With so many small stranded gas accumulations remaining in the Southern North Sea (currently numbering well over 100) and the days of larger finds in the U.K. sector largely behind it, operators are focusing hard on maximizing what they’ve got and extending the productive life of their existing infrastructure.

With planned near-term exploration activity on the U.K. Continental Shelf also at alarmingly low levels due to the industry downturn and general belt-tightening all around, attention is increasingly being paid to subcommercial tight reservoirs.

Many of these are only considered subcommercial because until relatively recently they have not had the latest production enhancing techniques applied to them, in addition to other factors such as distance from existing infrastructure, relatively small volumes and anomalous gas qualities.

Existing Technologies, New Techniques

Centrica Energy has successfully developed one such reservoir via a single subsea well, horizontal drilling and multistage hydraulic fracturing.

The company combined existing technologies with new techniques to bring onstream its Kew discovery in blocks 49/4c, 49/4a, 49/5a and 49/5b, lying about 3 km (1.9 miles) east of the Chiswick Field. Kew is also just 5 km (3.1 miles) from the U.K./Netherlands median line, on the other side of which lie many other small stranded gas reservoirs.

This low-permeability interbedded structure suffered from a lack of data and analogue wells for the design and execution of the fracturing treatments, meaning extended injection tests were needed prior to the execution of the stimulation process.

According to a technical paper given at the recent SPE ATCE event, chemical tracers were injected to maximize the data acquired from the well during the stimulation treatments and returns evaluated to assess the flowback of each individual fracture.

As this was a subsea development well, all the fracking operations had to be performed with the rig in place, meaning operational efficiency was paramount to keep the project economics on track.

Commercially Interesting

The Kew discovery itself—found in 1988 by Ultramar and acquired 100% by Centrica in 2006—became “commercially interesting” to develop, according to the paper, due to the industry’s technical improvements (multiple fracking), certain tax relief incentives and the option of tying it back to the existing Greater Markham area infrastructure to which the Chiswick Field was connected.

The initial well design for Kew contained five stages pumped at rates of up to 40 bbl/min. A fracture gradient of 2.3 psi/m (0.7 psi/ft) was forecast, which indicated an expected maximum effective stress of more than 5,000 psi. The proppant was then selected based on the determined minimum horizontal stress, cyclic stress loading and proppant pack permeability considerations.

As the field was being developed subsea, proppant flowback was seen as a significant risk, hence the use of resin-coated proppant to mitigate that risk. An encapsulating stress coating applied on a traditional curable resin-coated proppant protects it from premature curing until both the right temperature and closure stresses are applied. This makes certain that the proppant can be properly placed before the curable coating layer is activated for flowback control, and it also eliminates the potential consolidation of proppant in the wellbore, allowing an easier cleanout.

Centrica has adopted a policy of using resin-coated proppant on all North Sea fracture-stimulated wells given its success in previous projects.

Extensive Testing

Extensive testing also had been carried out to ensure compatibility between the proppant and the frack fluid. All of the tests were successful, with the proppant having little or no effect on the frack fluid properties and the fluid not influencing the ability of the proppant grains to bond together under stress and remain unbonded when no stress is applied. The proppant sizing also was designed to reduce any non-Darcy effect, which would give rise to an unwanted pressure drop in the near-wellbore region that would then impair production.

The fluid used for the treatments was a freshwater-based borate crosslinked guar with a polymer concentration of 35 lb of polymer per 3,785 l (1,000 gal) of fluid.

Execution Phase

During the execution phase only four of the planned five stages were placed due to the horizontal length of the well being less than originally envisaged due to drilling difficulties, with total depth being called early at 5,225 m (17,143 ft). These four stages were placed in the reservoir using more than 1 million lb of 16/30 resin-coated intermediate-strength ceramic proppant and more than 3 million l (800,000 gal) of fluid.

The isolation between consecutive stages was achieved using soluble fiber-enhanced sand plugs set at the end of each individual treatment.

The whole stimulation operation for the four stages took 18 days. One hydraulic fracture pumped every four and a half days in the Southern North Sea is considered extremely efficient for multistage plug-and-perf treatments, especially when factoring in the interventions required between stages, vessel sailing times and—a North Sea specialty—waiting on weather.

A dedicated purpose-built stimulation vessel also was used. The high level of efficiency achieved was based on Centrica’s previous multistage horizontal well experience and by ensuring good communication and coordination between the rig operations and the stimulation vessel team.

Chemical Tracers

To evaluate the cleanout efficiency of the fracks, chemical tracers were used for each treatment, with two tracers used for every stage, one in the pad and one in the slurry stage. A rigorous sampling schedule saw a total of 29 samples taken during the cleanup phase of the well. These showed good breaking of the frack fluid, with a higher volume of tracer recovered as expected from the slurry stages than from the pad.

A rig-based cleanup and well test saw minimal amounts of proppant produced during the test phase, which gave Centrica confidence in the resin coating curing process with the high drawdowns applied to the well. The cleanup also showed the deliverability of the well exceeded expectations, flowing on a constrained choke setting at about 1.3 MMcm/d (45 MMcf/d).

First Production

The Kew Field began first production in January 2014 and is flowing via a 3-km 6-in. flexible pipeline to the Chiswick unmanned platform facility, where the produced fluid is routed to the proppant removal facilities before commingling with the Chiswick fluids and flowing onward a distance of 18 km (11 miles) to the J6-A Markham platform in the Dutch sector.

The operational execution of hydraulic fracturing in subsea wells is always a challenging undertaking, the paper concluded. “The lack of data for hydraulic fracturing design was overcome through extensive diagnostic injection schedules, and all the four stages planned were successful and are contributing to the production of the well, as observed from the chemical tracer analysis and the excellent production results to date,” it stated.

The well continues to surpass Centrica’s expectations, delivering and producing at a healthy rate up to the present day.

Acknowledgment: Excerpts from this article have been taken from SPE paper 170852.