Browsing by Subject "Berea"
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Item Experimental analysis of the extension to shear fracture transition in Berea Sandstone(Texas A&M University, 2005-11-01) Bobich, Jennifer KayTo characterize low-pressure, brittle deformation in porous, granular rock, notchcut cylinders (30 mm neck diameter) of Berea Sandstone were extended in a triaxial apparatus from 10 to 160 MPa confining pressure at strain rates of 10-4 s-1 and 10-5 s-1. Acoustic emission counts were monitored when extending samples at a slow strain rate. Stress at fracture is characterized by the least compressive principal stress, ??3, and maximum compressive principal stress, ??1 (??1 = Pc). A change in strength dependence on pressure at Pc = 50 MPa corresponds to a change from pure macroscopic extension fracture to mixed-mode opening and shear fracture, and likely reflects the increase in mean stress that suppresses the propagation of extension fractures and the interaction between closely-spaced stepped cracks. Within the extension fracture regime (Pc < 50 MPa), ??3 at failure becomes slightly more tensile with an increase in Pc. At Pc > 50 MPa, ??3 at failure becomes more compressive with an increase in Pc and follows Coulomb behavior; however, the angle between the fracture surface and ??1 increases continuously with Pc. Fracture surfaces characteristic of the extension to shear fracture transition appear as linked, stepped extension fractures; the length of extensional segments decreases with increasing pressure. The onset of acoustic emissions and inelastic strain at fracture occurs at earlier points in the strain history with pressure, consistent with the Griffith prediction of the beginning of fracture growth.Item Further Investigation of Fluoboric Acid in Sandstone Acidizing Using ^(11)B and ^(19)F NMR(2014-05-01) Pituckchon, ArpajitAlthough fluoboric acid (HBF_(4)) has long been known as one of the low-damaging acid treatments for clayey sandstone formations, little is known of its chemistry which could explain the mixed results of fluoboric acid in actual field application. A better understanding of its limitations would contribute to an improved success rate in HBF_(4) stimulation application. The unique advantages of this acid system are the ability to reach deeper into formation to address damage at extended radius before spending, owing to its slow hydrolytic reaction to produce HF, as well as the stabilization and desensitization of undissolved fines with borosilicate. A more comprehensive understanding of how the chemistry of fluoboric acid and its reaction products affect silica and aluminosilicates is crucial to the design and optimization of fluoboric acidizing treatment. Through a novel application of ^(11)B and ^(19)F Solution State High Field Nuclear Magnetic Resonance (NMR) spectroscopy, chemical complexes involved in the reaction were defined. Various other experimental techniques were also employed in studies on the ability of hydrolyzed fluoboric acid to react with common clays found in sandstone at room and elevated temperatures, as well as coreflooding to investigate clay migration development. Analyzing fresh and spent acid with inductively coupled plasma (ICP) and ^(11)B and ^(19)F NMR helps identify reaction products and their distribution. A set of 12-3 mud acid experiments was done in parallel to serve as a reference to 3%- equivalent-HF fluoboric acid in aqueous-HCl solution. NMR results show complex mixtures of fluoborate species from HBF_(4) hydrolysis and products from HF-aluminosilicates reaction. The fresh HB_(F4) hydrolysis study at room temperature has confirmed retarded HF generation with presence of BF_(4)- and BF_(3)(OH)- and absence of BF_(2)(OH)_(2)- or BF(OH)_(3)- species . The effect of temperature on HBF4 reaction has also been studied to validate functionality of acid at 75?F and 200?F. A series of lab dynamic flow testing in Berea sandstone corroborates conclusions from lab experiments by showing decrease in permeability when treating Berea sandstone cores with HBF_(4) at 200?F. Fluoboric acid treatment is therefore not suitable for formations with approximate temperature of 200?F.