Effect of pre-test pressures and temperature on DST interpretation

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Abstract

Drill-stem tests (DST's) in porous media typically consist of a sequence of production and shut-in periods. Interpretation of DST's is often based on application of the Horner method to the shut-in periods. Such an analysis assumes that the wellbore rate is constant during the production period and that the pressure response during the shut in is affected only by that production period. For many DST's, however, these assumptions are no longer valid and other interpretation methods are required. For example, prior production and shut-in periods and/or pre-test pressures can significantly affect the pressure response of subsequent periods and cause erroneous parameter estimates. In this paper, we present a new DST analytical solution that can account for the effect of pre-test pressures, prior test periods, wellbore temperature variations, and other factors such as wellbore storage and skin on the pressure response. The solution is an extension of the solution presented by Correa and Ramey who derived a DST solution by writing a single time-dependent wellbore boundary condition. Application of the new solution to example DST data shows that estimates of formation pressure from the Horner analysis can deviate from the true formation pressure by several hundreds of psia, whereas permeability estimates can differ by a factor of two or three. The errors in the estimated formation parameters increase as the permeability is reduced, but can be significant even for large permeability reservoirs. These errors are more pronounced for longer pre-test pressure periods and when the pre-test pressures are much greater or smaller than the true formation pressure. The effect of wellbore temperature variations are also shown to be important for DST interpretation in certain cases when the permeability-thickness product is small.

Original languageEnglish (US)
Pages87-99
Number of pages13
Publication statusPublished - Jan 1 1997
EventProceedings of the 1997 SPE Annual Technical Conference and Exhibition. Part Omega (pt 1) - San Antonio, TX, USA
Duration: Oct 5 1997Oct 8 1997

Conference

ConferenceProceedings of the 1997 SPE Annual Technical Conference and Exhibition. Part Omega (pt 1)
CitySan Antonio, TX, USA
Period10/5/9710/8/97

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All Science Journal Classification (ASJC) codes

  • Fuel Technology
  • Energy Engineering and Power Technology

Cite this

Johns, R. T., & Ma, L. (1997). Effect of pre-test pressures and temperature on DST interpretation. 87-99. Paper presented at Proceedings of the 1997 SPE Annual Technical Conference and Exhibition. Part Omega (pt 1), San Antonio, TX, USA, .