The influence of biofilm development on the effective gas permeability in hydraulic fractures

Abstract

Shale gas is natural gas captured in low permeable shale formations, to be able to produce this gas the permeability of the reservoir needs to be enhanced. Hydraulic fracturing is used in low permeable reservoirs to increase the permeability and make gas production possible. Fracking fluid is pumped in the reservoir at high pressure to create high permeable fractures through which gas is produced. Fracking fluid used for hydraulic fracturing contains a lot of bacteria, attached to a surface these bacteria can create biofilms. A biofilm is a 3D structure of micro-organisms in a matrix of EPS consisting for more than 90 % of water. Previous studies have proven that biofilm development can decrease the water permeability (Gerlach & Cunningham, 2009). Less is known about the influence of biofilm development on the effective gas permeability. In this research the influence of biofilms on the effective gas permeability is investigated using a steady state gas flow through a porous sample containing methanotroph bacteria. A similar experiment with ethanol as the carbon source was done by Bottero et al (2010). A decrease in the effective gas permeability was noticed, however it could not be excluded that the permeability decrease was caused by ethanol condensation inside the sample. For this thesis the set-up as used by Bottero et al (2010) is improved to minimize the chance of condensation. A temperature control system is implemented and the carbon source is changed from ethanol to methane gas. The differential pressure and flow rate are logged to determine the permeability of the sample. The water content of the sample is based on the permittivity which is measured with a TDR cable tester. The multi section scatter function model for TDR analysis was adapted to make it applicable for this research (Heimovaara, Huisman, Vrugt, & Bouten, 2004). The parameters are selected with the DREAM algorithm created by Vrugt et al (2011). During the 32 days of the experiment the water content decreased due to desaturation resulting in a permeability increase. The bacteria produced only a minor amount of water, known because of the carbon dioxide production. In this experiment the gas permeability is not noticeably influenced by biofilm development, the water production by bacteria was lower than the desaturation rate. The sample was placed inside a low conductive plastic tube, unfortunately this made it impossible to separate biofilms from water based on measurements with the TDR cable tester.Reservoir GeologyGeoscience & EngineeringCivil Engineering and Geoscience