Frequently Asked Questions in Well Test Analysis

Abstract

Abstract Despite the tremendous progress achieved in well test analysis in the last twenty years, many important and practical questions that are frequently asked by practicing engineers have received little attention in the literature. Three of these are addressed in this paper. The first one is whether it is possible to obtain more information from build-up data than from drawdown data in a testing sequence. By introducing a new definition of the radius of investigation - as opposed to the radius of drainage - based on the ability to interpret data using pressure derivatives, we show that the information obtainable from a long build-up following a short drawdown is actually limited by the gauge accuracy and noises as the shut-in time increases. The second frequently asked question (FAQ) relates to the minimum amount of detailed rate history which is required in order to obtain correct pressure derivatives, as a truncated or averaged rate histories may modify the derivative shapes at late times and therefore induce erroneous interpretations. A new rule of thumb is proposed, which combines the most recent flow-rate history with an effective time based on a fraction of the cumulative well production. The last FAQ concerns the possibility to distinguish a non-uniform mechanical skin effect from a uniform one from well test data. Current interpretations with fully penetrating wells yield a single value for the mechanical skin effect, which implicitly assumes that damage is uniform over the well surface. Various skin distributions are investigated with a multilayered model to determine how a non-uniform skin distribution around the wellbore affects the interpretation. It is shown that non-uniform skin distribution can be identified during specific flow-regimes and therefore must be taken into account in the interpretation. Introduction Very significant advances have been made in well test analysis over the last twenty years with the introduction of a systematic interpretation methodology, pressure derivative analysis and new interpretation models. As a results, well test analysis has not only become more powerful, but also easier and faster to perform. There remains, however, a number of issues which are faced by practicing engineers in their day-to-day work, but have not received much attention in the literature. This paper aims at giving some answers to these frequently asked questions (FAQ) in well test analysis. FAQ # 1: Can we see more in a build-up than in a drawdown? This has been a subject a disagreement for many years. Some defend the concept that once you start producing a well, the pressure disturbance is felt everywhere instantaneously, and therefore, by shuting the well in indefinitely, you should be able to obtain all possible information on the reservoir, even that which has not been obtained in the drawdown. Others believe that "if you don't see it in the drawdown, you don't see it in the build-up". The answer to the question can be obtained from the concept of the radius of investigation of a well test, which is the distance at which a given feature of the reservoir can be interpreted, taking into account the influence of noises in the data and the rate history of the test. Radius of drainage The radius of investigation, as defined in this paper, is different from the radius of drainage which is routinely used in well test analysis. The radius of drainage has been the subject of many publications, mainly in the 1950's and early 1960's (summarized by Van Poolen1) with a few more recent ones2–4. They all propose similar expressions, of the form reD=1+D.tD0.5 in dimensionless parameters4 In most cases, the use of the line-source solution with an infinitely small wellbore radius simplifies this equation to reD=D.tD0.5. A summary of the various definitions is given in Appendix A. Radius of drainage The radius of investigation, as defined in this paper, is different from the radius of drainage which is routinely used in well test analysis. The radius of drainage has been the subject of many publications, mainly in the 1950's and early 1960's (summarized by Van Poolen1) with a few more recent ones2–4. They all propose similar expressions, of the form reD=1+D.tD0.5 in dimensionless parameters4 In most cases, the use of the line-source solution with an infinitely small wellbore radius simplifies this equation to reD=D.tD0.5. A summary of the various definitions is given in Appendix A.