Page 260 - Drilling Technology in Nontechnical Language

Chapter 10 – EVALUATION 251

8. Quantifying the effect of stimulation work done to the well.
(As mentioned briefly in chapter 4, stimulation refers to work

performed on a well in order to increase the production potential
[how fast hydrocarbons will flow into the wellbore at a given

pressure drop]. Techniques include deliberately fracturing the
formation using high pressures and pumping acid to create
larger channels.)
A well production test records the downhole pressure response over
time to changes in flow rate. Very accurate pressure gauges are placed in

the bottom of the well, and the well is allowed to flow through an orifice of

fixed size. This orifice is called a choke. It will take some time for the flow

to stabilize at a steady rate, and once the flow rate is stable, the well will

be flowed for a period of time. After that period of time, the well will be

shut in, and the pressure inside the well will build up, again over a period
of time. These pressures are measured and plotted on a graph of pressure

versus time, as shown in figure 10–12.

Fig. 10–12. Graph of well pressure vs. time during drawdown

When the well is closed in at the surface, and there is no flow from
the reservoir to the well, the pressure in the well equals the pressure in the

reservoir. When the well is opened, fluid flows out of the well because the

pressure in the well is greater than the pressure inside the test equipment.
This leads to a reduction in pressure at the bottom of the hole, and thus
there now is lower pressure in the well than in the reservoir. The difference
between reservoir and wellbore pressure is called drawdown. If there is

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