Abstract

Multiphase production systems are being considered as a development option for many fields on a worldwide basis. Engineers are faced with the challenge of selecting the best candidates to take full advantage of this novel technology. A review of the literature reveals that no guidelines have yet been published regarding this issue. This paper utilizes classical reservoir engineering methods to model the interaction of a surface installed multiphase pump with the reservoir and tubing. A derivative analysis of the backpressure equation is used to show the difference in incremental production among wells with different backpressure coefficients and exponents. A method was developed which combines well deliverability and decline data to forecast the impact of a multiphase pump on reserves, i.e. ultimate recovery. The results of this study indicate that:

  1. wells exhibiting a low wellhead backpressure coefficient (n) make poor candidates for multiphase pumping;

  2. plotting the derivative of the wellhead backpressure equation provides a means of defining a threshold, which must be reached before a multiphase pump, would provide value;

  3. liquid loaded wells may yield the best response to a surface installed multiphase pump; and,

  4. reduction of the backpressure on a well, through use of a multiphase pump, acts to increase the ultimate recovery for a well or field.

Introduction

Multiphase pumping has been shown to provide value in a number of different operating environments. These include heavy oil production in Venezuela,1,2,3 California4 and Italy,5 light oil production in Canada,6 Indonesia,7 and Trinidad,8 as well as gas condensate fields9 and subsea production.10 A summary of the various multiphase pumping technologies as well as a discussion of their worldwide applications was presented by Scott11 and Scott and Martin.12 All of these methods are installed on the surface to boost the pressure of the fluids produced from the wellhead. This study does not consider a particular multiphase pump technology, but rather the general method of using a multiphase pump to decrease wellhead pressure. The goal of this paper is to define methods that can be invoked to identify the best candidate wells for a reduction in wellhead pressure.

This paper utilizes classical reservoir engineering methods to model the interaction of a surface installed multiphase pump with the reservoir and tubing. First, bottomhole reservoir deliverability is analyzed to identify which reservoirs will best respond to a reduction in bottomhole pressure. The derivative of the backpressure equation is used to show the difference in incremental production associated with a reduction in wellhead flowing pressure among wells with different backpressure coefficients and exponents. Next, the expression for oil well deliverability at the wellhead developed by Thrasher, Fetkovich and Scott (1995)13 is utilized to consider the effects of tubing on candidate wells. The impact of a lower wellhead pressure on liquid loaded wells and tubing limited wells is discussed. Also, a simple economic model is proposed for comparison of the costs and benefits of a multiphase pumping project. Finally, the impact of backpressure on ultimate recovery is considered. The work of Fetkovich et al (1996)14 develops the relationship between the wellhead backpressure exponent and the decline type/ultimate recovery of the reservoir. Using this approach, a method was developed which combines well deliverability and decline data to forecast the impact of a multiphase pump on reserves.

System Analysis - Steady-State

The response of a well to a reduction in wellhead pressure, through use of a multiphase pump, depends on the overall production system. This includes the reservoir, formation completion and tubulars. The steady-state response of the reservoir and completion will first be considered through bottomhole analysis.

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