In the petroleum geology section, we learned how to calculate the volume of a reservoir using the geometry and porosity of a reservoir rock. This method is known as the volumetric method because it depends on knowledge of the reservoir geology and is based on a hypothesized volume of the formation. Since it is difficult to accurately estimate the characteristics of underground rock formations using limited data, the accuracy of this method is also limited.
Fortunately, more advanced estimation methods become possible once production from a well begins. By studying changes in production through time, reservoir engineers can predict the likely total amount of hydrocarbons a reservoir will produce. Let’s take a look at two of the more advanced methods for estimating total production. One is called the decline curve method, and the other is the material balance method.
Decline Curve Method
The decline method is based on the fact reservoirs in similar geologic environments experience similar patterns of production decline over time. Most wells produce a lot at the beginning, then they gradually slow down, but they can slow down for quite a long time. A graph of this production slow-down is called a decline curve.
There are different types of curves for different types of reservoirs or different situations. Once the type of curve is selected, it is fit to the available production data. If the curve fits the historical data well, the curve is extrapolated extended into the future to predict future production rates and ultimately recovery.
The advantage of this method is that it doesn’t require the development of a complex model. The disadvantage is that it is only accurate if the reservoir has been producing for some time. A year is a good start, but the more data available, the better.
That brings us to the most accurate method for estimating total oil and gas in place – the material balance method.
Material Balance Method
The idea behind the material balance method is that the pressure in the reservoir is related to the amount of oil and gas remaining in it. You can think of the reservoir like a pressurized tank; if you take a certain volume of fluid out, the pressure is going to drop by a certain amount. To use this method, you need to know two things: how much oil, gas and water have been produced and how much the pressure has changed as a result of this production.
You can do this for the entire reservoir if it is one continuous, interconnected body. But if the reservoir is separated into different isolated blocks by faults or pinchouts or anything else, then the material balance has to be done separately for each isolated part.
Material Balance is actually a surprisingly accurate bit of science if the data is good. You can reasonably estimate the total volume of hydrocarbons in place for a very large reservoir, even one with billions of barrels of oil. The downside of using this method is the vast amount of accurate data required to have any validity.
The basic material balance equation is shown below.
It is safe to say both the decline curve and material balance methods become more accurate as time and production go on as more data becomes available.
To sum up, a big part of what reservoir engineers do is determine how much oil and gas are in place in the reservoir and how quickly it can be produced from the wells. There are many different techniques developed to do that from simple back-of-the-envelope calculations to very complex computer simulations. At this point in time, if they have enough resources at their disposal, reservoir engineers are pretty good at making accurate estimates.
Images: “Estimation” by Jim Blecha; “Basic-Material-Balance-Equation” by Jon LaRue