Abstract
Operators and manufacturers of ESP equipment have generally overlooked the consequences of high productivity on the operation of Electrical Submersible Pumps (ESP's). The high productivity itself causes issues such as the difficulty to identify correct rotation when starting an ESP in a well for the first time. This issue manifests itself as a small drop in intake pressure, even at high flowrates, which can make it difficult to be certain that the pump is running in the proper rotation, even after both directions have been tried. In some cases, this ambiguity causes pumps to run for lengthy periods in the wrong direction until other symptoms, such as a rise in motor temperature suggest that the performance is not correct and reverse rotation is possibly the reason for the higher temperature rise. Hence, in this paper we will introduce the application of pressure derivatives as an approach to identify the correct rotation direction when a rotation check has been conducted and also, as a strategy to aim at identifying the correct rotation much more quickly than traditional methods. In both scenarios, pressure derivatives create a signature that unmistakably differentiates the correct from the reverse rotation, which demonstrates a sound basis for determining proper operation on start-up. Furthermore, since pressure derivatives allow calculation of flowrate in the tubing and annulus based on the rate of change of pressure, they can be used to detect events such as no-flow conditions caused by gas lock, or a closed valve on surface, or a change in productivity, for example from zone bridging and plugging/unplugging.
High well productivity can create other complex situations linked to ESP's such as what the authors call ‘negative apparent productivity’. In this situation, a high productivity index, along with a high gas-to-liquid ratio (GLR), and a large distance between the pump and the producing zone, can combine to produce a counterintuitive behaviour in intake pressure at start-up, frequency change and stop. This is the effect of a significant difference in the pressure profiles in static and flowing conditions. Likewise, in this situation, we will provide an example of an offshore well where this behaviour was observed and will explain the underlying mechanism that produces it.
