Viscous Displacement In Fractured Porous Media

Abstract

In some fractured reservoirs, a gas pressure gradient of the order of 3 -5 kPa/m may be established in the- fractures due to flow. Such a pressure gradient could result in recovery enhancement of the matrix oil. Several tests, are conducted to study viscous displacement in fractured porous media ,with artificial fractures. These tests are analysed by using a fully-implicit finite difference simulator with appropriate fracture capillary pressure. The results show that there is considerable recovery improvement due to viscous displacement. For a gas pressure gradient of 3 kPa/m, the matrix oil recovery increases by 10% of PV in one of the tests.

Introduction

In fractured porous media comprised of matrix blocks and a fracture network, gravity and capillary forces affect the two-phase flow in the matrix blocks and the fractures. Capillary forces play a major role in the interaction between the matrix blocks via the capillary continuity mechanism ⁽¹⁾ . Gravity forces affect the drainage performance of the matrix blocks and the reinfiltration process ⁽²⁾ . In addition to capillary and gravity forces, in certain cases, viscous forces are expected to affect the production performance of fractured reservoirs.

Gas injection in some fractured reservoirs with low oil viscosity (say 0.2 mPa.s and an effective permeability to matrix permeability ratio of less than say 30) may result in a small pressure gradient in the fractures between an injection and production well. The gas pressure gradient in the fractures away from the well could be of the order of 3-5 kPa/m. Such a pressure gradient in the fractures improves matrix oil recovery by reducing the matrix capillary threshold height for gravity drainage and the capillary end effect in the horizontal displacement. There is no published data in the literature on viscous displacement in fractured porous media.

The purpose of this work is to: I) provide experimental data on viscous displacement in fractured porous media for gas-oil displacement processes, and 2) analyse the data to examine the nature of displacement improvement from viscous forces. In the following, we first discuss the experimental set up and present the data, and then analyse the data using a finite difference simulator.

FIGURE 1: Viscous displacement experimental setup.(Available in full paper)

FIGURE 2: Matrix-fracture configurations used in the experiments (drawn not to scale). (Available in full paper)

Experimental

The apparatus schematic is depicted in Figure 1. It consists of a glass-walled case with metal top and bottom plates supported by metal framing. The metal framing allows the 0.95 cm thick glass plates to be forced against the rock faces. Design allows the setup to be tilted through 240 ° about a central horizontal axis. The glass case was sealed using fuel resistant room temperature vulcanizing fluorosilicone rubber. The bottom and the top end plates were made of 2.54 cm and 0.64 cm thick aluminum plates, respectively. Both plates provided connections for vacuum, ventilation, gas injection, fluid loading and drainage (see Figure 1). A valve mounted 6 cm above the bottom face of the coreholder allows gas to flow out freely without interfering with liquid flow.