Example 8-1 shows that different theoretical correlations give different answers. Additionally, some correlations give too small a value to use the critical rate in field operations. Therefore, an engineer has to choose the correlation to be used in field applications.

In this author's view, there is no right or wrong critical rate correlation. If production testing data is available, one can easily choose an appropriate correlation that fits the field data. For example, in a given field, a few wells can be selected. Each well that is producing water or gas can be choked down. In practice, this can be done by (1) reducing choke size, (2) reducing gas injection in a gas lift operation, or (3) reducing pump rate or flow capacity for a pumping well. Choking the well down increases bottomhole producing pressure. The rise in bottomhole pressure reduces reservoir drawdown. This in turn results in a reduction of total fluid production from a well. Once the change is made, the well can be allowed to stabilize and a water cut can be measured at the new rate. A plot of flowing tubing pressure against water and oil rate can be made. The production test can be conducted at several chosen settings, i.e., at several flowing tubing pressures (FTP). A plot of oil rate and water rate against FTP can be used to estimate critical production rate (see Fig. 8-2). As shown in the figure, if one increases FTP, at a certain FTP...