Automatic Optimization Of Parameters In Horizontal Well Test Analysis
Authors: Buitrago S. , Gedler G. 
Published: December 2007

The pressure transient well test analysis is a technique which allows the petroleum engineer to determine reservoir properties, such as permeability, porosity, the drainage volume of the reservoir, static pressure and, in general, to characterize or describe the reservoir-well system in order to indicate well damage or stimulation, fracturing or not of the well, the existence of faults or flow barriers, the approximate shape of the drainage area of the reservoir or the change of the reservoir lithological properties.

The aim of this work is to assist engineers in the analysis of pressure transient data from a horizontal well, calculating the parameters of the reservoir-well system. The basic idea is to find the set of reservoir and well parameters which provides the closest match between measured and model pressure data.

Thus, robust optimization algorithms able to calculate the global optimums in a reasonable number of function evaluations and to guarantee an adequate answer for any arbitrary set of pressure transient data is required.

This work reports a nonlinear global optimization technique able to locate the best set of reservoir and well parameters. The technique is a multi start type algorithm that combine a stochastic exploration of the domain and a heuristic calculation of a descent direction, in order to avoid stopping the algorithm at a local optimum. The methodology was designed to take advantage of a cluster of workstations.

Two type of objective functions were used to test the algorithm. The second type corresponds to a pressure transient data. In this case an excellent match between measured and predicted model pressures was obtained. All the runs underscore the non uniqueness of parameter estimation.

This methodology helps to reduce the engineers subjectivity when exploring the possible parameter values for the reservoir-well system, and to choose the more feasible physical solution among the possible optimum solutions.

Introduction

The pressure transient well test analysis is a technique which allows the petroleum engineer to determine reservoir properties, such as permeability, porosity, the drainage volume of the reservoir, static pressure and, in general, to characterize or describe the reservoir-well system in order to indicate well damage or stimulation, fracturing or not of well, the existence of faults or flow barriers, the approximate shape of the drainage area of the reservoir or the change of the reservoir lithological properties.

The aim of this work is to assist engineers in the analysis of pressure transient data from a horizontal well, calculating automatically the parameters of the reservoir-well system.

Pressure transient data from horizontal wells is (in many cases) practically impossible to analyze using conventional techniques such as straight line analysis or type curve matching. This is due to the complexity of the well and reservoir and reservoir geometry and the effect of multiple boundary on the wellbore pressure response.

While the assumption of the existance of one flow regime, which is radial, during the transient flow behavior of a vertical well is valid, it is not so for a horizontal well.

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