Experimental Investigation Of Effective Matrix Permeability And The Effect Of Soaking Time In Ultra-tight Shales

Open Access
Author:
Chakraborty, Nirjhor
Graduate Program:
Energy and Mineral Engineering
Degree:
Master of Science
Document Type:
Master Thesis
Date of Defense:
November 17, 2015
Committee Members:
  • Zuleima T Karpyn, Thesis Advisor
Keywords:
  • Ultra-Tight Shale
  • Permeability
  • Pulse Decay
  • Soaking Time
  • Shut-in
  • Hydraulic Fracturing
  • Gas
Abstract:
Fluid flow behavior through ultra-tight shale matrices is still poorly understood. Applications of classical concepts to these unconventional materials are proving to be insufficient and there is a need to generate data on the fundamental processes of fluid transport through nano-porosity networks. This work attempts to address this issue through an array of flow and materials characterization experiments. The context of this integrated petrophysical analysis is the examination of the impact of fluid leakoff and post stimulation shut-in or “soaking time” on effective gas permeability. Past research on fluid leakoff and soaking time has been predominantly conducted with relatively high permeability rocks and for short durations of up to 15 days. We present data on shales with nano-Darcy (10-6 md) permeability and have run experiments for up to 30 days. A pulse-decay permeability apparatus has been custom designed to accurately measure gas permeabilities down to 1nD and the results have been scrutinized in conjunction with X-Ray CT, SEM imaging, Mercury Porosimetry and XRD analysis. Results indicate that laminations and micro-fractures play an important role in fluid transport properties in these ultra-tight shales. There appears to be very little connected porosity at scales longer than a few millimeters, making clastic permeability virtually non-existent. The consequences of fluid leakoff are severe and the introduction of relatively small quantities of liquid into the rock matrix leads to orders of magnitude reductions in effective permeability. The impact of this leakoff fluid then evolves significantly as it is spontaneously redistributed through the rock matrix due to capillary imbibition. Base permeability or the initial absolute (single phase) permeability of dry sample is found to be key to understanding the consequences of soaking time – high permeability samples experience relatively minor permeability impairment due to leakoff and permeability recovery with soaking time while tighter samples experience permeability damage greater than 90% due to leakoff which continues to decline with soaking time. This raises the strong possibility that the practice of soaking in shale gas wells may be detrimental to long term production and ultimate recovery.