Selective zonal-isolation-stimulation reservoirs, with multiple sets of perforations, in the Norwegian North Sea challenges conventional bullhead approach.
Bob Murphy, Weatherford International Ltd.
In 2004, a selective zonal-isolation stimulation system underwent field trials when it was deployed in existing and underperforming wells. The system today is being used as part of a stimulation program1 in the Norwegian North Sea to evaluate its effectiveness in the chalk formations of a newly drilled well. The zonal isolation straddle packer system is run on Coiled Tubing (CT) to selectively stimulate individual sets of perforations while leaving others isolated.
INTRODUCTION
The wells were typically completed with long horizontal or high-angle sections often in excess of 3,000 ft (914 m) in length with several distinct sets of perforations and as often as ten separate, perforated intervals. To bring the wells into production with economically acceptable flowrate, the operator’s practice was to perform acid fracturing operations on newly completed wells. They consist of highly concentrated Hydrochloric (HCl) acid, bullheaded to the formation at a high rate while relying on frac balls to plug the more receptive perforated sections and direct the acid frac to the less permeable or potentially productive sections. These initial stimulation methods were simple in design, easy to execute and, for some time, were thought to be adequate to uniformly stimulate the various sets of perforations and provide an effective and economic production profile. Alternative methods of selective stimulation were considered either too costly or otherwise inappropriate compared to this conventional bullheading approach. The typical design of the acid frac operation involved pumping 1,200-3,000 bbl of 10-28 % HCl acid down the production tubing at rates up to 60 bpm, while diversion was to be accomplished with 5/8-in. frac balls.
This practice was maintained for several years. However, as production history was accumulated and production logs were routinely run to evaluate well performance, it became clear that most of the production on each well came from only a select few perforation sets while others were contributing little, if anything, to the overall production rate. It was evident that the relative differences in permeability, porosity and pressure along the length of the horizontal sections made fluid diversion in bullheading operations a very ineffective well-stimulation method, leaving much of the perforated sections essentially untreated while directing the bulk of the stimulant to the more receptive, and subsequently productive, sets of perforations.
DESIGN OF STIMULATION TECHNIQUE
Faced with this situation, the operator sought out a new stimulation technique-one that could be used to selectively treat the individual sets of perforations and thereby lead to a more homogeneous production profile. Consequently, this technique would also improve overall drainage patterns and water-flood operations. After a lengthy investigation of the available or possible alternatives, the operator chose a method of selective stimulation using a specially designed straddle packer system run on CT. This was the first operation using this method in the North Sea; however, before it could be deployed, several aspects of the proposed operation were examined.
CT-string design. Fracturing the carbonate formation required pump rates exceeding 8 bpm, thus requiring surface pressures approaching 7,400 psi during the gel-pad and acid stages. New CT strings would be required because these rates and pressures were beyond the operating limits of the CT strings stocked by CT vendors in Norway. The operator’s reservoir department provided more detailed input regarding the expected fracture pressures and the variations in perforation interval space out. This analysis led to a tapered 2 7/8-in QT-1000 CT string that could operate at the extremely high pump rates and pressures required to stimulate these zones. The string was designed to ensure that, upon stimulation of a particular zone, the minimum wall thickness across the CT gooseneck would be 0.175 in., providing maximized-pressure safety margins at the surface when pumping the acid stages. The CT string was subsequently designed for onshore measurement to accommodate both the depths and lengths of the intended perforation interval. To provide accurate pressure and flow limitations for the zonal stimulations, software modeling was used extensively to model each zone fracture.
The platform crane capacity was limited to 35 t while the complete CT drum weighed 55 t. The CT reel had to be split into two separate lifts, placed on the platform and eventually joined together on location, using a spoolable CT connector. To spread the deck loads on the platform, a CT- reel track-and-trolley system was designed which also helped position the equipment. A further refinement was made to accommodate two rotating joints, one on either side of the split reel, to provide both redundancy and additional flow area into the CT.
Redesign of packing elements. At the design and testing process, the Jet Frac straddle packer system’s specifications were designed to meet the requirements of the proposed operation. Slips were eliminated, as well as the need to manipulate the CT, to achieve full setting. To provide packoff and a high-pressure seal, the packing elements were set by back pressure acting against a spring-loaded piston area. The system needed to be capable of being set at a low flow rate, and while retaining the flexibility to achieve high rates with as low a pressure drop across the tool as possible, to minimize surface pressures. These requirements were met by a new design of injection valves incorporating a spring loaded sleeve that is actuated at 1,500-psi differential pressure. Sleeve actuation opens a flow area through the tool and into the packed-off area via a series of nozzles that can be size adjusted to suit well conditions and that can handle a 15-bpm rate at 1,500-psi differential pressure. The straddle system was designed so that it could be set and unset multiple times on the same run; the unsetting mechanism is activated either by stopping the pumps or setting down on the packers to activate an unloader at the top of the assembly. To monitor the frac process, the design included downhole memory gauges placed above and below the packing elements to record both pressure and temperature. This method would provide information on pressure interferences in the zone above or below and would also detect leaks in the event of tool failures.
The frac fluid was pumped from a stimulation vessel alongside the platform using a dynamic-positioning system. Frac fluid access to the tapered, high-strength 2 7/8-in. CT string was provided by a 3-in. coflexip hose and chiksan line.
FIELD TESTING AND APPLICATION
The first operational tests targeted perforation intervals analyzed using PLT logs on three existing wells, all of which indicated poor acid diversion and lack of stimulation during the initial bullheading treatment. A total of nine intervals in the three wells were selectively stimulated using the zonal-isolation packer. During these treatments, the pump rates through the CT were as high as 16 bpm (2.5 m3/min) and surface pressures at the gooseneck area reached as high as 7,400 psi (510 bar). Maximum differential pressure across the zonal-isolation packer reached as high as 5,250 psi (362 bar).
The 2 7/8-in. CT was rigid enough to enable maintaining accurate depth control using the injector head’s depth counter; as such, no depth logging and correlation runs were required. Also, the acid concentration was reduced from 28% to 10% with the lower acid strength providing more control, both on treatment diagnostics and extending reach into the formation. The results of these first operations on the three existing wells were generally above expectations and proved to be financially attractive, because combined production on the stimulated intervals increased by 31%, 3% and 273%, respectively, with operational payback achieved in five to seven days.
Initial application. After the success of the trial campaign, the operator decided to apply this stimulation technique to a new well to provide an offset comparison to the conventional bullheading approach. The first major application involved a newly drilled well that had a total of 16 separate perforated intervals needing treatment. The results from selectively stimulating the intervals using the straddle-packoff system on CT were expected to be considerably higher based on the results achieved in the initial tests. The equipment was mobilized and the zonal-isolation packer was run in the well on CT; it was set and unset successfully to allow for selective fracturing of 15 zones in one trip; the 16th zone was subsequently treated on a separate trip for several operational reasons unrelated to the stimulation system. Stimulation rates during the operation reached as high as 17 bpm (2.7 m3/min) with 7,000 psi (482 bar) injection pressure. In six of the treated intervals, differential pressure reached over 5,000 psi (345 bar) on the zonal-isolation packer and subsequent analysis determined that a standard bullheading operation would not have treated these intervals. The reproduced chart of pressure versus time clearly identifies those zones, Fig. 1.
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Fig. 1. Pressure vs. time plot for the treated intervals.
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RESULTS AND CONCLUSION
After applying the selective-stimulation frac procedure, the well has been producing above expectations at a relatively low production choke opening, and the production was nearly doubled compared to what would be expected from a standard bullheading operation.
An added economic benefit of this operation was, that by selectively isolating and treating the perforated intervals, the CT straddle-pack system reduced the total acid volume needed to treat the well by 5% when compared to typical bullheading applications. It was also evident that a campaign approach to such stimulation, rather than a single treatment, would have a major impact on economic viability by spreading the logistical costs over several jobs. 
LITERATURE CITED
1 Halvorsen, H., Ormark, K. and A. MacCleod, “High rate stimulation using coiled tubing zonal isolation straddle packer,” presented at the SPE/ICoTA Coiled Tubing Conference and Exhibition, Houston, Texas, March 23-24, 2004.
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