Type Curves for Analysis of Afterflow-Dominated Gas-Well Buildup Data

Abstract

A set of type curves is described that permits estimation of near-wellbore damage or stimulation from the afterflow-dominated portion of a pressure buildup test in a gas well. The result is portion of a pressure buildup test in a gas well. The result is an improved estimate of damage or stimulation relative to that obtained with constant fluid-property analyses of afterflow-distorted data. Introduction The conventional semilog plot of pressure buildup data is commonly used to determine formation permeability and, by means of the skin factor and flow efficiency, the degree of near-wellbore damage or stimulation. However, when these data are distorted by afterflow, other methods of analysis are required to extract useful information. One such method involves the use of type curves. Several sets of type curves are available, all of which work well when applied to oil wells and, in many cases, to gas wells. However, all were derived assuming constant fluid properties. For gas wells, this may result in inaccurate values of permeability and flow efficiency, particularly at low pressures when the gas compressibility is large. This paper presents an extension to gas wells of the "McKinley method" of type-curve analysis for afterflow-dominated data by including the effects of changing gas properties with pressure. While many approximations are involved, it will be demonstrated that, for gas wells, the new curves offer an improvement over McKinley's constant fluid-property curves. There is a penalty to be paid for this improved accuracy, however - the new types curves are more difficult to use than McKinley's. Therefore, they should be used only when (1) pressures beyond the afterflow region are not available for conventional semilog analysis, and (2) improved accuracy is desired relative to what would be obtained using the McKinley (or any constant fluid property) type curves to analyze gas-well buildup data. property) type curves to analyze gas-well buildup data. For rough estimates of damage or stimulation in gas wells, the easier-to-use McKinley curves will be shown to be adequate. With these new curves, it should be possible to obtain from afterflow-distorted gas-well buildup data a reliable value for near-wellbore permeability, and improved estimates of formation permeability and flow efficiency. This information is valuable in selecting workover and stimulation candidates, as well as in evaluating the effectiveness of workovers, stimulation treatments, and well completions. Development of the Type Curves Parameters Governing Afterflow Parameters Governing Afterflow When a flowing well is shut in at the surface, there will be a period of time during which appreciable flow into the wellbore will continue. This "afterflow" or "afterproduction" is a result of the wellbore storage capacity, which may be written F = 5.615 cw V,.............................(1) where cw = wellbore fluid compressibility, psi V = wellbore volume, bbl. The constant 5.615 converts F to cu ft/psi. JPT P. 915