Mechanism for Permeability Reduction by Small Water Saturation in Tight Gas Sandstones
Authors: Bybee Karen 
Published: February 2015

This article, written by Assistant Technology Editor Karen Bybee, contains highlights of paper SPE 107950, "Quantitative Mechanism for Permeability Reduction by Small Water Saturation in Tight Gas Sandstones," by Siyavash Motealleh, SPE, and Steven L. Bryant, SPE, University of Texas at Austin, prepared for the 2007 SPE Rocky Mountain Oil & Gas Technology Symposium, Denver, 16–18 April.

The full-length paper examines the possibility that a small change in water saturation can change the gas-phase permeability significantly in rocks with low porosity and very low permeability. The model of the grain-scale geometry of low-porosity sandstones is built from a dense random packing of spheres modified geometrically to simulate quartz over growth cementation.

Introduction

In tight gas sandstone, the productivity of one well sometimes is quite different from that of a nearby well. Wells also can be very sensitive to small amounts of water. Although the effect of water saturation on the effective permeability to gas has been the subject of numerous experiments, a fully mechanistic explanation has not been offered as to why the effect appears larger in tight gas reservoirs. The full-length paper explores the possibility that the grain-scale geometry of tight sandstone is responsible.

Small wetting saturation is mainly irreducible wetting phase that exists in two forms. One is volumes of water held in the smallest pores. The other is pendular rings held at grain contacts or liquid bridges held between two grains separated by a gap. The former forces gas to flow around the filled pores, decreasing the average connectivity of the gas phase. The latter reduces the area open to gas (the nonwetting phase) as it passes through a pore throat. It is possible to quantify the effects of these topological and geometrical changes on gas-phase permeability.

Method

Pore-Space Model. The geometry of pore space in porous media is very complex, and a large number of pores must be modeled to obtain reasonable predictions of macroscopic behavior. Direct simulation of flow within pore space is now possible, but physically representative network models of pore space provide an inexpensive alternative.

Delaunay tessellation of the sphere centers subdivides pore space and grain space simultaneously. The tessellation yields tetrahedra. Each face of a tetrahedron corresponds to a pore throat. The geometry of the throats is the primary control on permeability. Each edge of the tetrahedron corresponds to a grain/grain contact (if the spheres touch) or a gap between grains (if the spheres do not touch). The contacts/gaps can support pendular rings/liquid bridges of the wetting phase, depending on capillary pressure. The geometry of these rings/bridges and the geometry of the throats provide the link between small water saturations and effective permeability to the gas phase.

This rock model can be modified geometrically to simulate various rock-forming processes. Many modes of porosity reduction occur in tight gas sandstone. Only isopachous quartz cementation is considered in the full-length paper. The simplest model of this process treats the quartz cement as a coating of uniform thickness on all sediment grains.