North Slope Gas
In discussing North Slope gas there is more to the issue than the fact that the gas is a large energy resource to be removed whenever some means to move it is devised, be it the Alaska Natural Gas Transportation System or some other. The complication is that if oil production is to continue in the Prudhoe field excess natural gas will come out of the ground, and something will have to be done with it.
The reason for the excess natural gas is described by the University of Alaska's Dr. Michael Economides in the Summer 1981 issue of The Northern Engineer. According to Dr. Economides, a typical petroleum reservoir is stratified into zones containing natural gas, oil and water. Being the lightest of the three substances, the gas occupies the uppermost zone, and the water, being heaviest, occupies the lowest zone. The petroleum crude finds itself sandwiched in between the gas and water zones.
This is a somewhat oversimplified description because the oil zone does contain some gas in solution within the oil, and a certain amount of water exists throughout the reservoir. Therefore as oil flows from a producing well, some gas flows out with it. At present, 900 cubic feet of gas comes out of the ground with each barrel of Prudhoe oil. Each day at Prudhoe Bay, a billion cubic feet of natural gas is pumped back into the ground.
Continuing oil production at Prudhoe will decrease the thickness of the oil zone and decrease the pressure in the zone. Reinjection of the gas helps keep the pressure up, but, since oil is being removed, the pressure still drops somewhat with each barrel extracted. The decreasing pressure allows more gas to come out of solution. Thus by the time all the Prudhoe oil is extracted, the gas production will have averaged about 2700 cubic feet per barrel of oil.
Eventually--and it's only a few years away--the wells that used to produce mostly oil will produce mostly gas, so much gas that they no longer are economically productive oil wells. Also, the uppermost gas zone may expand in thickness down to the level of the bottom of the oil wells, and then they can only produce gas.
Injection of water into the reservoir will help keep up the oil production by squeezing the oil zone upwards. This works only until the top of the water zone reaches the bottom of the production wells. After that, the wells produce only water, and that's worth less than excess natural gas.
The upshot of all this is that the years immediately ahead will see more and more natural gas coming out of the ground at Prudhoe. The time will be reached when it is not feasible to reinject it into the reservoir. At that time the question is what to do with the gas.
One solution is to stop oil production; another is to take the gas away in a gas pipeline. The excess gas also can be flared away into oblivion.
Yet another possibility is described in Dr. Economide's article. Not original with him, the idea is to process the gas into methanol near the wellheads. This process uses up 30% to 40% of the contained energy, but what is left over can be transported in the existing Trans Alaska oil pipeline since that pipeline has sufficient capacity to carry both the Prudhoe oil and the methanol. Methanol is a useful petrochemical feedstock and it can be added to gasoline to make gasohol or be burned directly in proper engines. Proponents of the idea suggest that the cost of this alternative is many times less than construction of a new gas pipeline.
