A Systematic and Comprehensive Methodology for Advanced Analysis of Production Data

Abstract

Abstract This paper is a comprehensive presentation of ALL the methods available for analyzing production data, highlighting the strengths and limitations of each method. These methods include those developed by Arps, Fetkovich, Blasingame and Agarwal-Gardner, as well as a new method called the Flowing Material Balance. Some methods yield recoverable reserves, while others give hydrocarbons-in-place. Traditional (Arps) decline analysis (exponential or hyperbolic) gives reasonable answers in many situations, but has its failings, the most important one being that it completely ignores the flowing pressure data. As a result, it can underestimate or overestimate the reserves. The increase in electronic data measurements has made the flowing pressure as readily available as the flow rate. This enables the use of the more sophisticated methods of analysis, which take into account both the flowing pressure and the production rate. The sequential and systematic use of all these methods gives a consistent and more reliable answer. The strengths and limitations of each method are reviewed, from the simplest exponential decline to the Flowing Material Balance. Two examples are given, one that overestimates reserves, and one that underestimates them. An additional example illustrates the importance of knowing the dominant flow regime. Introduction Techniques of production data analysis have advanced significantly, over the past few years. There are many different methods available currently. However, there is no one clear method that always yields the most reliable answer. The first systematic approach to analyzing oil and gas production data was presented by Arps1 in the 1950's. A testament to the success of Arp's style decline analysis is its continued popularity today. One of the most attractive features of the Arps methodology is its simplicity. Because it is an empirical method, it requires no knowledge of reservoir or well parameters. The application of the method involves using an empirical curve match to predict the future performance of the well. Thus, it can be applied to production through any type of reservoir drive mechanism. In fact, a practical set of guidelines has been assembled through extensive field analysis, which suggests what curves belong with which type of reservoir2. One of the restrictions of Arps decline analysis, is its inability to disassociate the production forecast from operating constraints. In other words, the ultimate recoverable reserves, predicted by the Arps decline must inherently assume that historical operating conditions remain constant in the future. Another drawback of Arps is its limited applicability in the transient (or infinite acting) flow regime. In fact, Arps decline analysis is often misused for transient dominated production data, such as hydraulically fractured tight gas wells. Under such conditions, an analysis technique capable only of predicting ultimate recovery (such as Arps) has very limited application. Instead, other methods that use transient solutions should be used. Fetkovich2 was the first to extend the concept of using typecurves (previously reserved for welltest analysis) to the analysis of production data. The Fetkovich methodology uses the same Arps depletion stems to analyze boundary dominated flow, and constant pressure typecurves (originally developed by VanEverdingen and Hurst) for transient production. The most valuable feature of typecurves lies not in the analysis, but in the diagnostics. For example, the typecurve match can show whether or not production data is still in transient or has become boundary dominated (something that Arps decline analysis does not do). Like Arps, the Fetkovich method calculates expected ultimate recovery, but is constrained to existing operating conditions. The transient portion of the Fetkovich typecurves assumes constant bottomhole flowing pressure. Thus, for discontinuities in production data (such as extended shut-in or placing a well on compression) a segmented approach must be taken. Furthermore, if the well is rate restricted, such an approach will not work.