ABSTRACT

Solution gas drive reservoirs are characterized by rapid and continuous decline of reservoir pressure. This rapid and continuous decline of reservoir pressure causes direct decline of reservoir performance at early stages of the life of the reservoir. The principal source of energy which is gas liberation from the crude oil and the subsequent expansion of the solution gas as the reservoir pressure is reduced are inadequate to produce such reservoirs to their full capacities. Ultimate oil recovery from natural flow of a solution - gas drive reservoir makes it one of the least efficient primary recovery mechanisms. This leaves a substantial amount of remaining oil residing in the reservoir which must be produced.

Artificial lift technologies such as continuous gas lift, gas lift with velocity strings and positive displacement pumping is therefore employed at later phases of the reservoir’s life to increase the ultimate recovery which is what this project sort to do. Synthetic data based on material balance for a solution – gas drive reservoir is analyzed to predict its primary oil recovery based on which gas lifting, velocity strings technology and positive displacement pumping are suggested to be employed with respect to time at different stages of reservoir’s life.

Chapter 1

1.0 Introduction

1.1 Problem Statement

Solution gas drive also known as Dissolved gas drive or Internal gas drive reservoirs are characterised by a rapid and continuous decline of reservoir pressure. This reservoir pressure behaviour is attributed to the fact that no extraneous fluids or gas caps are available to provide a replacement of the gas and oil withdrawals (Tarek, 2001). This rapid and continuous decline of reservoir pressure causes a direct decline of reservoir performance at early stages of the life of the reservoir. Moreover, the principal source of energy which is gas liberation from the crude oil and the subsequent expansion of the solution gas as the reservoir pressure is reduced are inadequate to produce such reservoirs to their full capacities (Tarek, 2001). Ultimate oil recovery from natural flow of a solution gas drive reservoir (less than 5% to about 30%) makes it one of the least efficient primary recovery mechanisms (Tarek, 2001). The low recovery from this type of reservoir suggests that large quantities of oil remain in the reservoir and, therefore, solution gas drive reservoirs are considered the best candidates for secondary recovery applications.

Artificial lift technologies such as continuous gas lift, gas lift with velocity strings and positive displacement pumping method is therefore employed at later phases of the reservoir in order to increase the ultimate recovery. The main challenge is to know when to change existing production mechanism to a new one for optimum recovery. A production design has therefore been made in an attempt to solving this problem with respect to constraints such as maximum production rate, maximum drawdown, and available gas lift.

The flowing bottom-hole pressure required to lift the fluids up to the surface may be influenced by size of the tubing string (Lyons, 1996) and for that matter the time when tubing strings should be replaced as a function of cumulative production is necessary.

1.2 Method of Conducting the Project

Designing the natural flow and artificial lift tubing strings for the whole life of a well forms the tasks of this project. This is based on certain constraints such as maximum production rate, maximum drawdown, and available gas lift and horsepower requirement. Synthetic reservoir performance based on a material balance is the main data source for this project. A forecast of the production of oil as well as the time when tubing strings should be replaced as a function of the cumulative production is proposed.

1.3 Objectives

The objectives of this project are to:

·         Design natural flow and artificial lift tubing strings for the whole life of a well.

·         Forecast the production of oil as well as the time when tubing strings should be replaced as a function of both cumulative production and time.

1.4 Outline of this Project

The project consists of five (5) chapters. Chapter 1 defines the problem at hand, the method which the project follows and objectives. Chapter 2 presents a literature review of the topic as well as the technical terms that make up the topic. Chapter 3 introduces a thorough review of the material balance equation, methods of predicting primary oil recovery with emphasis on Muskat’s method which has been employed in this report. Application of the Muskat’s method is illustrated with a synthetic reservoir data. Chapter 4 comes up with the natural flow design as well as the artificial lift tubing strings with respect to the set constraints.