Unitization of producing oilfields in recent times in the interest of reduced operating costs has resulted in the centralization of production operations. In order to increase production rates and maintain reservoir pressure, secondary oil recovery schemes have been installed. Much of the initial secondary recovery work has been based on the injection of water or water based solutions to replace oil voidage and for reservoir pressure maintenance.
In conjunction with the unitization of the production facilities and consequent centralization, waterflood injection facilities have also been centralized and have resulted in large capacity injection pumping facilities. This trend to larger facilities has resulted in pump manufacturers developing larger capacity pumping units to meet the demand. It was not long ago that the larger positive displacement pumps common to waterflood service were 250 to 300 horsepower units whereas 600 horsepower units are now commonplace.
With this increased demand for higher injection capacity. the application of the centrifugal pump for high pressure service has become a consideration. One of the first applications of centrifugal pumping equipment for high pressure waterflood injection service was for offshore locations on floating and/or supported platforms. Reciprocating pumps and engines were difficult to utilize because of the inherent unbalanced forces of the reciprocating piston type machines and a resultant vibrational problem. For the offshore application it was not practical to provide the required mass or support mechanisms to take care of the unbalanced forces and subsequent vibration. Electrical motors were ruled out due to the unavailability of on site power at a reasonable cost.
These problem in combination with the cost of multi-platforms as opposed to a larger single facility resulted in the use of turbine driven centrifugal pumping units. The high horsepower/ weight ratios of the light industrial turbine drivers were extremely advantageous considering the design problems for offshore installations.
The centrifugal pumps originally utilized were basically conventional boiler feed pumps. In order to adapt these horizontal split case pumps to high speed operation and meet the high volume requirements it was necessary to produce pumps which were finely balanced and incorporated an improved seal design to withstand the increased wear and sealing problems of the high rpm operation.
The following paper is a discussion of the engineering and economic considerations in the application of the high speed centrifugal pump for waterflood injection service where high speed is defined as a pump shaft speed greater than 3600 rpm.
Pumping units utilized for waterflood injection service must be capable of the following basic requirements.
The hydraulic characteristics of the pump selected must be flex1ble enough to meet the varying capacity and pressure requirements of the proposed injection scheme for the life of the project. The typical hydraulic requirements are initially high capacity-low pressure to replace voidage, changing to lower capacity and higher pressures as reservoir fill up is achieved.
The pumping unit must have a high percentage availability where availability is defined as the percentage of the time the unit is operational.