Rock anisotropy and brittleness from laboratory ultrasonic measurements in the service of hydraulic fracturing

Abstract

Ultrasonic velocity anisotropy in the rock provides information of variability of the dynamic elastic moduli. Young's modulus and Poisson's ratio calculated from waves velocities can be used to determine brittleness index, which is usually used to predict rock susceptibility for hydraulic fracturing. This paper describes laboratory ultrasonic measurements carried out in order to improve hydraulic fracturing designing. The research was conducted over two types of rock: shale and limestone. The samples were cut out perpendicularly and parallel to the bedding planes. Next they were tested for effective porosity and mineral composition using XRD method. Directionally depended seismic velocities revealed noticeable anisotropy of laminated shale, caused by orientation of the bedding planes and weak anisotropy of limestone. Based on the velocities, dynamic elastic moduli and its anisotropy coefficients were determined. Calculations of brittleness index based on Young's modulus to Poisson's ratio relation and three types of mineral composition brittleness indexes, revealed strong variability in brittleness for both kind of tested formations. These results show, that different types of brittleness indexes should be used complementary, to better describe fracability of the rock. ARTICLE INFO

Hydraulic Fracturing in Enhanced Geothermal Systems—Field, Tectonic and Rock Mechanics Conditions—A Review

“…In formations of low v, the embedment phenomenon rises, which leads to a decrease in the conductivity of the fracture (e.g., [23][24][25]). Both E and v parameters allow determining susceptibility for fracking, defined by the brittleness index [22,[26][27][28][29][30]. The stress directions in the field could be in line with the regional stress map; however, the local fluctuations of the stress state caused by local geological/tectonic conditions often occur.…”

“…In formations of low v, the embedment phenomenon rises, which leads to a decrease in the conductivity of the fracture (e.g., [23][24][25]). Both E and v parameters allow determining susceptibility for fracking, defined by the brittleness index [22,[26][27][28][29][30].…”

“…Nevertheless, the extensive experience was gained in hydraulic and chemical stimulation operations, based on fracturing in unconventional deposits and matrix acidizing [15,89]. Research works were also carried out in the field of the design, environmental impact and performance of energized fluids for fracturing [90]; the embedment phenomena [24,25]; and the dynamic elastic parameters of the various types of rocks [27,30]. Their results may be useful in the implementation of a prospective Polish HDR geothermal collector program.…”

Laboratoryjne obrazowanie wielkości wgniatania ziaren podsadzki w ścianę szczeliny

“…W sytuacji podwyższonej zawartości minerałów ilastych, charakteryzują się one niskimi modułami Younga i wysokimi współczynnikami Poissona, co przekłada się na niskie wartości wskaźnika kruchości brittleness, tj. wysoką plastyczność (Moska et al, 2018;Kasza, 2019). Taka struktura szkieletu skalnego jest przyczyną niskiej przepuszczalności gazu w formacjach tego rodzaju.…”

Brittleness index of coal from the Upper Silesian Coal Basin

“…The concept of the brittleness index (BI) was developed in the 1960s, in response to the demand for a parameter describing a part of the elastic properties of the rock related to the deformation characteristics. Throughout the years, many authors used various geological and geophysical parameters to calculate BI based on the:  shape of stress-strain curve (Bishop, 1967;Hucka and Das, 1974;Hajiabdolmajid and Kaiser, 2003),  energy balance analysis (Tarasov and Potvin, 2013),  unconfined compressive strength and Brazilian tensile strength tests (Hucka and Das, 1974;Altindag, 2002),  mineral composition (Jarvie et al, 2007;Wang and Gale, 2009;Jin et al, 2014a;Jin et al, 2014b;Bała, 2017),  Young's modulus to Poisson's ratio relation (Grieser and Bray, 2007;Rickman et al, 2008;Luan et al, 2014;Bała, 2017;Moska et al, 2018;Wu et al, 2019),…”