The Use Of Pressure Build-Up Data In Pressure Transient Testing

Abstract

Introduction Over the past decade, the petroleum literature has been deluged by a seemingly endless variety of well test analysis techniques. Although some developments have occurred as refinements to the fundamental flow relationships, most have arisen from better models and more complex boundary conditions and flow geometries. Early on in the literature, steady state concepts were deemed adequate(1); apparently because of simple radial flow geometries, high permeability and strong reservoir drive mechanisms coupled with the use of pressure gauges with low resolution capacities. Gradually unsteady state techniques were introduced and graphical techniques evolved(2) to establish formation capacity, reservoir damage, etc. Today the well test analysts can be found working in conjunction with production engineers, drilling and completion engineers, reservoir engineers and explorationists alike. Recent developments in bottomhole pressure gauges with higher pressure resolving capabilities and the availability of computer resources have facilitated increasingly sophisticated pressure analysis techniques and yet very little has been written on how to collectively incorporate these new techniques into a unified and practical method of pressure transient analysis. It is the intent of this paper to provide one approach found to be very successful for analyzing the tests encountered. Also demonstrated is the use of pseudo pressure, pseudo time and a general method to correct build-up data for use with drawdown type curves. Although the focus is primarily on build-up analysis after a constant rate drawdown, the extension of these techniques to other types of analysis is obvious. A Unified Approach The plot of the logarithm of pressure, or pressure function, vs the logarithm of time, or time function, forms the basis of this technique (commonly referred to as the log-log plot). Because the axes of a log-log plot are of fixed dimensions relative to each other, the analyst has a standard frame of reference by which all times may be compared regardless of pressure behaviour. This facilitates quick qualitative assessment of the test through repeated pattern recognition developed through experience.High precision gauges have allowed the development of pressure derivative techniques(3) which greatly enhance qualitative test analysis due to the derivative's high sensitivity to changes in flow geometry. Further developments by Bourdet et al. (4) combined the pressure derivative response with the log-log plot to reduced the uniqueness of match to a limited number of alternatives within the scope and span of the data. Other advantages are the independence of the pressure derivative response to the initial shut-in pressure and the simple mannerin which pure flow geometries occur as straight lines. The conventional plots (plots of a pressure function vs a time function) can use data delineated by observing the pressure derivative response on the log-log plot to verify the well test interpretation. The authors do not endorse the approach of using the pressure and pressure derivative log-log plot alone to complete the analysis. The strength of the unified approach is the existence of consistency cross-checks between different methodologies to improve the quality of the answer. This reduces the uncertainties when type curve matching or conventional techniques are used independently.