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Quantifying the Mineralogical and Compositional Response of Shale Oil and Gas Rocks to Matrix Acidification

Alexander E. Gonzales and Callum J. Hetherington
Department of Geosciences, Texas Tech University, Box 41053-1053, Lubbock, TX 79401, USA

Unconventional oil and gas reserves hosted in shale formations have received much interest in recent years due to their potential to greatly increase global hydrocarbon production. There are, however, technical challenges to exploiting these reserves that demand varied approaches to hydrocarbon extraction. A significant challenge is stimulating and maintaining production from shale formations that have low to very low permeability. Hydraulic fracturing stimulation is a broadly applied technique wherein fluid and proppant is forced into the rock inducing brittle deformation, thereby increasing permeability. Hydraulic fracturing is commonly preceded by a matrix acidizing pre-flush. The acidizing pre-flush dissolves material resulting in increased porosity. Though the physical effects of the matrix acidizing pre-flush have been quantified, the mineralogical and compositional effects are less well understood. Core-plugs from several well-known shale formations have been halved, and one piece exposed to dilute HCl for comparison against a pristine untreated sample. Samples have been crushed, powdered, and mineralogically characterized by X-ray diffractometery (XRD) and the patterns evaluated and quantified by Rietveld refinement. Mass loss caused by exposure to HCl was ~2 %, which based on sample effervescence, was attributed to carbonate decomposition. This conclusion is loosely corroborated by interpretation of the XRD patterns. Compositional analyses of the samples by X-ray fluorescence (XRF) shows that abundances of CaO decreased slightly, while other major element abundances remain unchanged. Thermogravimetric analyses (TGA) will be used to monitor the compositional changes based on carbonate decomposition, as well as the impact of the acid treatment on sulfide and graphite speciation.


AAPG Search and Discovery Article #90164©2013 AAPG Southwest Section Meeting, Fredericksburg, Texas, April 6-10, 2013