Design, Interpretation, and Assessment of Short-Term Pressure-Transient Tests

Abstract

Abstract In order to meet most test objectives, conventional transient well testing usually requires long flow and shut-in periods. However, the current industry drivers demand short, cost-effective, and environmentally friendly test procedures, especially in exploration wells. This is particularly true in deepwater and arctic environments where conventional tests may be prohibitively expensive or logistically not feasible. While various short-term tests, test procedures, and interpretation methods are available for conducting successful short-term tests, clarity is lacking for specific applications of these methods. Some of these tests include surge testing, closed-chamber testing, slug testing, underbalanced perforating and testing, and back-surge perforation cleaning. This paper provides comprehensive evaluation of general closed-chamber tests, including general surge tests, and their comparison with special tests such as, FasTest,™ Impulse™ test, and slug tests. For each of these techniques, the review will examine:Test design, testing procedureTheoretical background of each of these techniquesMethod of data analysis including comparison based on both theoretical and practical considerations to determine the expected reliability, accuracy, and ease of analysis. A large portion of the paper will be devoted to field examples. Several actual case studies are analyzed using the various techniques, and results are tabulated and presented. The analyses of several of these examples will be presented in significantly more detail to compare techniques available to analyze the well-testing data obtained from surge testing, closed-chamber DST, slug testing of oil wells, underbalanced perforating and testing, and back-surge perforation cleaning. Introduction Advances in gauge, tool assembly, and telemetry technology have collectively paved the way for the conduct of short-term tests. Some of these tests may last for time periods as short as a few minutes. Techniques developed for analysis of these tests rely on modem gauge capability for accuracy and quick measurement of pressure change with time as well as accurate compensation for the effect of temperature. These methods have been well established in the literature and include short-term tests such as:DSTSlug testGeneral closed-chamber test (CCT)Surge Test"Shoot-and-pull" test, which is similar to the backsurge testFasTest (essentially a surge test/CCT)Impulse Test (also essentially a surge test/CCT). All the above tests with the exception of the slug test are similar in nature. Fluid flows into a limited volume chamber where an increasing back pressure causes the influx from the formation to decline. The decline in rate is very fast and is difficult and many times impossible to calculate. In many of these tests there is no differentiation between the flow and build up periods. This dictate the development of specialized techniques that turns accounts for this tests characteristic. In a slug test, however, flow is not against atmospheric pressure but against increasing hydrostatic head as fluid accumulation takes place. It is usually possible to calculate the rate production of fluid into the wellbore. This would allow for the use of classical analysis approaches if one wishes to. The goal of this paper is to provide comprehensive evaluation of general closed-chamber tests, including general surge tests, and their comparison with special tests such as FasTest, Impulse and slug tests. The paper will analyze the practical considerations of the various tests and analytical techniques to determine the expected reliability, accuracy, and ease of analysis.