Estimation of fracture compliance from tubewaves generated at a fracture intersecting a borehole

Understanding fracture compliance is important for characterizing fracture networks and for inferring fluid flow in the subsurface. In an attempt to estimate fracture compliance in the field, we developed a new model to understand tubewave generation at a fracture intersecting a borehole. Solving the dispersion relation in the fracture, amplitude ratios of generated tubewave to incident P-wave were studied over all frequency ranges. Based on the observations from the model, we propose that measuring amplitude ratios near a transition frequency can help constrain fracture compliance and aperture. The transition frequency corresponds to the regime where the viscous skin depth in the fracture is comparable to its aperture. However, measurements in the high frequency limit can place a lower bound on fracture compliance. Comparing the model to a previously published VSP dataset, we argue that compliance values of the order 10[superscript −10] −10[superscript −9] m/Pa may be possible in the field

Deformation mechanics of quartz at a single asperity under hydrothermal conditions

Thesis (S.M. in Geophysics)--Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences, 2010.Cataloged from PDF version of thesis.Includes bibliographical references (p. 92-97).Pressure solution is a naturally occurring deformation process in fluid-bearing rocks, with implications for sediment consolidation rates and deformation in the mid to upper crust. The process involves dissolution at asperities under load; diffusion along grain-to-grain contacts; and precipitation in the pore space. The kinetics of the process and the identity of the rate-limiting step in quartzose rocks are still a subject of debate and depend critically upon the structure of the inter-granular boundary during deformation. To investigate the mechanisms and kinetics of pressure solution and the effects of changes in boundary morphology, we continuously monitored deformation and the evolution of boundary morphology at a Y-cut convex quartz lens that was pressed against a flat surface of Y-cut quartz under hydrothermal conditions (425 0C, 150 MPa fluid pressure, 80-435 MPa effective normal stress) in a see-through optical pressure vessel. The pore fluid was de-ionized water, which became saturated with quartz during the initial heat-up period. Several different boundary morphologies were used, including polished flats, etched and roughened flats, and an island-channel structure fabricated by plasma etching. The island-channel structure had square pillars of quartz, either 4 micro meter or 8 micro meter on a side, surrounded by an inter-connected grid of open channels of comparable width. Deformation at the interface between the quartz flat and lens (i.e., inter-granular convergence) was observed in only one case: the 4pum island-channel structure. In that instance, pillars within the contact region were eroded at a rate that decreased with time, and SEM photomicrographs after the experiment showed significant cataclasis within and adjacent to the pillars. The total transient convergence (0.15i0.02pum) observed during the 472 hour period was less than the original pillar height (0.55±0.005pim). For all surface morphologies, the shapes of the surface structures in both the loaded and unloaded regions evolved, perhaps owing to capillarity forces. Although an inter-connected island-channel structure was initially imposed upon some of the experiments, this open, fluid-filled boundary structure was seen to be a transient phenomenon that was largely destroyed during convergence. Thus, there was no evidence for a dynamically stable interface, as previously suggested.by Sudhish Kumar Bakku.S.M.in Geophysic

Seismic imaging of hydraullically-stimulated fractures: A numerical study of the effect of the source mechanism

We present a numerical study of seismic imaging of hydraulically stimulated fractures using a single source from an adjacent fracturing-process. The source is either a point force generated from the perforation of the casing of the well or a double-couple as is typically observed from the induced microseismicity. We assume that the fracture is sufficiently stimulated to be imaged by reflected seismic energy. We show for a specific monitoring geometry of hydrofracturing that not only different waves (P and S) but also different source mechanisms from the same region form an image of different parts of the target fracture and thus add complementary information. The strategy presented here might be used as an additional monitoring tool of the hydrofracturing process.Massachusetts Institute of Technology. Earth Resources Laborator

Fracture characterization from attenuation of Stoneley waves across a fracture

Fractures contribute significantly to the permeability of a formation. It is important to understand the fracture distribution and fluid transmissivity. Though traditional well logs can image fractures intersecting the borehole, they provide little information on the lateral extent of the fractures, away from the borehole, or the fluid transmissivity. Experiments in the past demonstrated that fracture compliance can be a good proxy to fracture fluid conductivity. We describe a method to estimate fracture compliance from the attenuation of Stoneley waves across a fracture. Solving the dispersion relation in the fracture, transmission coefficient of Stoneley waves across a fracture is studied over all frequency ranges. Based on the observations from the model, we propose that measuring the transmission coefficient near a transition frequency can help constrain fracture compliance and aperture. Comparing attenuation across a finite fracture to that of an infinitely long fracture, we show that a bound on the lateral extent of the fracture can be obtained. Given the limitation on the bandwidth of acoustic logging data, we propose using the Stoneley waves generated during micro-seismic events for fracture characterization.Eni-MIT Energy Initiative Founding Member Progra

Sequential approach to joint flow-seismic inversion for improved characterization of fractured media

Seismic interpretation of subsurface structures is traditionally performed without any account of flow behavior. Here we present a methodology for characterizing fractured geologic reservoirs by integrating flow and seismic data. The key element of the proposed approach is the identification—within the inversion—of the intimate relation between fracture compliance and fracture transmissivity, which determine the acoustic and flow responses of a fractured reservoir, respectively. Owing to the strong (but highly uncertain) dependence of fracture transmissivity on fracture compliance, the modeled flow response in a fractured reservoir is highly sensitive to the geophysical interpretation. By means of synthetic models, we show that by incorporating flow data (well pressures and tracer breakthrough curves) into the inversion workflow, we can simultaneously reduce the error in the seismic interpretation and improve predictions of the reservoir flow dynamics. While the inversion results are robust with respect to noise in the data for this synthetic example, the applicability of the methodology remains to be tested for more complex synthetic models and field cases.Eni-MIT Energy Initiative Founding Member ProgramKorea (South). Ministry of Land, Transportation and Maritime Affairs (15AWMP-B066761-03

Fracture characterization from seismic measurements in a borehole

Thesis: Ph. D., Massachusetts Institute of Technology, Department of Earth, Atmospheric, and Planetary Sciences, 2015.Cataloged from PDF version of thesis.Includes bibliographical references (pages 219-227).Fracture characterization is important for optimal recovery of hydrocarbons. In this thesis, we develop techniques to characterize natural and hydraulic fractures using seismic measurements in a borehole. We first develop methods to characterize a fracture intersecting an open borehole by studying tubewave generation and attenuation at the fracture. By numerically studying the dispersion relation for fluid pressure in the fracture, we show that the tubewave measurements made in the transition regime from low to high frequency can constrain fracture compliance, aperture and length, while measurements made in the high-frequency regime can place a lower bound on fracture compliance. Analysis of field data suggest a large compliance value (10- 0m/Pa) for a meter-scale fracture and supports scaling of fracture compliance and applicability of scattering based methods for fracture characterization on a reservoir scale. We next study Distributed Acoustic Sensing (DAS), a novel Fiber Optic (FO) cable based seismic acquisition technology. We relate DAS measurements to traditional geophone measurements and make a comprehensive study of factors that influence DAS measurements. Using a layered borehole model, we analytically compare the sensitivity of DAS measurements to P- and S-wave incidence at arbitrary angles for the cases when the FO cable is installed in the borehole fluid or when cemented outside the casing. In addition, we study the azimuthal placement of the cable, the effect of cable design, and the effect of environmental conditions on time-lapse measurements. We show that DAS is a reliable tool for time-lapse monitoring. Finally, we analyze time-lapse DAS Vertical Seismic Profiling (VSP) data collected during a multi-stage hydraulic fracture treatment of a well drilled into a tight gas sandstone reservoir. We develop a processing workflow to mitigate the unique challenges posed by DAS data and propose methods for DAS depth calibration. We observe systematic and long-lived (over 10 days) time-lapse changes in the amplitudes of direct P-waves and nearly no phase changes due to stimulation. We argue that the time-lapse changes cannot be explained by measurement factors alone and that they may be correlated to the stimulated volume. Though the current geometry is not ideal, DAS is promising for hydraulic fracture monitoring.by Sudhish Kumar Bakku.Ph. D

Expression of Genes for a Flavin Adenine Dinucleotide-Binding Oxidoreductase and a Methyltransferase from Mycobacterium chlorophenolicum Is Necessary for Biosynthesis of 10-Methyl Stearic Acid from Oleic Acid in Escherichia coli

In living organisms, modified fatty acids are crucial for the functions of the cellular membranes and storage lipids where the fatty acids are esterified. Some bacteria produce a typical methyl-branched fatty acid, i.e., 10-methyl stearic acid (19:0Me10). The biosynthetic pathway of 19:0Me10 in vivo has not been demonstrated clearly yet. It had been speculated that 19:0Me10 is synthesized from oleic acid (18:1Δ9) by S-adenosyl-L-methionine-dependent methyltransfer and NADPH-dependent reduction via a methylenated intermediate, 10-methyelene octadecanoic acid. Although the recombinant methyltransferases UmaA and UfaA1 from Mycobacterium tuberculosis H37Rv synthesize 19:0Me10 from 18:1Δ9 and NADPH in vitro, these methyltransferases do not possess any domains functioning in the redox reaction. These findings may contradict the two-step biosynthetic pathway. We focused on novel S-adenosyl-L-methionine-dependent methyltransferases from Mycobacterium chlorophenolicum that are involved in 19:0Me10 synthesis and selected two candidate proteins, WP_048471942 and WP_048472121, by a comparative genomic analysis. However, the heterologous expression of these candidate genes in Escherichia coli cells did not produce 19:0Me10. We found that one of the candidate genes, WP_048472121, was collocated with another gene, WP_048472120, that encodes a protein containing a domain associated with flavin adenine dinucleotide-binding oxidoreductase activity. The co-expression of these proteins (hereafter called BfaA and BfaB, respectively) led to the biosynthesis of 19:0Me10 in E. coli cells via the methylenated intermediate

Fracture compliance estimation using borehole tube waves

We tested two models, one for tube-wave generation and the other for tube-wave attenuation at a fracture intersecting a borehole that can be used to estimate fracture compliance, fracture aperture, and lateral extent. In the tube-wave generation model, we consider tube-wave excitation in the borehole when a P-wave is incident on the fracture. The amplitude ratio of the pressure due to the tube wave to that of the incident P-wave is a function of fracture compliance, aperture, and length. Similarly, the attenuation of a tube wave in the borehole as it crosses a fracture intersecting the borehole is also a function of fracture properties. Numerically solving the dispersion relation in the fracture, we study tube-wave generation and the attenuation coefficient as a function of frequency. We observed that measuring amplitude ratios or attenuation near a transition frequency can help constrain the fracture properties. The transition frequency corresponds to the regime in which the viscous skin depth in the fracture is comparable to its aperture. Measurements in the high-frequency limit can place a lower bound on fracture compliance and lateral extent. We evaluated the applicability of the tube-wave generation model to a previously published VSP data set and found that compliance values of the order 10[superscript −10]–10[superscript −9]  m/Pa are likely in the field. These observations support scaling of fracture compliance with fracture size.Eni-MIT Energy Initiative Founding Member Program (Eni Multiscale Reservoir Science Project