The Optimization Approach of Casing Gas Assisted Rod Pumping System
Authors: Han Guoqing , Wu Xiaodong , Zhang He , Zhang Xishun , Ling Kegang 
Published: March 2015

The free gas evolved in the pump chamber results in a low rod pump working efficiency, and it can even lead to a failure. A common and effective solution is to install a downhole gas separator before fluid entering the chamber, which can divert the free gas to the annulus. If we can re-inject the diverted gas back to the tubing at a shallower depth above the pump, the flowing gas is then re-combined with the liquid and decreases fluid density. Consequently, the injected gas also creates additional lifting drive for the liquid. A new technology based on this concept has been developed and called Casing Gas Assisted Rod Pumping (CGARP). This paper firstly presents an analytical model to optimize the overall lifting performance and minimize the operating expenditure. It is especially useful in producing hydrocarbon at high GOR.

As the gas is re-combined with the liquid above the pump installation depth, the hydrostatic pressure gradient is reduced consequently. However, if the gas reinjection valve is placed at a shallow depth, the well segment at reduced fluid density is subsequently short, so the contribution of gas lift is restricted. Vice versa, if the gas reinjection valve is placed at the depth close to the pump, it requires high pressure to open the gas injection valve, so the gas reinjection can happen infrequently and the production rate is unstable. This paper has proposed a genetic optimization method to maximize the overall production system efficiency. A multi-variable vector has been defined, which includes pumping speed and depth, mechanical power, rod string diameter and length, surface stroke length, downhole separator efficiency, as well as gas reinjection valve depth. The optimized object can be the system lifting efficiency or Net Present Value, which must be a function of this vector in the constraint of mass and momentum conservations.

This work has been applied as the primary guide for four oil producers with rod pump installed in Jilin field, China. The average system lifting efficiency and production rate have been increased by 20% and 15% respectively. This analytical model has enhanced the field performance. Most importantly, the same concept can be applied for other pump-assisted wells.

Introduction

For oil fields with high GOR, gas lift method can be an ideal candidate (Redden, et al., 1974; Herald, 1987). However, because of the high capital expenditure, limitation of available gas, and complexity of the surface system, the rod pumping system has been generally adopted in field. On the other hand, for the high GOR fluid, the release of solution gas inside the rod pump can notably deteriorate its working performance. As an effective and common solution, the Downhole Gas Separator (DGS) or anti-gas pump can restrict the free gas entering the pump and thus improve the pump efficiency (McCoy and Podio, 1989; Dottore, 1994). Unfortunately, the separated gas is usually discharged through the casing, and later mixed with the liquid flow lines at surface. As a result, the energy of this casing gas is not utilized above the pump setting point.

A new concept of Casing Gas Assisted Rod Pumping (CGARP) system has been introduced earlier (Liu et. al 2007). The free gas separated after DGS can be re-injected from the annulus into the tubing above the pump installation depth. After re-combining the gas with the liquid, the fluid density is reduced. Thus, the producer can have an enhanced production rate in the favor of both pump- and gas-lift assistances, as shown in Fig.1.