ESP compounds have different sizes and can be assembled in a variety of combinations. These combinations must be carefully determined to operate the ESP with production requirement, downhole conditions, material strength and temperature limits, etc. to select the optimum size of compounds.
To determine the required number of stages of the pump to produce the anticipated capacity; just divide the Total Dynamic Head (TDH) by the Head developed by Stage.
The Head developed per stage is deducted from the published performance curve which shows the discharge head developed by the pump. It is an experimental curve given by the manufacturer and obtained with fresh water at 60 F under controlled conditions detailed in API R11 S2. Refer to the articles “Pump Performance Curves – part 01” and “Pump Performance Curves – part 02” for more details.
Once calculated, divide the TDH by the Head developed per stage to get the Total Number of Stages required to produce the anticipated capacity.
In order to select the most suitable pump, Refer to the pump selection data table in the manufacturer’s catalog for pump type, range and pump performance curve. Based on expected fluid production rate and casing size, select the pump type which will be operating within the recommended operating range and nearest to the pump’s peak efficiency.
When two or more pump types have similar efficiencies at the desired production rate, the following recommendations should be considered to select the most adaptable pump to the well conditions:
The shape of the pump performance curve:
The ability of a pump to adapt to changes in well performance depends on the characteristic shape of the pump performance curve. A pump with a steep characteristic (i.e. large change in head with respect to flow rate) is less suited to a well with poorly defined inflow performance (IPR), especially if it is intended to produce with a fixed drawdown. For such pumps, a small loss in IPR translates to a large fall in pump intake pressure and may result in gas locking. Conversely, the head produced by a pump with a flatter characteristic will change less for a given change of flow rate and can therefore be used over a wider variety of IPR’s with limited changes in intake pressure.
The presence of free gas in the tubing above the pump changes the fluid density, consequently reduces the required discharge pressure. Also the performance of centrifugal pumps is considerably affected by the presence of free gas in the pumped fluid. The pump starts producing lower than normal head as the produced GLR (Gas to Liquid Ratio) at the pumping conditions increases beyond a critical value. The critical value of the ratio or percentage of free gas present at the pump intake to the total volume of fluid depends on the pump impeller design (typical critical values are shown in the article “ESP: Gas handling device “). Therefore, it is essential to determine the percentage of free gas by volume at the pumping conditions in order select the proper pump and gas handling device (if required).
Percentage of free gas by volume:
Assuming that Solution GOR (Rs), Gas Volume Factor (Bg) and Oil Formation Volume Factor (Bo) are known, the total volume of fluids and the percentage of free gas released at the pump intake should be calculated.