Field reaching peak production after 16 years
Lilly Skartveit Bergsvik
Mark R. Norris
- Valhall historic production overview [31,029 bytes]
- In a typical completion at Valhall today, an 8 1/2-in. horizontal hole section in excess of 1,000 meters is completed with a well-cemented 5 1/2-in. heavy wall liner. [26,434 bytes]
In addition, the simultaneous operation concept (SIMOPS) applied in conjunction with the completion technique, has accelerated production, significantly reduced the rig day schedule, and offered a cheaper rig-less alternative, which provides improved economics for this type of completion.
The Valhall Field is an Upper Cretaceous, chalk anticline, forming an overpressured, undersaturated oil reservoir located in the southern part of the North Sea. The field is characterized by its high porosity (35-50%), high oil saturation ( 95 %), and permeabilities ranging from 2-10 mD.
The field is reaching peak production after 16 years. Since the first well came on stream, the completion method at Valhall has been an evolving process, mainly driven by major stability problems in both the chalk pay and overburden.
The field has sustained a remarkable increase in productivity during the last three years, and the production is increasing faster than when the field was first put on stream. Two of the most important reasons for this change are the horizontal well completions and the reduction in time to complete and stimulate new wells.
Fractured horizontal wellsIn a typical completion at Valhall today, an 8 1/2-in. horizontal hole section in excess of 1,000 meters is completed with a well-cemented 5 1/2-in. heavy wall liner.
Each zone is perforated in 5-ft intervals with up/down oriented guns, and stimulated with tip screen out proppant fractures, spaced approximately 170 meters apart. A permanent bottomhole pressure and temperature gauge is installed in the tubing.
The fracture spacing has been optimized with respect to risk-weighted economics, based on simulated rates from a history-matched numerical model. A 1,000-meter well with seven fractures yields initial production rates as high as 10,000-12,000 b/d, compared to 2,000-4,000 b/d of earlier horizontal well completion techniques.
Simultaneous operationsOnce the liner and production tubing are set in place, subsequent completion operations associated with the multiple fracture treatment do not require a derrick, and can instead be performed by 2 3/8-in. coiled tubing.
On the Valhall WP platform, a self-contained coiled tubing package performs the completion operations, involving perforation, stimulation, clean-out, and isolation of each zone on one well, while a jackup rig is drilling the next. Since the SIMOPS start ed in 1996, there has been continuous improvement with respect to equipment and working environment, as well as completion and stimulation technology.
The operating envelope of 2 3/8-in. coil tubing was extended to achieve the record 6,000-meter depths, and the reach is currently being extended to 7,200 meters.
A total of 220 operating days were saved from the jackup drilling rig schedule in 1996/1997 by performing these operations in a simultaneous standalone mode, using SIMOPS. As a result of this, 2 MM bbl additional oil was produced in 1997 alone. In addition, the costs are significantly reduced, since the SIMOPS spread costs are only one-third that of the jackup drilling rig spread cost.
Zone isolation plugsIt was recognized early that developing a technique to achieve zonal isolation without using mechanical plugs, would not only save cost and time, but also enable underbalanced clean-outs in low pressure wells, while ensuring ideal curing conditions for excess proppant left in the wellbore after the fracture treatment was utilized.
Although proppant plugs have been used for many years in vertical wells, this had not been tested in horizontal wells before. The technique to set a plug is as follows:
- (1) Underdisplace the treatment by 15 bbls
(2) Shut down for five minutes to let the fracture start closing
(3) Pump one bbl at 0.5 bbl/min to dehydrate the proppant slurry against the perforations. These squeeze cycles are repeated until a stable BHP of about 3,000 psi above the closure pressure is obtained. The first successful proppant plug was set in February 1997, and currently, the success rate for obtaining proppant plugs is 90%, saving approximately 1-1.5 days per fracture cycle.
Recycling proppantWaste is minimized by re-cycling proppant in separate zones. As a result of the aggressive tip screen out frac design, an average of 30,000 lb of proppant remains in the wellbore and needs to be removed and disposed of after each treatment.
Due to the resin coating on the proppant, the material is classified as chemical waste, requiring expensive onshore disposal. How ever, laboratory testing and numerical sim ulations indicated that pumping this proppant in separate re-cycle fracs, with a tail-in of new resin coated proppant, would not only pay out, but also add revenue. Today, three of the horizontal wells have a separate recycle fracture.
Dual fracturingThe completion and stimulation process at Valhall is evolving in a continuous effort to further improve value by increasing productivity and reducing cost. One of the techniques under optimization is limited entry dual fracturing in tip screen-out mode.
So far, Amoco Norway has completed three dual fracturing treatments in a horizontal well; however, there is insufficient data available to fully evaluate these treatments.
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