Analysis of Pressure Fall-off Tests in High Capacity Water Injection Wells
Authors: Falade G.K. , Namba T. , El-Hadidi S. 
Published: December 2007

The use of a simple data correction procedure which involves the adjustment of bottom-hole flowing pressure prior to a Pressure Fall-Off (PFO) test has been illustrated. The procedure has proved very effective for cases where the early time pressure data are marked by well bore secondary skin effects, which are commonly seen in high capacity water injection wells. The results of field cases considered, including those that may otherwise be considered as normal tests, showed that it is good Engineering practice to routinely apply the new procedure for all PFO tests in water injection wells.

Introduction

Pressure response profiles observable during PFO tests of water injection wells are sometimes very peculiar, offering no direct clues as to the predominant well bore phenomenon at early test times. This is more so for wells with limited perforations subjected to very high water injection rates. The pressure profiles of PFO tests on these wells are characterized by a substantial separation between the Pressure and the Pressure Derivative curves on the diagnostic (Log-Log) plots. Also the short time pressure response profiles are usually not amenable to analyses using established techniques for the flow regimes observable at short times. The long time pressure and derivative profiles are more descriptive.

In view of the fact that PFO tests of water injection wells are of limited application in evaluating permeability of a given reservoir, it would appear that, in dealing with PFO tests of water injection wells, the emphasis should be on Interpretation with a view to understanding the fluid flow phenomenon at the near well bore region, and the reservoir rather than dwelling too heavily on routine analysis for reservoir parameters. We are therefore concerned with down-hole flow and static phenomena and their possible effects on reservoir dynamics involving such reservoir properties as average pressure and skin factors. In the circumstance, it is important that the short time pressure data be left unencumbered as much as possible so that all possible flow regimes can be evaluated without undue error.

PFO in Water Injection Wells

In order to meaningfully design or interpret a PFO test in a water injection well, it is important to understand, the factors governing the dynamics of this kind of test. We consider an injector at the center of a five-spot pattern. In its simplest form, we can assume that the water front moves radially outwards pushing the oil ahead of it. If we further assume that there is no interference between the injector and the producers within the pattern, we can in general expect to have three distinct zones within the drainage radius of the injector. These zones are made up of the following:A water swept region around the well bore where only water is flowing: designated as the water bank.An Oil/Water transition zone immediately ahead of the water bank where both oil and water are flowing.A virgin reservoir region of unswept oil where only oil is flowing designated as the oil bank.