CHAPTER ONEINTRODUCTION1.1 Background of the study. Before a well can produce oil or gas, a borehole is drilled from the surface to the oil and gas pool or reservoir rock. The borehole must be stabilized with casings cemented in place. A small diameter tubing string is centered in the wellbore and is sometimes held in place with packers. This tubing will carry the oil and gas from the reservoir to the surface. Reservoirs are typically at elevated pressure because of the underground forces that surrounds them. The driving force which causes these fluids to move out of the reservoir comes from the compression of the fluids that are stored in the reservoir. The actual energy that causes a well to produce oil results from a reduction in pressure between the reservoir and the producing facilities on the surface. Early in its production life, the underground pressure will often push the hydrocarbons all the way to the surface facilities. Depending on reservoir conditions, this “natural flow” may continue for many years. The production capacity of an oil well is a key surveillance factor in monitoring the well’s performance which is dependent on a number of other factors which include; fluid properties and composition of the oil itself, gas oil ratio, water cut, reservoir characteristics and completion strategy/design (Beggs et al, 1991). The well performance consists of the inflow performance relationship (IPR) which involves flow from the reservoir into the wellbore and the outflow performance which involves flow from the wellbore up the tubing to the surface production facilities. Constant monitoring of the well performance is of paramount importance in the oil and gas industry as it is critical to the crude oil production obtainable from such wells and the equivalent profit margins that will be generated from the production (Hernandez et al, 2001). Monitoring of the well’s performance also help the engineer to determine the economic limit and amount of oil production recoverable from such wells. Once a well is produced down to its economic limit due to pressure depletion, increase in water-cut the pressure differential which is the driving energy becomes insufficient in producing the fluid (oil) to the production facilities (Beggs et al, 1991). Gas lift method once applied at the required injection gas rate can supplement the reservoir energy to drive the oil to the surface. The introduction of lift gas to a non-producing or low producing well is a common method of artificial lift. Naturalgas is injected at high pressure from the casing into the wellbore and mixes with the produced fluids from the reservoir . The continuous aeration process lowers the effective density and therefore the hydrostatic pressure of the fluid column, leading to a lower flowing bottom-hole pressure (Pbh). The increased pressure differential induced across the sand face from the in situ reservoir pressure (Pr), given by (Pr − Pbh), assists in flowing the produced fluid to the surface. The method is easy to install, economically viable, robust, and effective over a large range of conditions, but does assume a steady supply of lift gas. At a certain point, however, the benefit of increased production due to decreased static head pressure is overcome by the increase in frictional pressure loss from the large gas quantity present. This has the effect of increasing the bottom-hole pressure andlowering fluid production.
1.2 statement of problem Movement or transport of oil to the production facilities requires energy to overcome friction losses in the systems and to lift the products to the surface. The production system can be relatively simple or can include many components in which energy or pressure losses occur. The production rate or deliverability of a well can often be severely restricted by the performance of only one component in the system due to pressure losses. As mentioned earlier, within the life of a reservoir, there is a time when the available reservoir pressure is unable to lift the produced fluid to the surface. This is mainly as a result of pressure depletion , increased water production (water cut). Also, the need may arise for producing companies of oil and gas to maximize the production of oil and gas at the current installation facilities and reservoir condition. Most times, the efforts of the companies are directed to a medium and long term project to maximize the factor of recovery (production of oil to the minor possible cost), and in the short term to accelerate the recovery of the recoverable reservations. These efforts are usually realized during the economic limit period of the well in which the energy for production (pressure draw down) is insufficient for the required production rate. It becomes necessary to optimize the production system of the well to increase the economic recovery and to meet up with the increasing global energy demands.