Wellbore Transmissibility from Afterflow-Dominated Pressure Buildup Data

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Abstract
This paper shows how pressure buildup data distorted by afterflow can beanalyzed directly to give a numerical value for effective wellboretransmissibility. Because such data are directly indicative of wellbore-limitedproductivity, the method is helpful in assessing productivity, the method ishelpful in assessing the need for and the effectiveness of a well stimulation. Alternatively, it is a method for interpreting that portion of a pressurebuildup curve not subject to Horner analysis. The method does not require any estimates of formation or wellboreproperties for the analysis. The procedure consists of first plotting pressurebuildup from flowing plotting pressure buildup from flowing pressure as afunction of shut-in time on a log-log pressure as a function of shut-in time ona log-log scale and, second, matching the data points to one of a set ofparametric type curves. From this match, a numerical value is calculated forwellbore transmissibility. Finally, the shape of the buildup curve indicateseither wellbore damage or stimulation. The paper describes the generation and use of type curves with computed dataillustrating the effects of wellbore damage or stimulation. A variety of fieldexamples demonstrate the practical utility of the method. practical utility of the method Introduction Afterflow, or afterproduction, is the continued influx of fluid from aformation into a wellbore after the well is shut in. The greater the storagecapacity of the wellbore, the longer the duration of afterflow. Such afterflowprecludes a conventional analysis of a pressure buildup curve so long as theafterflow rate is greater than a few percent of the production rate prior toshut in. Consequently, production rate prior to shut in. Consequently, afterflow has generally been regarded as a nuisance. Yet, it is reasonable tosuppose that such data contains valuable information. Certainly most of thepressure change occurs during afterflow provided it is present; hence, pressuremeasurements during afterflow should pressure measurements during afterflowshould be more accurate than at later times. Also, the pressure change in thewellbore during afterflow should reflect a sort of dynamic balance between twolimiting factors:the capacity of the wellbore to store fluid andtheease with which the formation supplies fluid to the wellbore. This secondfactor, which is indicative of wellbore transmissibility, is the one oftechnical importance since it determines productivity. However, most of thepublications dealing with afterflow are concerned either with estimating itsduration or with correcting pressure buildup data taken during its pressurebuildup data taken during its duration. Gladfelter, et al. gave a method forcorrecting pressure buildup data as early as 1955. Ramey has provided equationsto estimate the duration of afterflow.

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