Page 264 - Introduction to Petroleum Engineering

DECLINE CURVE ANALYSIS 251

I =1 2 3 4

J =1

2

3

4

FIgURE 13.1 Illustration of a bubble map.

area of interest. For example, cumulative oil production for a well at a particular
point in time can be plotted as a circle (or bubble) centered on the well location
displayed on a map. If this is done for several wells as in Figure 13.1, the resulting
map will provide a graphic comparison of the relative amount of cumulative oil pro-
duction at each well. The location of wells in the figure is specified in terms of the I,
J indices. The radius of the circle indicates the magnitude of the variable. In our
example, a large circle indicates large cumulative oil production relative to other
wells with smaller circles.
Bubble maps can be used to look for trends in the distribution of a variable. For
example, if a small bubble representing the gas production rate at a well is surrounded
by a set of large bubbles in an area, the small bubble may represent an anomalous
measurement or a well that is damaged. In this case, a workover could increase the
gas production rate at the well.

13.2 DECLINE CURVE ANALYSIS

Decline curve analysis (DCA) is an empirical technique for predicting oil or gas
well production (Arps, 1945; Towler, 2002; Economides et al., 2013). The tech-
nique fits a curve to measurements of flow rate as a function of time. Some reser-
voirs, such as oil reservoirs with strong water influx, have enough energy to sustain
relatively constant oil production rate for an extended period of time. As a rule,
production flow rate declines with time once a well is completed and production
begins.
An exponential equation has been used to predict future production by fitting the
exponential equation to historical decline rates for many production wells. Although
the exponential equation provides a good fit of production rate as a function of
time for some wells, a hyperbolic equation provides a better fit of the decline in