T2. What causes Oil Field Production Decline and Depletion?

This is one of a series of somewhat technical articles that I plan on putting up on Sundays, in which I describe, in somewhat simple terms, some of the technical aspects of fossil fuel production. Because they are simplified some aspects may be glossed over, but for those that would like to explain some of the aspects in more detail, please feel free to do so (or to correct any apparent mis-statements).

After the algal remains from which crude oil would come first became trapped in the sands and mud of the time, those beds were successively covered over the eons, until the heat and pressure generated by rocks that had been deposited above the original sediments, transformed the original fluid into oil. The original bed, which we call a source rock, also contained some water and, as geological time passes, the oil would rise above the water, and start to migrate up through the overlying rock towards the surface. The oil and water could do this, because rock is not a complete solid, but rather has many small cracks and passages in it, that we define as the rock permeability. As a general rule rocks that are made up of larger grains (such as a sandstone) are more permeable than those with finer grains (such as a limestone). I’ll come back and talk about that more in a later post. But some rocks have virtually no permeability and so, as the oil moves upwards and it reaches one of these, which we will call cap rocks, then the oil movement stops, and the oil is trapped in the underlying rock, which is normally called the reservoir. It helps, in this case, if the movement of the rocks over time has caused the rocks to bend into a dome, or fold, which geologists call an anticline.

Simple slice through the ground showing, from the side, how a fold can capture oil that has been pushed up, by water, from a source rock and trapped by the overlying cap rock.

Now, when we find this pool of oil, which is really not a big open space filled with oil, but rather a rock with lots of small holes or pores in it, (the porosity) that are each filled with oil, we would like to get the oil out. To do this we drill a hole, which we call a well, down to the oil bearing rock, and through it to the top of the underlying water. The well might be somewhere around 6 inches in diameter, for the sake of illustration, and the rock might be some 6,000 ft below the current surface of the ground. And for the sake of making the arithmetic that follows a little easier, we will assume that the rock that has the oil in it starts out with the oil layer (or column as it is called) being some 100 ft thick.
Slice through the side of the rock showing the well in the oil bearing rock (the reservoir)

The pressure of the overlying rock on the reservoir rock can be simplified to around 1 pound per square inch (psi) for every foot the rock is below the surface. So that at 6,000 ft the rock pressure is 6,000 psi. (which you might think of as the weight of a full sized pickup or two resting on a single square inch – not something you want your hand under).

The pressure squeezes on the oil in the rock, so that when the well appears, with a lower (and controlled pressure) in the well, the difference between the two pressures causes the oil to flow to the well. The amount of oil that flows into the well depends on the difference in pressure between the oil pressure in the rock and in the well, the permeability of the rock and the length of the well that is actually in the oil bearing rock (the 100 ft shown above).

Time passes and the oil flows into the well, often for a number of years, as it flows the pressure in the oil drops a little, but also the water level below the oil rises, as the oil above it disappears. (And in many cases that I will talk about in a later post the company will pump water through other wells under the oil, so that it will keep the pressure in the oil high enough to make it flow out of the well faster).

But the amount of oil that comes out of the well is controlled by the length of the oil column in the well, and after a certain amount of time the water level has risen.

Later stage of well production only 20 ft of oil left

Now as the water level has risen, and the amount left has been reduced (depleted) the amount of oil produced a day will reduce (decline) because the rate is controlled by the length of the column. In this example, very crudely we will be getting 20% (20/100) of the production that we got from the well when it was first started. (Actually because of other factors that I will cover in later posts it will likely be a bit less than this.) As a very rough rule this decline for the overall field starts in at about the point that half the oil in the reservoir that the well is tapping has been removed, and the decline rate increases from that point on. There is not a lot that can be done with a conventional vertical well to stop the decline in production once the well starts to pump out the oil, and when there are a lot of wells successively being developed and then pulling oil out of a reservoir, then the overall production rate from the field very often will follow a bell-shaped curve.

In other words, as the wells first start to produce the field (which is what one of these large underground pools of oil is called) the production of oil will rise, it will have a relatively steady increase until as it approached full production, the increase tails off and there is a short period of steady production. Then as the oil columns get shorter in more and more wells the decline in production sets in and the remaining time that oil can be economically recovered from the field becomes more finite. (Though in some fields residual production of a barrel or two a day can continue for years).

The average decline in production rate from wells in a field is a matter of some debate, but for conventional quasi-vertical wells which have been the main method of getting oil out of the ground until about ten years ago, the rate, during the steady decline phase, has been assumed to be 4%. That rate is now thought to be increasing, but there are other factors that control that, and we’ll leave that discussion until another day.