Immiscible water alternating gas (IWAG) injection has effectively managed produced gas at the Kuparuk River Field; increasing field oil rate and recovery. Although IWAG economics are very favorable, this pilot study was undertaken to ascertain whether separate water and gas injection lines to the drill sites and on-pad distribution systems can be eliminated by simultaneously injecting water and gas in waterflood lines.
A significant problem in simultaneous water and gas injection is the phase separation which occurs at branches in the injection pipe network. The higher density phase, because of its greater momentum, will tend to flow in the straight or least deviated pipe branch, while the lower density phase will tend to flow into the most deviated branch of pipe. Studies conducted at ARCO's Exploration and Production Technology Research Center in Plano, Texas; indicated that static mixers positioned at the pipe branches could achieve equal distribution of the phases to both runs of the pipe branch.
In the pilot study gas was injected into an 8" waterflood line which supplied two drill sites. A static mixer was placed at the junction for the two drill sites. Static mixers were also placed in the 4" injection manifold of one of the drill sites, the other drill site had no mixers in the injection manifold. Gas was distributed to both drill sites. At the drill site with static mixers in the injection manifold, gas was distributed to the wells on the injection manifold; while at the drill site without static mixers, phase separation occurred and only one well on the injection manifold received gas.
The Kuparuk River Field is located on the Alaskan North Slope, approximately forty miles west of the Prudhoe Bay Field. The wells produce from two physically independent sands, informally named the A and C Sand members, of the Kuparuk River Formation, a lower Cretaceous, shallow marine sandstone. Since startup in 1981, the field has experienced a variety of development processes including primary production, waterflood, gas storage, a water alternating immiscible gas injection (IWAG) project, a water alternating miscible gas injection project, peripheral development, and infill drilling.
Immiscible water alternating gas (IWAG) injection has effectively managed produced gas, increasing oil production rate and recovery (approximately 2% OOIP). Its success has led to IWAG expansion to 18 drill sites. The current long range plan provides for more IWAG expansion.
IWAG expansion requires a high pressure gas line to the drill site (approximately 1 MM$/mile) and an on-pad gas distribution system (approximately 3 MM$/drill site). In addition to capital investments, IWAG also incurs operating costs for converting wells from gas to water injection or vice versa. The average conversion cost is about 2M$.
In the current IWAG Process approximately 3% hydrocarbon pore volumes (HCPV) is injected during each gas half cycle. This procedure has several disadvantages. Successive cycles of gas injection create a preferential pathway for gas in the reservoir, which yields a lower sweep efficiency than desired. The poor sweep efficiency manifests itself in elevated formation gas oil ratios (FGOR) in producing wells adjacent to the injection wells.
Due to gas compression limitations at the central production facilities, the elevated FGOR may exceed the optimum FGOR for the compression facilities. When this situation occurs the producing well is shut in to production.