Abstract
Gas reservoirs are mainly produced by depletion with an aquifer rise; reservoir simulation requires two main SCAL inputs: the amount of trapped gas by the aquifer (residual gas saturation: Sgr) and the relative permeability to water due to aquifer flooding. As it is quasi impossible to predict aquifer strength, the primary SCAL input for reservoir simulation is the Sgr. The recovery factor is directly defined by initial and residual gas saturations. In fact, the residual gas saturation Sgr highly depends on the initial gas saturation Sgi and there is no universal petrophysical parameter governing the shape of this curve. This relationship can be described by several different models (Land, Aissaoui…). While Land’s model is widely used, the Aissaoui model better fits the experimental results (Suzanne et al. 2003), at least for homogeneous sandstones. For a given threshold of initial gas saturation Sg0, this relationship typically exhibits a plateau at high Sgi>Sg0 and an increasing linear trend at low Sgi<Sg0. The challenge here is to properly estimate the value of the Sg0 threshold. Classical laboratory method would require one experiment per point in the Sgr/Sgi plot, and therefore can be achieved in a matter of months. Here we propose a laboratory method allowing the acquisition of the Sgr/Sgi curve in a few days. The proposed method combines centrifugation and capillary rise under imaging. First, the centrifuge allows creating a saturation profile along a sample; measured by NMR. Then, capillary rise is used to capture Sgr under NMR monitoring. By adding NMR imaging, this technique allows combining the benefits of centrifugation to explore a wide range of Sgi; and the ease and cost effectiveness of capillary rise to measure the resulting Sgr. Therefore, at a timescale close to a traditional capillary rise, the proposed technique avoids Land extrapolation and provides a direct measurement of Sgr in a wide range of Sgi. As an additional benefit, the combination of NMR and centrifuge can provide at the same time a direct measurement of capillary pressure, providing information on the gas in place and potential imbibition process in the reservoir.
Highlights
With stronger and stronger economic and environmental drives, gas reservoirs are becoming more attractive
The Richemont outcrop sample was submitted to a spontaneous imbibition in the dedicated setup and the capillary rise was monitored by NMR
The two reservoir samples were submitted to a spontaneous imbibition in the dedicated setup and the capillary rise was monitored by NMR
Summary
With stronger and stronger economic and environmental drives, gas reservoirs are becoming more attractive. At the discovery of a gas reservoir, it is essential to evaluate the amount of Gas In Place (GIP). This is commonly done by using the primary drainage capillary pressure curves. In some cases like a rise of the aquifer previous to the discovery, the reservoir can be undergoing an imbibition process at discovery. In this case, imbibition capillary pressure curves are required to describe the gas saturation between the original free water level and the current free water level. With the pressure drop and the subsequent encroachment of the aquifer into the gas reservoir, water traps gas.
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