Why Plot the Equivalent Time Derivative on Shut-in Time Coordinates?

Why Plot the Equivalent Time Derivative on Shut-in Time Coordinates? David Anderson; David Anderson Fekete Assoc. Inc. Search for other works by this author on: This Site Google Scholar Colin Lyle Jordan; Colin Lyle Jordan Fekete Assoc. Inc. Search for other works by this author on: This Site Google Scholar Louis Mattar Louis Mattar Fekete Assoc. Inc. Search for other works by this author on: This Site Google Scholar Paper presented at the SPE Gas Technology Symposium, Calgary, Alberta, Canada, April 2002. Paper Number: SPE-75703-MS https://doi.org/10.2118/75703-MS Published: April 30 2002 Cite View This Citation Add to Citation Manager Share Icon Share Twitter LinkedIn Get Permissions Search Site Citation Anderson, David, Jordan, Colin Lyle, and Louis Mattar. "Why Plot the Equivalent Time Derivative on Shut-in Time Coordinates?." Paper presented at the SPE Gas Technology Symposium, Calgary, Alberta, Canada, April 2002. doi: https://doi.org/10.2118/75703-MS Download citation file: Ris (Zotero) Reference Manager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex Search Dropdown Menu nav search search input Search input auto suggest search filter All ContentAll ProceedingsSociety of Petroleum Engineers (SPE)SPE Unconventional Resources Conference / Gas Technology Symposium Search Advanced Search AbstractThe (semilog) derivative of pressure data from oil and gas wells is a widely accepted and highly useful tool for reservoir characterization. For buildup pressure data, this derivative is calculated with respect to equivalent time. It is generally assumed that since this derivative is obtained using equivalent time, it should be plotted on a coordinate axis that uses equivalent time. However, in the presence of boundaries, this style of plotting distorts the shape of the derivative during late-time, to the point where it bears very little resemblance to the original shape of the drawdown derivative. Moreover, the shape of the buildup derivative is strongly affected by the duration of the preceding drawdown.This paper investigates the shape of the equivalent-time-derivative when plotted against both equivalent time and real time (shut-in time) coordinates. Using computer generated models, it shows that a plot based on real time produces a buildup derivative response that is much closer in shape to the drawdown derivative, thus resulting in more accurate diagnosis of reservoir characteristics. This was true regardless of reservoir type, geometry or producing time. The advantages of using real time as a plotting coordinate are, a more readable diagnostic plot and a more easily recognized shape because of its similarity to the drawdown.Our research extends the work of Spivey (1999). This study investigated numerous reservoir types including homogeneous with boundaries, composite, fractured and dual porosity, and it also covered a wide practical range of producing times.IntroductionA great deal of literature exists on the use of derivative typecurves for buildup tests. The general consensus in the literature is that a buildup pressure derivative, calculated with respect to equivalent time, should be plotted against equivalent time to give a shape that is similar to the drawdown derivative. This procedure works provided that the radial flow assumption is valid and producing time is sufficiently long. However, in cases where boundaries, heterogeneities, or fracture flow exist in conjunction with short production times, the opinions of some experts vary.Spivey et al (1999) suggest plotting the buildup pressure derivative calculated with respect to shut-in time against shut-in time for all reservoir cases1. Although this method allows the welltest analyst to develop a complete set of buildup type-curves that do not rely on the pre-assumption of radial flow, the resulting curves bear little resemblance to their drawdown counterpart for many reservoir configurations. More importantly, the shapes of these typecurves depend on producing time, thereby adding to the complexity of the diagnostic analysis.Onur and Satman (1998) suggest using the conventional plotting method (equivalent time derivative plotted against equivalent time) for all cases, except when producing time is short, in which case they propose to plot the equivalent time derivative against shut-in time2. Although this method provides buildup type curves that are similar in shape to drawdown type curves, it requires the analyst to decide what is a "short" producing time. Indeed, an experienced welltest analyst can usually identify the effects of a "too short" production time (or a significant rate change that occurs very close to shut-in) on a plot of equivalent time buildup derivative against equivalent time. Upon identification of these equivalent time effects, the analyst would then re-plot the equivalent time buildup derivative against real shut-in time.The method we propose eliminates the need for the analyst to be able to identify producing time effects from the diagnostic plot, but preserves a set of buildup typecurves that are very similar in shape to drawdown typecurves for a wide variety of reservoir configurations. The method is simple: always plot the equivalent time buildup derivative against shut-in time, regardless of the duration of the preceding drawdown period. Keywords: drillstem/well testing, fluid dynamics, equivalent time derivative, scenario, upstream oil & gas, field example, producing time, flow in porous media, real time, permeability Subjects: Reservoir Fluid Dynamics, Formation Evaluation & Management, Flow in porous media, Drillstem/well testing This content is only available via PDF. 2002. Society of Petroleum Engineers You can access this article if you purchase or spend a download.

Fetched live from OpenAlex and de-inverted. Abstracts are not stored in this database: the inverted indexes are 8.6 GB of the frame’s 9.3 GB of text, and the host has 13 GB free.