Abstract

This work presents the results of a systematic study of wellbore pressure response for solution gas-drive systems, during drawdown and buildup tests run under multiphase non-laminar flow conditions.

Under laminar flow conditions, it is well known that the sandface pseudopressure correlates multiphase results with the liquid solution for a constant oil rate drawdown. Also, the calculated wellbore pseudopressure using the gas-oil ratio at shut-in time, is the appropriate function to analyze build-up tests. However, there is no information in the literature regarding the use of this function when non-Darcy flow in the reservoir is important.

This study shows that during a drawdrown test, due to nonlaminar flow there are repressurization effects around the wellbore, which may create an undersaturated zone. This situation has a strong influence on the pseudopressure behavior, preventing the presence of a semilogarithmic straight line during the transient period. During the shut-in period, the influence of the inertial effects in the reservoir is smaller than that during the drawdown period. Thus, apparent semilog straight line, both in terms of pseudopressure as well as in terms of wellbore pressure, are developed. This above result is important, because it allows us for these conditions to use the available methodology for the slightly compressible non-laminar liquid flow case. Therefore, we can detect the presence of inertial effects in the reservoir from a cartesian graph of pressure derivative versus the inverse of the square root of time, and obtain an estimate of reservoir permeability from the ordinate to the origin of this graph. From these results and an approximate value of total skin factor, we obtain a practical procedure to determine the skin condition present in the wellbore. This is the most important result of this work. The analysis of a published field example showing non-laminar flow conditions is also presented.

Introduction

Most of the studies on well testing considering non-laminar flow have been devoted to dry gas systems, where due to the high mobility of the gas it is expected to have high-velocity flow around the wellbore. However, recently Refs. 6 and 7 showed that the presence of non-Darcy flow may have a significant effect on the pressure response, even for liquid systems.

Considering the non-laminar flow problem as one of variable radial permeability, as a perturbation of the corresponding laminar response, and using Oliver's solution, Ref, 6 proposed a diagnostic and evaluation methodology of inertial effects in the reservoir, analyzing their influence on the pressure response through a single well test. Thus, from the results of Ref. 6, it is expected to have significant inertial effects on the pressure performance in solution gas-drive systems.

It is well known that under multiphase laminar flow conditions, the pressure response can not be directly used with the methods developed for slightly compressible liquid flow and that it is necessary to compute appropriate functions of pressure and saturations (pseudopressures), in order to correlate multiphase flow responses with the liquid solutions. However, for solution gas-drive reservoirs under non-laminar flow conditions, this result has not been explored in the literature.

P. 317

This content is only available via PDF.
You can access this article if you purchase or spend a download.