Impact of fines type on gas flow using 3D micro-computed tomography

Abstract

Methane hydrates are promising unconventional energy source with natural reserves above 1,000 Trillion m3. Hydrates are found in saturated sandy sediments, where multi-phase flow of gas takes place through sediment's pores, physical phenomenon at pore-scale controls flow properties. Gas permeability is highly affected by fines type due to migration, clogging and bridging reducing gas flow. Fines migration has huge influence on storage and recovery of oil and gas resources from the subsurface. There is a knowledge gap of fines effects on gas production from sandy sediments, especially at pore-scale, in homogenous porous media. Therefore, there is a need to model and quantify fines impact on multi-phase flow using 3D reconstruction to better understand gas recovery systems. The pores in which gas flow through are reconstructed using 3D synchrotron X-ray micro- computed tomography images of sand sediments, at resolution of 3.89 micron per voxel. Sand was mixed with fines and deposited in test columns. Kaolinite and Montmorillonite clay fine particles were added in different columns, each system was scanned at four stages with varied saturations of brine as CO2 gas was injected in real time while scanning. Images were processed for 3D visualization, segmentation and quantification for each system. Effects of swelling and non-swelling clays were characterized to understand multi-phase flow of gas at pore-scale. Findings revealed that fines accumulate at sand-brine and brine-gas interfaces. As fines concentration increased, gas percolation decreased. Further increase in fines concentrations resulted in blocking local gas flow causing pressure variations enough to create fractures that allows gas to escape and percolation to rise. In unconsolidated media, the pore space geometry will change due to sand grains movements. At high concentrations, different fines types produce altered gas flow regimes, Kaolinite which is non-swelling clay resulted in fractures while montmorillonite which is swelling resulted in detached gas ganglia entrapping the gas. Generally, increasing fines reduces gas percolation and further injection of gas reduced permeability. The finds herein are critical in understanding the impact of fines migration during gas flow in sand, they can be applied to characterizing and predicting two phase properties of unconsolidated sediments. The qualitative and quantitative findings achieved in this work in 3D, agree with other observations in recent literature on 2D micromodels. Understanding the fluid physics in homogenous sand systems is an essential step to quantify fines impact on heterogeneous rock systems. The work helps in improving flow characterizing at pore scale, to mitigate to formation damage problem and advance reservoirs upscaling capabilities. Copyright 2019, Society of Petroleum Engineers.