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Transcript for Wrestling with Uncertainty, segment 10 of 16
In the history of a play the first wells drilled tell the tale. By this time geologists have narrowed the risk by using all the innovative techniques of exploration analysis, but uncertainty still remains. If any one of several essential geologic clues has been misread, the chance of finding oil or gas plummets, but if the first well strikes a sizable pool, a process begins. With these first wells production engineers estimate how much the field might produce. If the accumulation seems large enough, wells are added. As new estimates of productivity are made, they often turn out to be much larger than originally expected. A field that was initially estimated to produce ten million barrels, for example, may ultimately produce five or even ten times that amount. This process is called field growth. It reflects in part the way our understanding evolves as a field develops. As the area of the field expands, wells may penetrate pods of oil or gas that were not recognized at the time of the original estimate, and new pools may be tapped near the original accumulation. If their petroleum resources are accounted for as part of the original field, its reserves increase.
Technological innovations also contribute to field growth. With pumping alone we get, on average, only about twenty percent of the oil available in the pool. As the natural pressures in the field are released, production declines, but if we pump water at high pressure into selected wells, we can push oil through the reservoir, extract it from others, and boost recovery by ten percent or more or we can pump in carbon dioxide which works like blood thinner in a heart patient and helps the oil flow more smoothly through the rock, and in some large fields where the oil may be as thick and waxy as shoe polish we inject steam to melt the oil and push it out.
The magic of horizontal drilling has also added significantly to field growth. Individual wells can now draw petroleum from a much greater volume of rock, increasing productivity and reducing cost, but even with these improved techniques, we recover on average only about thirty percent of the oil available. We might be able to recover much more but only at greatly increased cost so much of the oil is likely to remain the ground forever.
Still, we see that fields have histories, and as more are added to a play, the play develops a history as well. It's born, it grows and then gradually declines. These historical trends teamed with the experience and intuition of project geologists drive the assessment.