For more than half a century, well testing has provided valuable information about reservoir volumes and productivity. But most well tests are performed on naturally flowing wells. The reason is that, to get interpretable data indicative of true reservoir conditions, the well must achieve pseudo steady-state flow conditions. However, in the past few years, great success has been achieved while performing pressure transient analysis on wells produced by electric submersible pumps. Hupecol LLC, the Colombian joint venture company of Houston American Energy Corp., took advantage of this technique to solve a production problem on one of its wells in Colombia. The Hupecol-operated RQ-D1 well is in the Rancho Quemado field, about 100 miles east of Bogota. When the RQ-D1 well failed to produce to expectations, monitoring and pressure testing in real time yielded spectacular results. Because Hupecol operates several fields in the area and has extensive experience from offset wells, it was surprising when the RQ-D1 well did not produce to expectations. In fact, the ESP specified and installed in the well was sized for considerably more production than the well was making. Accordingly, company engineers sought to diagnose the problem and seek a practical and economic solution.