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Article Title:Decline Curves Analysis
Date:2002/08/20
Body:
As a part of gas reservoir development, one of the most important requirements is to estimate and forecast gas recoveries and production rates for individual wells or entire fields. Different technique has been developed in the past to obtain this information.
Decline curves can be characterized by three factors (1) initial production rate, or the rate at some particular time, (2) curvature of the decline, and (3) rate of decline. These factors are a complex function of numerous parameter within the reservoir,wellbore ,and surface-handling facilities .Formation parameter of porosity, permeability, thickness, fluid saturations, fluid viscosity ,relative permeability, reservoir size will spacing compressibility producing mechanism and fracturing will all contribute to the character of the decline curve .Welbore conditions such a whole diameter ,formation damage ,lifting mechanism, solution gas ,free gas, fluid level, completion interval, and mechanical conditions will have their effect on the decline curve too. The factors that directly affect the decline in gas production rate are (1) reduction in average reservoir pressure and (2) increases in the field water cut in water –drive field.
Generally, only information about production rates versus time (production history) is available to initiate any evaluation of the reservoir.
Among the techniques used for this propose, type curves have been found quite accurate to forecast gas well performance in absence of known reservoir parameters.
Various methods have been developed and published in the literature for estimating reserves ,and these methods range from the basis material balance methods to decline/ type curve analysis techniques.
Decline curve analysis appears to be a very useful method for performing future projections and evaluating original gas in place and reserves.
This project is a discussion about Arp’s Empirical equation, meaning of b (exponent decline) and prediction of that and also numerical solution of the parameter of decline curve analysis.
Arps explained that decline curve analysis began as a rate forecasting technique by extrapolation the production trend. he presented empirical relation ship to evaluate rate behavior versus time .
Fetkovich developed a type –curve matching procedure for decline curve analysis .he integrated analytical solution for oil transient flow with practical decline equations.
Carter generated other type curves dealing with the change of gas properties wit pressure .Studying gas wells, Fraim and Wattenbarger established a normalized produce time function to force the gas reservoir date to match liquid solution for constant Pwf, which was introduced by Fetkovich .The goal of this project is introducing the parameters of the decline curve analysis ,how can we estimate those and also numerical solution of those.
Application of the decline curve analysis:
Technique is used for performing future projections and evaluating original gas in place and hydrocarbon reserves .These estimates are needed to determine the economic viability of the project development.
Used to determine the remaining production life of the well or the entire field.
In addition with this technique, can estimate individual well flowing characteristics, such as formation permeability and skin factor.
Decline curve analysis for gas field is generally different with the conventional type curve matching, especially in the early time period.
Decline curve analysis offer an alternative to volumetric and material balance methods and history matching with reservoir stimulation for estimating original gas in place and gas reserves.
Application of decline curve analysis technique to gas reservoirs is most appropriate when more conventional volumetric or material balance methods are not accurate or when sufficient data are not available to justify complex reservoir stimulation. material balance methods require estimates of stabilized shut-in bottom hole pressure(BHP’s); however in low permeability reservoirs where long time are needed for stabilization ,accurate shut-in BHP’s often are not available.
Unlike volumetric methods that can be used early in productive life of reservoir, decline curve analysis cannot be applied until some development has occurred and production trend is established.
An advantage of the decline curve analysis and material balance calculations is that these methods estimates only the gas volumes that are in pressure communication with an my ultimately be reserved by the producing wells .Volumetric estimates of gas in place and reserves how ever and based on the total gas volume in place ,part of which may be uncoverable with the existing wells because unidentified reservoir discontinuity or heterogeneities .
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