ABSTRACT

A reliable rock classification in a carbonate reservoir should take into account petrophysical, compositional, and elastic properties of the formation. However, depth-by-depth assessment of these properties is challenging because of the complex pore geometries and significant heterogeneity caused by diagenesis. Common rock-classification methods in carbonate formations do not incorporate the impact of both depositional and diagenetic modifications on rock properties. Furthermore, elastic properties, which control fracture propagation and the conductivity of fracture under closure stress, commonly are not accounted for in conventional rock-classification techniques. We apply an integrated rock-classification technique, based on both depositional and diagenetic effects that can ultimately enhance (1) assessment of petrophysical properties, (2) selection of candidates for fracture treatment, and (3) production in carbonate reservoirs.

We apply the conductive and the elastic self-consistent approximation theories to estimate depth-by-depth volumetric concentration of interparticle (e.g., interconnected pore space) and intraparticle (e.g., vugs) pores, as well as elastic bulk and shear moduli, in the formation. This process takes into account the impact of shape and volumetric concentrations of rock components on electrical conductivity and elastic properties.

We document a successful application of the introduced technique in two wells in the upper Leonardian carbonate interval of Veterans field in west Texas. The identified rock types were verified using thin-section images and core samples. We estimate elastic moduli as well as interparticle porosity with average relative errors of approximately 8% and 10% compared to the core measurements, respectively. Furthermore, the well-log-based estimates of permeability and water saturation are improved by approximately 50% and 20%, respectively, after considering rock classification. Finally, we explain that the fracture propagation failure in the second well (i.e., well B) could be the result of relatively lower Young’s modulus in the rock class corresponding to fracture locations.

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