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.