A Field Test of Nitrogen WAG Injectivity
Authors: Greenwalt Wayne A. , Vela Saul , Christian L.D. , Shirer J.A. 
Published: December 2007

A one-well field test of nitrogen WAG (water alternating gas) injectivity was conducted to determine whether a reduction in water injectivity would occur after injection of nitrogen. A 40% reduction was observed immediately after nitrogen injection. Following three short WAG cycles, water injectivity increased to its pretest level with injection of a large volume of water.

Introduction

The Jay/LEC field in Florida and Alabama is producing under unitized waterflood operations. Several papers describe this field's historical development. The field test described in this paper was undertaken as part of an engineering evaluation of injecting nitrogen and water alternately into the Jay/LEC field Smackover reservoir to determine the effect on a miscible gas tertiary process. Several operators of miscible WAG injection projects have encountered a reduction in water injectivity after gas injection. in two projects, water injectivity decreased significantly and operators speculated that either the precipitation of asphaltenes, trapped residual gas saturation, or movement of fine granules of reservoir rock caused declines in injectivity. These projects were unable to achieve oil production rates forecast because more than the anticipated time was required to inject planned water volumes. If a reduction in water injectivity occurs because of trapped carbon dioxide saturation, the high solubility of carbon dioxide in water should allow water injectivity to increase rapidly during the succeeding water injection cycle. In addition, injection wells may experience a slight increase in permeability and porosity due to the solution of carbon dioxide in water forming a weak acid. In a nitrogen WAG project, the low solubility of nitrogen in water will prevent the trapped gas saturation from changing significantly during the water injection phases of WAG injection. Several documented WAG projects using low-water-soluble gases have observed significant reductions in water injectivity, while significant reductions in water injectivity when high-water-soluble gases are used are not documented. Laboratory experiments were performed with cores from the Jay/LEC field in an attempt to determine whether a seduction in water injectivity would occur if nitrogen and water were injected alternately into the Smackover reservoir. The results of these experiments were inconsistent. Changing rock wettability and movement of formation fines may have contributed to the inconsistent results. Since laboratory experiments to provide a basis for predicting field injectivity were unsuccessful, and in light of reported poor field performance, a field test was carried out to determine whether nitrogen WAG injection into the Jay/LEC Smackover reservoir would reduce water injectivity. During the field test initiated in Aug. 1978, reservoir injectivities and transmissibilities of water and nitrogen were observed during three cycles of nitrogen followed by water injection into a Jay/LEC field well. The conventional Homer analysis interpretation technique did not provide definitive solutions. A technique consisting of plotting transmissibility vs. square root of shut-in time provided a useful tool for observing fluid banks in the reservoir and evaluating changes in water and gas injectivities.

Design

To ensure the selection of a representative injection well and simplify interpretation of bottom hole pressure measurements, the following criteria were established.Reservoir development in the injection well should be limited to one relatively thin zone.The ratio of permeability-thickness to injection rates during the injectivity test should be representative of rates in injection wells if a field wide project is carried out

JPT

P. 266^