Constraints on mineralization, fluid‐rock interaction, and mass transfer during faulting at 2–3 km depth from the SAFOD drill hole

Abstract The results of combined analytical and experimental studies involving simulated multiple bit-tooth penetration into rock are incorporated into a drilling rate equation for roller-cone bits assuming rather idealized downhole conditions. In particular, it is assumed That the rock behaves statically in a ductile fashion during bit-tooth penetration and that the rock chips are instantaneously removed from the bottom of the drill hole. The general analysis demonstrates an application of plasticity theory for the rock/bit-tooth interaction to The formulation of an upper limit on rotary drilling rate. Introduction Extensive experimentation involving single and indexed bit-tooth penetration into rock in a confining pressure environment has demonstrated that the pressure environment has demonstrated that the chip formation process is of a ductile, or pseudoplastic, nature at sufficiently low differential pseudoplastic, nature at sufficiently low differential pressures so as to be of interest in rotary drilling. pressures so as to be of interest in rotary drilling. Coincident with the experimentation, analytical consideration has been given to the theoretical problems of single and indexed bit-tooth penetration problems of single and indexed bit-tooth penetration into rock. In general, the analyses have assumed that the rock behaves statically in a rigid-plastic fashion and obeys the Mohr-Coulomb yield criterion. The quantitative comparison between experimental and calculated values of bit-tooth load required for chip formation has been remarkably good for a variety of rocks commonly encountered in drilling and at simulated differential pressures as low as 500 to 1,000 psi. Results obtained recently for indexed bit-tooth penetration indicate that the work (or energy) penetration indicate that the work (or energy) required to produce a unit volume of rock chip can be minimized by a proper combination of bit-tooth spacing and bit-tooth load for a given rock type and differential pressure. By utilizing this information, it is possible co formulate a drilling rate equation, at least in a preliminary fashion, for a roller-cone bit performing under rather idealized downhole conditions. In particular, through the use of characteristic dimensionless quantities pertinent to a roller-cone bit and to indexed bit-tooth penetration, interrelationships among bit weight, rotary speed, rotary power, bit diameter, rock strength and bit-tooth shape and spacing can be explicitly expressed. In the formulation of the equations, however, it is assumed that the rock chips are instantaneously removed from the bottom of the drill hole and that the rock behaves in a ductile manner during bit-tooth penetration. In addition, the effects of bit-tooth load application And penetration by a yawed tooth at an oblique angle are neglected. Although the analysis is presented in the light of some rather restrictive conditions, it does demonstrate a method of applying fundamental rock/bit-tooth interaction data, obtained by combining the results of analysis and experiment to the formulation of a drilling rate equation for rotary drilling. INDEXED BIT-TOOTH/ROCK INTERACTION PREVIOUS RESULTS PREVIOUS RESULTS The mechanics of bit-tooth/rock interaction under simulated conditions of borehole environment have been extensively described in a number of papers. In particular, the effects of differential papers. In particular, the effects of differential pressure, mechanical properties of rock, pore fluid, pressure, mechanical properties of rock, pore fluid, bit-tooth shape and spacing, rate of bit-tooth load application and dynamic filtration below the bit-tooth have been investigated experimentally. From a sequence of experiments, it was demonstrated that, for dry rock at atmospheric pore pressure, the mode of chip formation exhibits a transition, with increasing confining pressure, from predominantly brittle to predominantly ductile. SPEJ P. 443

Download Full-text

Strain shadow “megapores” in mid-crustal ultramylonites - local, transient reservoirs servicing the granular fluid pump?

10.5194/egusphere-egu21-10343 ◽

2021 ◽

Author(s):

Florian Fusseis ◽

Craig Allsop

Keyword(s):

Mass Transfer ◽

Transport Properties ◽

Shear Zones ◽

Grain Boundary Sliding ◽

Shear Direction ◽

Rock Interaction ◽

The Matrix ◽

Granular Fluid ◽

The Individual ◽

Individual Grains

Shear zones are important conduits that facilitate the bidirectional migration of fluids and dissolved solids across the middle crust. It is a relatively recent revelation that mylonitic deformation in such shear zones can result in the formation of synkinematic pores that are potentially utilised in long-range fluid migration. The pores definitely influence a shear zone&#8217;s hydraulic transport properties on the grain scale, facilitating synkinematic fluid-rock interactions and mass transfer. Our understanding of how exactly various forms of synkinematic porosity integrate with the kinematics and dynamics of shear zones is still growing. Here we show a previously undescribed form of synkinematic porosity in an unweathered, greenschist-facies psammitic ultramylonite from the Cap de Creus Northern Shear Belt (Spain). The sizeable, open pores with volumes > 50k &#181;m3 appear exclusively next to albitic feldspar porphyroclasts, which themselves float in a fine-grained, polymineralic ultramylonitic matrix that likely deformed by grain size-sensitive creep and viscous grain boundary sliding. The pores wrap around their host clasts, occupying asymmetric strain shadows and tailing off into the mylonitic foliation. A detailed analysis using high-resolution backscatter electron imaging and non-invasive synchrotron-based x-ray microtomography confirms that the pores are isolated from each other. We found no evidence for weathering of the samples, or any significant post-mylonitic overprint, unequivocally supporting a synkinematic origin of the pores.&#160;We propose that this strain shadow porosity formed through the rotations of the Ab porphyroclasts, which was governed by the clasts&#8217; shapes and elongation. The ultramylonitic matrix was critical in enabling the formation of pores in the clast&#8217;s strain shadows. In the matrix, the individual grains were displaced mostly parallel to the shear direction. As a consequence of clast rotation it can be expected that, in the strain shadows, matrix grains followed diverging movement vectors. As a result, phase boundaries in the YZ plane experienced tensile forces, leading to the opening of pores. We infer that this tensile decoupling among matrix grains established a hydraulic gradient that drained the matrix locally and filled the pores with fluid. The fact that the strain shadow pores remained open in our samples suggests a chemical equilibrium with the fluid. Pore shape and volume will have been subject to continuous modification during ongoing matrix deformation and clast rotation.This form of synkinematic porosity constitutes a puzzling, yet obvious way to maintain surprisingly large pores in ultramylonites whose transport properties are otherwise likely determined by creep cavitation and the granular fluid pump (Fusseis et al., 2009). We envisage that the strain shadow megapores worked in sync with the granular fluid pump in the ultramylonitic matrix and, while the overall porosity of ultramylonites may be small, locally, substantial fluid reservoirs were available to service fluid-rock interaction and fluid-mediated mass transfer. Our findings add another puzzle piece to our evolving understanding of synkinematic transport properties of mid-crustal ultramylonites and fluid-rock interaction in shear zones at the brittle-to-ductile transition.

Download Full-text