The law can also be rewritten into a relationship between pressures and volumes before and after a cer-
tain isothermal change
, = pressure of gas at state 1 respectively 2, and
, = specific volume of gas at state 1 respectively 2.
Gas extraction removes mass without changing the volume of gas in the different states, i.e.
As a result, pressure must fall in order to maintain the balance. From Darcy's law (Equation 3.3.1) it fol-
lows that decreasing pressure leads to decreased flow rates, if all other things are equal. Consequently,
extraction of gas from a reservoir will result in declining production with time, in other words a deple-
The situation becomes more complicated with oil extraction or other forms of production strategies,
but the general situation is the same as in the simplified gas reservoir case. In fields where production
strategy is to maintain reservoir pressure, for instance by water or gas injection, the extracted volumes of
oil and water will remain relatively constant through the life of the field, in agreement with the material
balance equation (Satter et al., 2008). However, the oil production will ultimately fall and water produc-
tion increase as more and more injected water begins to diffuse into the production wells. As the reservoir
depletes, the well will eventually produce too much water to be economically viable, despite the fact that
reservoir pressure might still be high. The ratio of water compared to the volume of total liquids produced
is referred to as water cut. In mature fields the water cut can reach very high levels, up to 90% and more
has been recorded in parts of China (Pang, 2008). American oil fields in Kern Bluff and Mount Poso ar-
eas reports water cut of over 99% (California Department of Conservation, 2007).
To summarize, it is necessary to differentiate politics and economics from physical factors affecting
decline rates. This is perhaps best shown with a few real life examples based on simple exponential de-
cline functions (Figures 4.2 and 4.3). Curve fitting can be improved dramatically when data points dis-
turbed by sudden political or economical events are ignored.
28 The law can also be rewritten into a relationship between pressures and volumes before and after a cer- tain isothermal change (4.1.3) where , = pressure of gas at state 1 respectively 2, and , = specific volume of gas at state 1 respectively 2.