Use of PITA for Estimating Key Reservoir Parameters
Authors: Rahman N.M.A. , Pooladi-Darvish M. , Santo M.S. , Mattar L. 
Published: March 2010

Well testing is sometimes reduced to perforating the well in underbalanced conditions, capturing the pressure data and then analyzing it. The primary objective of a Perforation Inflow Test Analysis (PITA) is to estimate the initial reservoir pressure, permeability and skin for evaluating a future development strategy. However, special analytical procedures are required for analyzing the data.

In this study, a systematic analytical procedure for estimating meaningful reservoir parameters from perforation inflow tests will be presented. Two major aspects of data interpretation will be discussed: diagnostic analysis and modelling. A straight-line approach is taken to analyze the early-time and late-time data. A special diagnostic technique is required for detecting and estimating positive or negative skins. A distinctive feature of PITA is that it does not require calculation of the inflow rates. Note that the same approach can be applied in overbalanced situations, when there is fluid efflux, rather than fluid influx.

When employing straight-line analysis techniques, acceptable estimates of initial reservoir pressure, permeability and skin are only obtained if the test duration is sufficient to achieve radial flow. This is usually not a problem in high permeability reservoirs. However, in low permeability reservoirs, the test duration required to reach radial flow can be prohibitively long. In these cases, the test is often terminated during the transition from wellbore storage to radial flow. Consequently, acceptable estimates of initial reservoir pressure, permeability and skin can only be obtained by extending the analysis into modelling. A rigorous technique for estimating the radius of investigation during the test will be discussed.

Introduction

Well tests have been the primary and most reliable means of characterizing reservoirs for decades. However, there has been a growing trend over the last several years to search for alternatives that could yield the desired information in less time in a more environmentally-friendly manner, and at a cheaper cost than conventional well tests. The desired change has inevitably been towards tests of shorter duration(1–4). Although it is accepted that results from short tests with small radii of investigation may not be as reliable as those from conventional well tests, it is reasonable to accept that they could be of value in assisting with strategic decisions about field development when an increased margin of error can be tolerated. In offshore wells, in addition to the potentially exorbitant cost of testing (several millions of dollars), the drive towards green (shorter) tests is fuelled by environmental considerations, such as requirements for restricted flaring of hydrocarbons. In Alberta, and elsewhere in North America, the driving force towards inexpensive tests is the marginal economics of low deliverability wells. Either way, there is an increasing trend towards these green tests to replace conventional well tests. One such green test consists of simply allowing the well to flow into the closed wellbore after perforating when there is an underbalanced wellbore condition. As the fluid from the reservoir enters the wellbore (with a fixed volume), the wellbore pressure builds up.