Abstract

Residual gas saturation is considered to be a key factor in evaluating gas recovery from a lean gas reservoir invaded by water. Residual gas saturation is known to be dependent on both pore network characteristics and initial gas saturation. In this paper, sixty experimental relationships between initial gas saturation (Sgi) and residual gas saturation (Sgr) are presented for a large set of sandstone samples and checked against former empirical Sgr-Sgi laws. Previously, no more than two authors have presented more than twenty full experimental Sgr-Sgi relationships.

The core plugs are selected from two different sandstone gas reservoirs and from Fontainebleau sandstone outcrop. Porosity and permeability plugs, respectively range from 0.06 to 0.25 and from 0.1 to 2000 mD, with Sgrm values ranging from 0.04 to 0.65.

Two-phase Sgr is achieved by controlled evaporation -spontaneous imbibition methods. The main results are:

  1. Two-phase Sgr-Sgi experimental relationships have piecewise linear form, characterized by two parameters: the maximum residual gas saturation, Sgrm, and the saturation corresponding to the intersection of the two segments, Sgo.

  2. Sgo was found to be dependent on the amount of microporosity and different from Swir. The constant Sgr region confirms that the microporosity does not trap gas. The linear Sgr/Sgi region corresponds to gas trapped in the macroporosity.

  3. None of the hyperbolic laws is able to describe correctly the observed experimental behavior. Jerauld's and the simplified version of Land's relationships are the poorest estimates of Sgr as a function of Sgi.

  4. The best relationship to describe Sgr-Sgi curves is the piecewise linear Aissaoui's law. It results in a significantly better agreement with experimental data than Land's original formulation. Both forms rely on two fitting parameters.

  5. Our experimental results show that porosity (or permeability) and the amount of microporosity along with the initial gas saturation control Sgr values.

Introduction

During depletion of gas fields, the aquifer often encroaches into the reservoir, and residual gas saturation (Sgr) is used to estimate microscopic recovery. Residual gas saturation is known to be dependent on both pore network characteristics and initial gas saturation (Sgi). Maximum residual gas saturation (Sgrm) values vary between 0.05 and 0.95 according rock characteristics1,2. Yet the economic impact of Sgr on gas reservoirs can be extremely high.

Many studies have attempted to understand gas-trapping mechanisms according to various research axis: experimental method, wettability, rock characteristics and initial gas saturation influence. First, Geffen3 established that residual gas saturation measured in the laboratory on core samples is the same as in a gas reservoir. The effect of water flooding rates on Sgr was found to be negligible3,4,5. Katz6 showed that the residual gas left behind the moving water front remains constant and equal to that obtained during the measurement of capillary pressure. Several authors demonstrated that Sgr obtained by water flooding and spontaneous imbibition are very close3,4,7, provided the reduction in Sgr due to diffusion is disregarded5. The effect of the type of displacing liquid was also found to be negligible3,8,9. The same Sgr values were obtained whatever the pressure and temperature prevailing during the core test3,5,7,10.

The results mentioned above prove that simple experimental conditions may be representative of gas trapping in reservoirs. As the objectives of this study are to gather a substantial number of experimental results over a large range of rock characteristics, simple experimental conditions are preferable. In this work, residual gas saturations are obtained by spontaneous imbibition at ambient conditions on samples.

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