Evaluating the Impact of Multiphase Flow Properties on Formation-Tester Pressure Transients
Authors: Gómez Juan D. , Torres-Verdín Carlos , Proett Mark A. , Ma Shouxiang 
Published: September 2016

Spherical- and cylindrical-based analytical models are frequently used to estimate reservoir flow properties from Formation-Testing (FT) measurements. Solutions to these equations are developed under restrictive assumptions which compromise their reliability over a wide range of field conditions. On the other hand, a Wellbore Numerical Simulator (WNS) enables improved understanding of complex flow systems (Escobar Gómez et al., 2016). In general, numerical models implement full multiphase flow algorithms which are robust but computationally expensive. Such condition often makes the numerical simulation problem less appealing and the estimation of properties more non-unique, especially for Pressure Transient Analysis (PTA). Understanding the circumstances under which total flow properties dominate over individual-phase flow properties prompts the implementation of a singlephase type model for faster and simpler FT-PTA. Starting from a black-oil model, this paper develops a simplified multi-dimensional single-phase pressure equation applicable for multiphase FT data analysis, which is shown to be valid over a wide range of petrophysical conditions.

To conduct the work, a multi-dimensional, single-well, finite-difference model is developed for forward modeling of FT operations. The formulation of the algorithm allows simulating either single- or two-phase flow. Additionally, the numerical code couples an analytical mudcake growth model to simulate the process of mud-filtrate invasion.

We perform radial numerical simulations to quantify the impact of multiphase flow on otherwise singlephase equivalent pressure transients. The study includes effects of pressure-dependent oil properties and two-phase relative permeability. Findings from synthetic cases indicate that such multiphase effects on pressure transients are negligible except in the presence of extreme pressure drops and substantial spatial variations in mobility; for a wide range of testing and reservoir conditions, the pressure signal is mostly governed by combined rock-fluid properties. For PTA purposes, the latter behavior justifies the implementation of a single-phase type model for fast and accurate flow-regime identification and estimation of total mobility.

The above results are confirmed with a field study where measurements were acquired in a multiphase environment. Common practices in numerical interpretation of FT measurements would suggest the need for multiphase algorithms. Nonetheless, excellent pressure history matching is attained with the developed WNS in single-phase mode. Such results confirm that, under suitable testing (and reservoir) conditions, FT pressure transients chiefly respond to spatial variations and total flow properties.

Our work confirms the feasibility of identifying testing conditions such that the recorded pressure is negligibly affected by individual-phase flow properties. The single-phase equivalent model developed in this paper, applicable under such conditions, provides fast and accurate quantification of flow zones without a-priori multiphase flow information.