Abstract

Imbibition runs at different overburden pressures have been performed on a SFE2-8707 sample from the Travis Peak formation (slot and solution pore type), with an approximate porosity of 7% and gas permeability of 22 to 37 d, using our physical model. The experiment continued for several days until the water flow rate reached the steady state values. The single phase water flow experiments were performed at several inlet and overburden pressures, following the imbibition runs. The results from imbibition and single phase water flow experiments showed that under different overburden pressures, due to the porous structure changes, considerable amounts of pore closing and opening occurred which resulted in an overall increase in permeability of the sample. However, from single phase gas flow experiments an overall decrease in permeability of the sample was observed. Furthermore the single phase transient experiments compared well with steady state runs. This proves the reliability of transient runs which results in significant saving in time and expense for core analysis of tight core samples.

Introduction

A large portion of gas reservoirs lies in unconventional fields. Among these unconventional resources of gas, Sharer and O'Shea estimated that over 500 trillion cubic feet of natural gas may be recoverable from low permeability sandstones located in the Eastern and Western United States. There are several experimental studies for the measurements of flow properties in tight sand media as well as the petrographic, petrophysical and insitu analyses of tight, sandstones; such as Jones and Owens, Walls et al., Spencer, Chowdiah, Soeder, Soeder and Chowdiah, Narahara and Holditch, Holditch, Robinson and Whitehead. The tight sand porous media may be grouped in the following three distinct categories based on pore geometry:

  1. The classic tight sand is basically a conventional sandmade tight by precipitation of minerals in the pore throats. This type usually has the highest permeability as well a slow stress-dependence of permeability among the tight sandstones.

  2. Slot and solution pore types are the most common tight sandstones. The narrow and flat slot pores were generated from the reduction of the primary porosity, which occurred along the boundaries of adjoining quartz overgrowths. The flat, poorly-supported structure of the slots gives the high stress-dependence of permeability in this kind of tight sand.

  3. Matrix supported grain types are the least common of the tight sandstones. The sand grains are supported in an ultra-fine microporous matrix. This type of pore geometry tends to occur in the lowest permeability tight sand and has a high drop in permeability under increasing stress.

Our experiments have been performed on a sample of the second category, slot and solution pore type, which is sensitive to net stress. In this study we particularly focused on the effect of the variation of the net stress on this kind of sample.

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