Pressure-Derivative Approach to Transient Test Analysis: A High-Permeability North Sea Reservoir Example (includes associated papers 15270 and 15320 )

Abstract

Summary A recently developed method of transient test analysis, which uses the derivative of pressure data vs. time rather than the pressure/time data alone, is discussed in general terms with regard to its application to very-high-permeability reservoirs. The extension of this method to the various well/reservoir system models considered by several authors is summarized. The procedure involves the combined use of existing type curves in both the conventional dimensionless pressure form (PD) and the new dimensionless pressure pressure form (PD) and the new dimensionless pressure derivative grouping ( ). The procedure proved theoretically and through examination of data from the Oseberg field to be a powerful tool in transient test analysis. The identification and powerful tool in transient test analysis. The identification and interpretation of reservoir heterogeneities, both linear and dispersed, as well as earth tidal effects, which are often not easily discemible through existing methods, are greatly facilitated by the new method. Introduction The interpretation of transient test data is the main source of information on the dynamic behavior of a reservoir, expecially if permeability is high, because a test can investigate an appreciable volume of the formation. In recent years, the science of transient well test interpretation has progressed rapidly, most notably through the increased use of type curves, the introduction of new reservoir models, and the advent of computer interpretation packages. These advances greatly augmented the conventional analysis approaches that are based on the homogeneous reservoir system and manual, graphical methods. However, many engineers still have reservations about the usefulness of type curves and the reliability of test information from anything but the simplest responses. Improvements in well-testing techniques-especially the introduction of the latest generation of electronic bottomhole pressure (BHP) transducers and recorders have dramatically improved data quality and, consequently, the amount of information that can be obtained from test analysis. An analysis method that deals directly with rate of pressure change, rather than absolute pressures, greatly pressure change, rather than absolute pressures, greatly simplifies the main problems of interpretation, model diagnosis, and evaluation of parameters. Consequently, much more confidence can be placed in the results obtained with the derivative approach. This paper covers extensions of the derivative approach to various models proposed by different authors. its application to high-permeability >1 darcy) reservoirs is demonstrated. All previous work on the subject has been confined to reservoirs with permeabilities less than 100 md. Field examples are used to illustrate tile application's validity. Interpretation of Transient Tests Traditional and Derivative Theory. Practical transient test interpretation methods have become polarized in recent years between two analysis techniques - conventional and global. The former basically consists of fitting straight lines to data regions (for example, in the Homer analysis) or their multirate extensions or superpositions (see Appendix A). The latter involves the use of various type curves to include the entire data set in the process of reservoir/well system diagnosis. Flow-regime identification, and evaluation of system parameters. It is commonly accepted that great confidence in interpreted results is obtained by an iterative combination of the two techniques that starts with the global approach. The recently documented pressure-derivative approach has combined the most powerful aspects of the two previously distinct methods into a single-stage interpretative plot. The most useful traditional type curves depict, on a log-log scale, the evolution of the dimensionless pressure, PD, with the dimensionless time group pressure, PD, with the dimensionless time group. For the basic, most common model of a well with wellbore storage and skin in a homogeneous reservoir, the individual curves are dependent on the wellbore condition group. These type curves have two main drawbacks, especially when associated with high-permeability reservoirs. First, there is the uniqueness of the diagnosis. The various curves have similar shapes, particularly when the effect of wellbore storage is very short-lived, as in the highly permeable Oseberg field. JPT p. 2023