Permeability Alteration of Sandstone Using A High-Energy Liquid Compound
Authors: Heath Larman J. , Huff Ray V. 
Published: December 2007

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Abstract
This study was done to determine the technical feasibility of the thermal alteration of sandstone by the in situ burning of a high-energy compound. A proprietary liquid monopropellant was burned in several sandstone cores to effect an increase in permeability. This report summarizes the results of preliminary tests.
Attempts were made to burn the propellant in 13 different cores, each saturated with the combustible liquid. Burning was complete in three of the cores, while in three other cores burning was only partial. The increase in permeability for the three completely burned cores ranged from 109 per cent to 384 per cent and showed an average increase in air permeability of 216 per cent. Results of these tests, based on recorder response, indicated a maximum uniform heating of the rock matrix to a temperature of about 455 deg.C.

Introduction
Most gas-storage and gas-producing reservoirs have adequate capacity but would benefit from improved deliverability. Daring peak demand periods, the lack of formation permeability adjacent to the wellbore often limits the flow of gas. By altering the permeability of the sandstone around the wellbore, productivity can be increased.
Sandstones are composed primarily of crystal phases or modification of silica. There are seven principal crystalline phases of silica, but the three most common ones are quartz, tridymite and cristobalite. Quartz, the phase most generally found in nature, can be converted to tridymite or cristobalite by heat. The stable forms of these silicas are: quartz below 867 deg. C, tridymite to 1,470 deg. C and cristobalite to the liquid form at 1,723 deg.C. Within each of these crystal modifications there are reversible changes or inversions which can be effected by the rapid cooling or heating of the crystal through a fairly definite temperature value. For example, the low-high inversion of quartz occurs at 573 deg. C. Daring the heating and also during the modification of silica crystals there are changes in volume. The density of quartz is 2.65, that of tridymite is 2.26 and that of cristobalite is 2.32. The transformation of quartz to tridymite causes an increase in volume of 17.3 per cent. In most volume changes, fracturing or spalling takes place. In addition to its effect on silica, heat can effect not only the dissociation of carbon dioxide from carbonates but also the complete irreversible dehydration of clays.