This paper presents rigorous methods to analyze and interpret production rate and pressure data from oil wells using type curves to perform decline curve analysis. These methods are shown to yield excellent results for both the variable rate and variable bottomhole pressure cases, without regard to the structure of the reservoir (shape and size), or the reservoir drive mechanisms. Results of these analyses include the following:Reservoir properties:Skin factor for near well damage or stimulation, sFormation permeability, kIn-place fluid volumes:Original oil-in-place, NMovable oil at current conditions, Np, movReservoir drainage area, A
We have thoroughly verified these analyses and interpretation methods using both synthetic data and numerous field examples. In addition, we provide illustrative examples to demonstrate the ease of analysis and interpretation, as well as to orient the reader as to what are the benefits of rigorous decline curve analysis.
The importance of performing accurate analysis and interpretation of reservoir behavior using only rate and pressure data as a function of time simply can not be overemphasized. In most cases, these will be the only data available in any significant quantity, especially for older wells and marginally economic wells where both the quantity and quality of any types of data are limited. The theoretical application of this technique is for newer wells, at pressures above the bubble point, although we show that the methods described here can be accurately applied at any time during the depletion history of a particular well.
The development of modem decline curve analysis began in 1944 when Arps published a comprehensive review of previous efforts for the graphical analysis of production decline behavior. In that work, Arps developed a family of functional relations based on the hyperbolic decline model for the analysis of flow rate data.
Arps' efforts provided a variety of results; including the exponential, hyperbolic. and harmonic rate decline relations that we use today for empirical decline curve analysis. Due to the simplicity and consistency of this empirical approach, the Arps relations remain a benchmark in the industry for the analysis and interpretation of production data.
The utility of the Arps relations is the applicability of the hyperbolic family of curves to model a wide variety of production characteristics. In addition, the simplified analysis of exponential and hyperbolic data trends (such as the graphical techniques provided by Nind) maintain the popularity of the Arps relations.
The application of the Arps relations typically includes a semilog plot of rate versus time where the hyperbolic cases yield gently declining curves which have the straight-line, exponential decline case as a lower limit. Nind provides the development and illustration of plotting functions for the graphical analysis of rate data for the general hyperbolic decline case as well as the exponential decline case.
Another attraction of the Arps relations is their use in graphical as well as functional extrapolation. Many analysts rely uniquely on the Arps relations for performance predictions. often without realizing the empirical nature of such extrapolations. In this work we will use exponential decline case as a basis for estimating movable oil at current conditions, Np, mov. We will demonstrate that this approach can be derived theoretically for the case of a well produced at a constant bottomhole flowing pressure. We will also show that this approach works for wells which are not produced at such restrictive conditions.