Impact of New-Defined Criteria on Evaluation of Depth of Investigation for Radial and Transient Linear Flow

Abstract

The depth of investigation (DOI) is a fundamental parameter in many reservoir engineering calculations, whereas there is no generally accepted industry standard and evaluation method so far. Inconsistencies or errors will arise when unconditionally using various formulae to calculate the value of DOI. This paper investigated the key issues about DOI in terms of production rate, pressure cut-off value, especially the selection of evaluation criteria. New relations for DOI evaluation with variable DOI criteria are developed based on the solution of diffusivity equations of radial flow and transient linear flow. We propose a new DOI evaluation technique for radial and transient linear flow by using a flow-rate-ratio or pressure-drop-ratio based criteria. The validity of this method was demonstrated with synthetic radial flow and transient linear flow cases. This new technique reveals the independency of DOI on production rate and gauge resolution, ant explains the discrepancies between historical methods. For transient linear flow the new technique explains the difference of DOI coefficients between constant-rate and constant-FBHP conditions and unified the two in one relation by introducing ratio-based criteria. New DOI formulas can be derived easily according to different values of self-defined criteria. Meanwhile it is robust and compatible with other historical methods. The new technique provided in this paper overcomes the inconsistencies among various DOI equations and can significantly improve the practicality and effectiveness of DOI evaluation.