ASSESSMENT OF SAFE AND COST EFFECTIVE METHODS TO MAXIMIZE PRODUCTION (PROFITABILITY) FROM A GAS-LIFTED FIELD
1.1 Background of Study
After the completion of a given well or group of wells, they are then put under production. During this phase of operation, every operator looks for means to minimize operating cost and maximize cumulative oil production in the most cost-effective manner for the entire field. This stage of operation is what is generally termed production optimization. A true optimization requires an operator to take a logical look at the field’s production systems from the sub-surface to surface facilities.
Production optimization implies striking a balance between production deliverability of the wells and demand which basically aim at increasing the rate at which a well flows fluid from the reservoir without restriction to the surface storage tank(s). One of the most common means of conducting production optimization is through nodal analysis. This is normally done to optimize production from single wells or other smaller production systems. Large complex systems demand a much more sophisticated approach to predict the response of a large complicated production system accurately and to examine alternative operational scenarios efficiently. Beggs (1991) stated that optimization is directly dependent on some functions. The functions may be a single variable or more than one variable (multivariate optimization). A well is said to be optimized when it is producing at optimum conditions with minimum problems (Bath, 1998).
Most wells upon completion in oil producing sand formations will flow naturally for some period of time. Production at this stage will be initiated by the existing reservoir pressure. This reservoir pressure will provide all the initial energy needed to bring fluid from the well to the surface. As the well produces, this energy is consumed and at some point, there will no longer be enough energy to bring fluid to the surface. The well at this state, will cease to flow. When this happens, there is need for the well to be put under some form of artificial lift method in order to provide the energy needed to bring the fluid to the surface. It should be pointed out that artificial lift systems can also be used in de-watering of gas wells to sustain production.Basically, there are two methods of artificial lift systems. These are: pumping system (electrical submersible pump, sucker rod etc.) and Gas lift system.
There are different key factors that are considered prior to artificial lift installation in the field which include analysis of the individual well’s parameters and the operational characteristics of the available lift systems. For the different pumps and lift systems available to the oil and gas industry, there are unique operational/engineering criteria particular to each system, but they all require similar data to properly determine application feasibility. Such as the inflow performance relationship, liquid production rate, Gas liquid ratio, water cut, well depth, completion type, well bore deviation, casing and tubing sizes, power sources etc. Each of the artificial lift systems has economic and operating limitations that rule out it consideration under certain operating conditions.
An extensive overview of artificial lift design considerations was presented by Clegg et al. (1993). Clegg mentioned some economic factors such as: revenue, operational and investment costs as the basis for artificial lift selection. Ayatollahi et al., (2001): Selection of the proper artificial lift method is critical to the long-term profitability of the oil well; a poor choice will lead to low production and high operating costs.For the purpose of this work, Gas-lift method will be considered with a view to optimizing production from an oil well and hence optimal production from the field.
1.1.1 Gaslift system
Gaslift is the method of artificial lift which utilizes an external source of high pressure gas for supplementing formation gas in order to reduce the bottom-hole pressure and lift the well fluids. The mechanism of gas-lift is fairly simple. Gas is injected into the tubing string to lighten the liquid column and decrease the bottom-hole pressure, which allows the reservoir to push more fluids into the wellbore. At the same time, increased flow rates in the tubing string and surface flow lines result in higher back pressure on the well and adjacent wells that share a common flow line. This in turn causes a reduction in well production rates. Therefore, lift-gas has to be carefully allocated to achieve maximum efficiency. The primary consideration in the selection of a gaslift system for lifting a well or group of wells is the availability of gas and cost of compression.
Of all artiﬁcial lift methods, gaslift most closely resembles natural ﬂow and has long been recognized as one of the most versatile artiﬁcial lift methods. Because of its versatility, gaslift is a good candidate for removing liquids from gas wells under certain conditions. Again, Production of solids will reduce the life of any installed device that is placed within the produced ﬂuid ﬂow stream, such as a rod pump or ESP. Gas-lift systems generally are not susceptible to erosion due to sand production and can handle a higher solids production than conventional pumping systems. In addition to the above mentioned advantages, gaslift systems can also be employed in deviated wells without mechanical problems.
Gas compressors are usually installed for gas injection or as booster compressors. There are various methods of injecting gas into a well during gas lifting operations. But the most commonly practiced method is the continuous flow gaslift system. Here, the utilization of gas energy is accomplished by the continuous injection of a controlled system of gas into a rising stream of well fluids in such a manner that useful work is performed in lifting the well fluids.
It is important to note that a number of factors affect the performance of a well. An understanding of these factors will allow the designer of a given production system to appreciate the need to obtain all available data before his design work begins. Some of the most common factors that will be considered in view to production optimization are discussed below:
1.1.2 Productivity Index (PI) and well Inflow Performance Relationship
Accurate prediction of the production rate of fluids from the reservoir into the well-bore is essential for efficient artificial lift installation design. In order to maximize production of oil from a gas lifted system, it is often necessary to determine the well’s production. The accuracy of this determination can affect the efficiency of the design.