Combined Artificial Lift System - An Innovative Approach

Abstract

Abstract Artificial lift methods such as gas lift and electric submersible pump offer great advantages in improving vertical flow performance and oil well productivity. Each method has its own limitations and disadvantages as well. A combined artificial lift system is proposed to improve oil well productivity and to overcome as many of the disadvantages as possible. The objective is to minimize total energy requirements to maximize production. Major benefits of combined artificial lift are to reduce initial capital and operation costs, by optimizing compression gas and electricity requirements and to reduce abandonment pressure leading to higher recovery. Main applications areas include new and developing fields mature fields, fields with low gas supply, low PI and deep wells. Introduction Millions of barrels of oil remain virtually inaccessible in reservoirs because of the limitation on the abandonment pressures. Hopefully the new advent of technology in artificial lift mechanism will enable the production engineers to recover a part of the oil left behind by lowering the abandonment pressures. Reservoir pressure declines with time due to net extraction of fluids. With the declining pressure, the operating artificial lift mechanism eventually fails to lift the wellbore fluids to the surface and consequently, the well dies. Therefore, it becomes necessary to switch to a more efficient lifting technology that can operate at lower pressures and thereby extend the well life. When all economic lifting methods have been tested in accordance with the availability of surface facilities, the reservoir is said to have reached the optimum abandonment pressure. Inflow performance (Figure 1) is proportional to drawdown pressure (P), i.e., the difference between reservoir static pressure and the flowing bottom-hole pressure (Pwf). A positive flow results if the difference tween the bottom hole flowing pressure and the sum of the pressure losses due to friction and the hydrostatic fluid column is greater than the tubing head pressure, or: (1) If the criteria of equation 1 are not met, the well cannot produce under natural conditions, and hence an artificial lifting mechanism is required to bring additional energy. Common lifting mechanisms are:–Gas lift and its derivations–Mechanical pumping (sucker rods, progressive cavity)–Electric submersible pumping. Numerous articles related to Multiphase flow and artificial lift can be found in the literature. A few of these examples are cited. As reservoir conditions change with time, artificial lift quantities have to increase in order to maintain proper fluid production. A continuous depletion of reservoir pressure will cause the Pwf level sufficiently low as to make the conventional lifting inefficient and uneconomic. These situations are ideally suitable for combining different lifting practices, such as gas lift and ESP for improved utilization of lifting methodology. Additionally, surface facilities such as, gas compression and/or electric power may vary from site to site. Gas usage varies dynamically in accordance with the market demand and corporate business strategy thereby affecting the field performance. On the other hand, some secondary recovery projects may turn-out to be uneconomic when the investment cost of artificial lift using ESP or gas lift are considered. As is described later in the paper, a combined artificial lift methodology results in reduced equipment size and therefore, a reductions in capital investment. simpler infrastructure and reduced operating costs.