Nitrogen miscible flooding case studies
Nitrogen miscible injection is a process whereby nitrogen gas is injected into an oil reservoir above the Minimum Misciblity Pressure (MMP) to create a miscible vaporizing drive and increase the oil recovery factor.
Discovered in 1970, the Jay field produces primarily from a Jurassic-aged Smackover carbonate that is heavily dolomitized and has complex lithology. The entire pay interval was cored in virtually all the wells to provide an accurate geologic description and aid in unitization efforts.
Reservoir properties are as follows:
- The productive area is 14,415 acres
- Average depth is 15,400 ft subsea (SS)
- Porosity is 14%
- Permeability is 35 md
- Swi is 12.7%
- Oil gravity is 51° API
- Oil viscosity is 0.18 cp
- Pri is 7,850 psi
- Tr is 285°F
- Net-to-gross ratio is 0.27
- The dip is 3°
The field was unitized in 1973, and waterflooding began 4 days later to arrest pressure decline. Miscible N2 injection was started in 1981. The minimum miscible pressure (MMP) for many solvents and the light oil in this reservoir is well below 7,000 psi. N2 was selected over methane and CO2 because of cost and supply considerations. Delay in methane sales was unattractive. Using CO2 would have required a long pipeline from central Mississippi with accompanying costs and right-of-way complications.
The field was developed initially with 89 wells on 160-acre spacing. Selective infill drilling later in poorer sections of the reservoir (both areally and vertically) improved the sweep of injected water. This reduced the average spacing to 140 acres per well.
The waterflood was implemented with a 3-to-1 line-drive pattern using low-salinity water from a water-source well. Produced water is also injected. Peak water injection rates reached 250,000 B/D.
Nitrogen is produced by three air-separation units. Produced nitrogen is recovered with cryogenic units and reinjected. The injection rate peaked at 86 MMcf/D. Water-alternating-gas (WAG) injection is used; the WAG ratio varies by pattern, as dictated by ongoing surveillance of producer oil rate and gas/oil ratio (GOR) performance. Current plans call for the injection of a 0.4 hydrocarbon pore volume (HCPV) bank.
The oil production rate reached 100,000 BOPD in 1973 and was sustained at or above that level through 1979. In 2002, the field was producing 10,500 BOPD at a 95% water cut. The gas production of 80 Mcf/D is approximately 75% N2. Reservoir pressure has been maintained at 7,500 psi. At this pressure, even high-water-cut wells continue to flow, negating the need for artificial lift.
Estimated ultimate recovery is 60% of original oil in place (OOIP), with recoveries approaching 70% in the upper section of the reservoir. Primary recovery resulting from fluid expansion and solution-gas drive was projected to be 17% of OOIP. Waterflooding increased this by 60%, and the miscible project will add another 7 to 10% of OOIP, with recovery in the upper part of the reservoir approaching 13%.
The reservoir surveillance program includes monitoring the WAG ratios, injection-to-withdrawal ratios, and profitability of each pattern. Tools used in the surveillance processes include a history-matched, fully compositional fieldwide simulation model based on an updated geologic model that includes sequence stratigraphy, geostatistical methods, and a styolite model. This finely gridded model includes an updated fluid characterization. These refinements enabled well-by-well matches of production and pressure data for all 137 wells in the field. The updated model has been used to optimize N2 distribution and investigate several operational changes that are scheduled for implementation and will extend field life by more than 10 years.
- Langston, E.P. and Shrier, J.A. 1985. Performance of Jay/LEC Fields Unit Under Mature Waterflood and Early Tertiary Operations. J Pet Technol 37 (2): 261–268. SPE-11986-PA. http://dx.doi.org/10.2118/11986-PA
- Lawrence, J.J., Maer, N.K., Stern, D. et al. 2002. Jay Nitrogen Tertiary Recovery Study: Managing a Mature Field. Presented at the Abu Dhabi International Petroleum Exhibition and Conference, Abu Dhabi, United Arab Emirates, 13-16 October 2002. SPE-78527-MS. http://dx.doi.org/10.2118/78527-MS
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