Conceptual modeling of a Fractured Reservoir: Where are we heading towards?
1. Is it possible to secure the actual three-dimensional (and NOT the two-dimensional areal features - in the absence of ‘hydraulic-connectivity with the ensured connected paths of least resistance’) data on fracture attributes such as fracture-density, fracture-length, fracture-width, fracture-spacing and fracture-dip from a real field scenario?
2. If it is feasible, what would be the ‘scale’ at which, these details (fracture attributes) could be expected?
3. If the associated scale of the deduced fracture attributes remains much greater than macroscopic-scale, then, will it be feasible for us to deduce a reasonable REV (associated with the fractured rock reservoir) in order to consider the associated parameters/variables that will probably vary smooth and continuous so that the conventional calculus could be applied with ease? For such conceptual models, whether, stochastics (or the fractals) is ‘the only means’ to characterize the reservoir heterogeneity?
4. In fractured reservoirs, to what extent, the conventional hydro-static pressure and gravity would decide the resultant draining of oil – at the relatively later stage of the production cycle? Also, during the early-phase, whether the expansive force of the gas associated with the oil – will any more remain - the principle source of propulsive energy? Even, if it is so, do we really have a control over the expansion of gas – within the high permeable fracture - which will mitigate the velocity of oil; and in turn, which will try to suppress the frictional resistance? [Assuming that the velocity of oil within the high-permeable fracture cannot be so slow; and subsequently, the relatively higher oil velocity may not allow the gas to flow more rapid – unlike, as observed in conventional sandstone reservoirs.]
Dear Suresh Kumar Govindarajan, your question is very multifaceted to answer, I will try just only a geophysical ("seismic") point of view to help you, in focus points 1 and 2.
It is usual that fractured reservoirs are heterogeneous at different scales (length, width, density, spacing, dip) in a real scenario. Even with millions of grid blocks, numerical models may not be able to exactly simulating the pressure transient performance of continuously and discretely NFRs (Natural Fracture Reservoirs) containing variable conductivity fractures. Following the first question that you invite to support, “…Is it possible to secure the actual three-dimensional and NOT the two-dimensional…” such as geophysicist, 3D seismic tools are the only ones to illuminate in real THREE-D the fracture systems and estimate these attributes, such as length, width, density, spacing. In three-dimensional 3D reflection seismic surveying, the sound detectors are spread out over an area and the sound source is moved from location to location through the area, to generate a subsequent volume of common depth point stacked reflections.
Also, it is critical to understand the vertical and lateral resolution of 3D seismic, a function of the design, acquisition, processing, and interpretation, on one side, and the depth and natural geological conditions that your study case has. Therefore, the seismic resolution is the real boundary or edge to understand the fracture properties in a real scenario in an oil and gas field.
An acoustic or elastic fracture model derived from seismic is useful but limited. Limited by seismic resolution, both, vertical and lateral, it means, the ability to identify individual peaks on a seismic trace with the top and base of geological unit or bed, or to see fine-scale features in map view. Both kinds of resolution are based on local velocity near the feature of interest and frequency content of the seismic wavelength. Also, remember that seismic data always contains noise as well as signal, the signal-to-noise ratio describes how strong the signal is relative to the random noise.
Fracture density, relative fracture density, for example, has been reported to give a good amplitude signature by dimming over areas of increased fracturing, and in 3D seismic, it may be possible to map this amplitude behavior and identify fracture trends. Also, complex-trace seismic attributes, geometric-interpreters-dependent are useful to illuminate the fracture systems: coherence volume, amplitude extraction, spectral decomposition, curvature, instantaneous-frequency, and a battery of composite combinations. But remember: the seismic interpretation of geological detail simply was unjustified by the resolution of the seismic data; you must understand the limitations of the seismic data to avoid pitfalls. Once a reservoir-characterization problem is identified, fracture characterization, in this case, we geoscientists should be able to select and combine attributes that can better delineate features of interest, choosing only attributes sensitive to the geological problem to be tested.
Let me share this excellent paper: “Fractured reservoir modeling and interpretation”, by Kuchuk, Biryukov, and Fitzpatrick, International Petroleum Technology Conference, Malaysia, 10-12, December 2014.
Best regards, Mario E. Sigismondi
Thank you P. Contreras for sharing your experience in the field. It is certainly necessary for cooperation between researchers. The microseismic is one instrument to evaluate a fractured reservoir, but it is a tool that does not give us whole imaging of the subsurface, it is valid only just around the well / s that we do it. It means you have good details or resolution near the well site surveying, but not a regional - scale.
On the other hand, I am surprised not only by the P. Contreras´ recommendations numbers but also by disciplines outside of this specific question. If you read between lines, these recommendations come from professionals specialists in disciplines outside the field of Geology and Geophysics, such as nanotechnology, materials chemistry, polymer chemistry, industrial engineering, conservation agriculture, sustainable agriculture, cropping systems, agronomy, soil physics, soil science, crop, organic agriculture, salinity, neuropsychopharmacology, psychopathology, clinical assessment, addiction psychiatry, comorbidity, psychoeducation, applied math, real analysis, complex analysis, ordinary equations, nonlinear analysis, history science, differential equations, fractional calculus, political science, social science, human mobility, business administration, economics, quantitative social research, manufacturing engineering, pre-school education, rehabilitation medicine, neurology, teacher education, pedagogic theory ...
Are Contreras´s recommendations a factual contribution to this geophysical/geological question and in primary instances, people without obvious experience in this specific field?
Best regards, Mario E. Sigismondi
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