Decline Curve Analysis Using Type Curves: Case Histories

Abstract

Summary The purpose of this paper is to present case history studies that demonstrate methods of analyzing rate-time data to predict future production and to determine reservoir variables. Constant wellbore pressure analysis techniques are demonstrated, using pressure analysis techniques are demonstrated, using existing qDd - tDd type curves along with developing new qDd - tDd type curves from actual field data. Case histories for individual oil and gas wells are presented, along with groups of wells in a field and presented, along with groups of wells in a field and total field studies. The field studies include a one-well full water drive field, a low permeability solution gas drive field, and a field with both primary and secondary (waterflood) history. Field primary and secondary (waterflood) history. Field shutins and backpressure changes are shown to retrace the early time rate data as would be expected from superposition principles. Reservoir variables developed from a total field rate-time match are compared to early well pressure buildup analysis results. Comparisons are excellent. This work not only demonstrates the technique of analyzing rate-time data, it also presents a method whereby a reservoir or formation dimensionless type curve can be developed from rate-time field data. The resulting type curve can then be used to forecast wells or fields in the same reservoir or formation. Because such a type curve is dimensionless, changes in stimulation, spacing, and reservoir properties can also be accounted for. Introduction Since the original presentation in 1973 by Fetkovich of the paper "Decline Curve Analysis Using Type Curves", many successful applications have been made with declining rate-time data using the type curve approach. Case history studies of individual oil and gas wells, of groups of wells in a field, and of total fields are presented in this follow-up paper. Additional papers dealing with the constant wellbore pressure solution which also include the depletion period have since been published to aid analysis and understanding of what we published to aid analysis and understanding of what we now call "advanced decline curve analysis." In essence, decline curve analysis is a forecasting technique: rate-time data is first history matched on an appropriate type curve after which a forecast is made. Complex simulation studies proceed similarly. This paper demonstrates that by using basic reservoir engineering concepts and knowledge we know what direction to take, what type curve(s) to choose and where the rate-time data should fit. Decline analysis must work since it is founded on basic fluid flow principles, the same principles as used in pressure transient analysis. The problem most engineers have had and will continue to have with decline curve analysis is bad, erratic, or insufficient data. Careful attention to obtaining accurate flow rates, flowing pressures and downtime should help solve the problem. A good rate-time analysis will not only give the same results as conventional pressure transient analysis but will also allow a forecast to be made directly at no cost in lost production. For low permeability stimulated wells in particular, pressure buildup testing could be eliminated in many cases as being of little value or economically unjustifiable because of the resulting production loss when compared to what can be obtained from properly conducted constant wellbore pressure drawdown tests. pressure drawdown tests. RATE-TIME TYPE CURVE ANALYSIS CONCEPTS The Radial Flow Solution The fundamental basis of advanced decline curve analysis is an understanding of the constant wellbore pressure solutions and their corresponding log-log pressure solutions and their corresponding log-log type curve plots, which is the inverse of the constant rate solution. Fig. 1 is a composite of the analytical constant wellbore pressure solution and the Arps exponential, hyperbolic and harmonic decline curve solutions all on a single dimensionless type curve. The depletion stem values of b range between 0 (exponential) and 1 (harmonic) which are the normally accepted limits.