves in the upper zone, restimulating the lower zones, a second run with the i-Shift shifting tool to open all the sleeves, and putting the well back on production. 

The operator ran a rotating jet nozzle to clean the wellbore with coiled tubing and allow the shifting tool to be run. The i-Shift shifting tool was then run, and sleeves in the first zone were closed. An acid stimulation vessel was brought to the rig, and the third and fourth zones were stimulated using dissolvable balls. After the stimulation and flow testing, a second run was made with the i-Shift shifting tool to reopen the sleeves in the first zone. The full intervention program was performed successfully, with the sleeves functioned without any issues, and the restimulation of the third and fourth zones was performed according to the program. The well was put back on production and reached the levels achieved before the mud invasion, and it continues to produce according to expectations.

Case history 2
In 2009 a North Sea operator drilled a horizontal oil producer well. The well was completed with 10 stages using 10 i-Valve systems. After a short period of production, the well was acid stimulated zone by zone by mechanically closing and reopening the i-Valves. After producing for seven years, an increase in water cut and a decrease in well pressure caused the well to stop producing, and the well was shut in.

The operator decided to work with NOV to initiate an intervention program to recover production in the well. It was determined that while several zones were still contributing to production, water cut in other zones needed to be shut in. Additionally, further stimulation in between existing zones would contribute to more production. An intervention program was designed with wellbore cleanup using a jetting assembly or bit/motor, a run with the i-Shift shifting tool to close all 10 i-Valves, perforating and then proppant frac zones of interest between the i-Valves, a second run with the i-Shift shifting tool to open sleeves in zones contributing to production and without significant water cut, and putting the well back on production.

All i-Valves were successfully closed during a single run, and a pressure test of the well was performed to verify there were no leaks. Zones of interest between the i-Valves were then perforated and proppant fractured. After a cleanup run was made to remove any remaining proppant from the well, the i-Shift was run to reopen three i-Valves. The operation was performed as planned, and the well was returned to production at significantly improved rates.

Conclusion
Completions will only continue to grow in difficulty as the industry drills more wells and explores new geological territory. With completion-related expenses often making up almost two-thirds of a well’s cost, the need for iterative improvement in this highly consumptive and capital-intensive area has become more and more apparent. NOV continues to focus on flexibility and reliability with its stimulation sliding sleeve product line, recognizing the varying challenges of global operations. North American shale wells require different equipment functionalities from North Sea wells, and working with a partner who understands these differences and uses technology solutions appropriately will help operators outperform the competition in a crowded marketplace.

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