High-Velocity Gas Flow In Liquid-Saturated Porous Media And Its Visualization
Authors: Noman R. , Archer J.S. 
Published: March 2010

Deviation from Darcy's law is known to occur for high-velocity fluid flow through porous media due to inertial and turbulence effects. In the presence of a second phase, this deviation increases even further. The present study was carried out to investigate the effect of a static liquid phase on the inertial coefficient for high-velocity gas flow through consolidated sandstones.

Experimental work was carried out on core samples at various liquid saturations, up to the irreducible minimum. The inertial coefficient was found to increase with increasing liquid saturation though not as rapidly as had been reported for carbonates. Furthermore, no minimum value of the inertial coefficient could be found at any particular saturation. A method is proposed to enable the value of the inertial coefficient to be estimated for saturated cores.

A method has also been developed/or visualizing the wetting and non-wetting phase distribution during the high-velocity flow process using coloured epoxy resin for locating the two phases.

Introduction

For fluids flowing at high velocity through a porous medium, it is well known that Darcy's law does not correctly predict the pressure drop. This is due to inertial and turbulence effects becoming significant thereby causing extra pressure drop. These phenomena are usually encountered in gas wells where the velocity near the wellbore can become very high. The pressure gradient is also at its highest in this region. In gas condensate wells for example, liquid saturation could build up due to retrograde condensation. In the presence of a liquid phase the non-Darcy effect in the gas can increase considerably, thereby causing a loss of productivity. High liquid saturation in the vicinity of a wellbore may also be caused by acidizing or mud filtrate invasion.

The increase in the inertial effect in liquid saturated porous media was clearly demonstrated by Gewers and Nichol(l) who carried out experimental measurements on micro-vugular carbonates with static liquid phase. Their work was later extended by Wong(2) for a mobile phase using the same cores. The work on carbonates(l) showed that the inertial coefficient (often wrongly termed as "turbulence factor") actually decreases on increasing the liquid saturation from 0% to 10% and then rapidly increases with any further increase in saturation. It was also reported(2) that the inertial coefficient increases by as much as eightfold on increasing the liquid saturation from 40% to 70%. It has been assumed(3) that the same behaviour would also be observed in sandstones. Due to lack of information in the literature the present work was carried out to check if this assumption is true. This study using sandstones, therefore, extends the earlier work carried out by Gewers and Nichol on carbonates.

To have a proper understanding of the effect of liquid saturation on the inertial coefficient it is necessary to have a knowledge of the distribution of the liquid phase within the pores. For this purpose, a method using coloured epoxy resins has been developed to observe the two phases at a microscopic scale.