NATURAL FLOW AND TUBING STRING DESIGN FOR A WATER DRIVE RESERVOIR
1.1 BACKGROUND TO THE STUDY
Fluids are stored in the reservoir and must be produced to the surface facilities in order to be measured, treated and finally sold or discarded. The flow of fluids from the reservoir towards the final processing facility is divided into three phases: Recovery, Lift and Gathering. Recovery refers to the flow of fluids from the reservoir into the well bore; Lift refers to the flow of fluids from the bottom of the well bore to the surface wellhead and Gathering refers to the flow of fluids from the wellhead through the gathering network towards the production facility. Recovery is used in a broader sense a referring to the production including the lift and gathering processes. Lift and gathering process will influence the final recovery of hydrocarbons and must be included in a proper economic analysis.
The flow rate from a well depend on the energy level of the reservoir and the energy losses of the fluids as they flow from the reservoir towards the surface facilities. In order to increase production flow rates we may use processes or systems to either increase the energy level or to facilitate the flow of hydrocarbons. Those systems or processes may be used in the reservoir or in the production tubing or gathering system. The recovery of hydrocarbons may then be classified as: Primary where no process or method is used to increase energy level or facilitate the flow of hydrocarbons inside the reservoir; Secondary and Tertiary where methods are used to increase energy level and or to facilitate the flow of hydrocarbons in the reservoir.
The lift and gathering may also be classified as: Natural flow – No process or method used to increase energy level or facilitate the flow of hydrocarbons in the production system; Artificial lift– when processes are used to increase the energy level or facilitate the flow of hydrocarbons inside the well bore; Boosting – When processes are used to increase the energy level or facilitate the flow of hydrocarbons downstream of the wellhead. The recovery of hydrocarbons is classified in the following categories: Primary Recovery (also called Primary Production); Secondary Recovery (also called Secondary Production); Tertiary Recovery (also called Tertiary Production or Enhanced – EOR or Enhanced Production or Improved – IOR or Improved Production). Those categories are usually associated with a method or recovery (or production) used – Primary recovery uses the pressure and displacement of hydrocarbons without any external process using solely the reservoir drive mechanism, secondary recovery supplements the natural drive effects on pressure maintenance and displacement by water injection or water flood and natural gas injection ; and Tertiary recovery supplements the natural drive by modifying the properties of the fluids by chemical floods, miscible displacement and thermal methods.
Each reservoir is composed of a unique combination of geometric form, geological rock properties, fluid characteristics, and primary drive mechanism. Although no two reservoirs are identical in all aspects, they can be grouped according to the primary recovery mechanism by which they produce. It has been observed that each drive mechanism has certain typical performance characteristics in terms of :Ultimate recovery factor, Pressure decline rate, Gas-oil ratio, Water production. The recovery of oil by any of the natural drive mechanisms is called primary recovery. The term refers to the production of hydrocarbons from a reservoir without the use of any process (such as fluid injection) to supplement the natural energy of the reservoir For a proper understanding of reservoir behaviour and predicting future performance, it is necessary to have knowledge of the driving mechanisms that control the behaviour of fluids within reservoirs. The overall performance of oil reservoirs is largely determined by the nature of the energy, i.e., driving mechanism, available for moving the oil to the well- bore. There are basically six driving mechanisms that provide the natural energy necessary for oil recovery: Rock and liquid expansion drive, Depletion drive, Gas cap drive, Water drive, Gravity drainage drive, Combination drive.
In this work the design of natural flow and artificial lift tubing strings for the whole life of a water drive reservoir will be carried out using data based on synthetic reservoir performance based on a material balance. The effects of reservoir properties on the life of the well will also be investigated. Constraints such as maximum production, maximum drawdown, limitations on surface facilities capacities, as well as available gas lift and horsepower will be imposed.