A Conceptual Study of Dual Lift Design in Ultradeep Wells

Abstract

For many years rod pumping has been a desirable artificial lift method for on-shore oil wells. Today, in addition to rod pumps, other artificial lift methods such as gas lift and electrical submersible pumps (ESP) are also available for assisting well production. While rod pumps have been proven to be one of the simplest and most economical choices, when it is not sufficient for high-rate wells, harsh-fluid production, or deep wells, operators turn to other lift methods to produce hydrocarbons to the surface. The selection of appropriate artificial lift methods revolves around many factors, including economics. This paper presents a conceptual and feasibility study of combination of two methods of lifting for production of moderate volumes of oil from deep or ultra-deep onshore wells. It proposes to use an ESP and a sucker rod pumping unit simultaneously to lift fluids. The argument is that to lift from a deep well, with only ESP the cost and housing limitation of the wellbore may become a vital problem. Lifting the hydrocarbons to a certain depth by an ESP system, and then using the rod pump to transport the fluid to the surface may be a solution for deep and ultra deep wells. One of the critical issues in this concept is that rod pump lift is an intermittent lift, but ESP is a continuous lift. This issue has been address with the design of a chamber in between the two systems to accommodate the two lift devices and ensure continuity. This paper presents an example with detailed calculations to provide the technical support to the idea. Even though this approach has not been applied in the field, once proven feasible, the dual lift method can bring a solution with economic value to artificial lift for ultra-deep onshore oil wells. Introduction Artificial lift is a means to assist oil well production. It is applied when the wellbore pressure drop is too high, and the pressure at the bottom of the well (supplied from the reservoir) cannot lift the desired amount of fluid naturally to the surface. The main methods of artificial lift include gas lift and pump lift, such as sucker rod pump, plunger pump, positive capacity pump and electrical submersible pump (ESP). While gas lift aims at lowering the pressure gradient in the tubing, pump lift increases production rate by lowering the bottomhole flowing pressure and boosting the pressure with the pump. There are two types of pumps in artificial lift, positive displacement pumps (sucker rod pumps) and dynamic displacement pump (e.g. electrical submersible pump). The rod pump is, probably, the oldest method of artificial lift and has proven to be a very desirable lift method due to the simplicity and minimum requirement of maintenance, for many years rod pumping has been a very desirable artificial lift selection for on-shore oil wells. As the technology has allowed operators to go deeper and produce at higher rates many other artificial lift methods have been developed. A modern example is ESP systems. Compared with a rod pump, an ESP can handle higher flow rates, and is more tolerable of the produced fluid. Selecting the appropriate artificial lift method is an essential but well-understood practice in production engineering. The decision of artificial lift methods revolves around many factors, predominately economics and technical complexity. The fundamentals have been discussed in detail in many publications (Allen and Robert, 1978, Economides, et. al., 1994, Lea and Bearden, 1999, and Naguib, et. al., 2000), and have been followed in the field applications. As energy demands have increased and technologies advanced, deep and ultra deep wells have been increasingly drilled. The industry faces the challenges in drilling and completion of these wells, as well as producing such a well with appropriate artificial lift. While rod pumps are limited in deep wells, the conventional design of ESP will also have wellbore space and cost concerns. To the point, in some deep wells only one lift method may not satisfy the production requirement. This paper discusses a possibility of combining two forms of artificial lift methods for deep well production.