Implicit Solutions and Precision Pressure Measurements Provide Reliable Transient Analyses
Authors: Rodgers John S. , McArthur Bill W. 
Published: December 2007

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Abstract
An investigation is presented in regard to the influence of solution technique and instrument sensitivity on transient pressure analyses.
One of the actual pressure buildup cases included illustrates the effects of two different pressure-gauge sensitivities on conventional pressure-gauge sensitivities on conventional evaluations of mobility, skin effect and extrapolated pressure. The results of these evaluations emphasize the importance of using sensitive pressure gauges in all types of transient pressure tests.
Another area investigated includes the effect of solution technique and precision of pressure measurement on the analyses of pressure measurement on the analyses of transient data influenced by reservoir boundaries. Two theoretical pressure buildup cases and one actual pressure buildup case are analyzed to illustrate how solutions of distances to partially closing boundaries and boundary shape are obtained by an implicit technique. This method employs a formulation of the shut-in pressure in terms of the extrapolated pressure (at "infinite" shut-in time) and the pressure (at "infinite" shut-in time) and the effects of nonclosing boundaries. Application of this method leads to an implicit solution of the diffusion constant (in terms of boundary distance) by establishing a relationship between the precision of pressure measurement and an observed departure time. Theoretical buildup data are generated in terms of the extrapolated pressure, the diffusion constant and a selected pressure, the diffusion constant and a selected boundary shape and distances. Measured and theoretical buildup profiles are compared in a systematic variation of distances for each boundary shape selected. The minimum standard deviations for each shape are then compared to determine which shape(s) and respective boundary distances can be considered for a most probable reservoir description. For an actual case studied, it is shown that pressure data measured with insensitive gauges pressure data measured with insensitive gauges can have a large band of standard deviation which contains all the minimum or "best fit" deviations of the different boundary shapes. In these situations a reservoir description cannot be inferred from the analysis without supporting information from other sources.