Pore Size and Ultrasonic Velocity: Lessons From Miss Lime Reservoirs

Abstract

This article investigates the relationship between rock properties (composition, porosity, and pore architecture) and dry ultrasonic P-wave velocity (VP) of 14 samples representing three facies of the Mid-Continent Mississippian-age Limestone (Miss Lime) units of North–Central Oklahoma. Generally, in carbonate rocks, what drives VP, in addition to bulk porosity (ϕ) and composition, is not straightforward to determine. In this data set, when samples are categorized based on their facies and composition (quartz fraction), VP shows a better trend with dominant pore size rather than ϕ. Results show the dependence of elastic properties on texture and highlight a need for incorporating pore-size distribution in seismic models used for seismic interpretation of low-permeability reservoirs such as the Miss Lime.

How Nanopores Influence Dry-Frame VP Pressure Sensitivity

“…Almost all existing studies on V P pressure sensitivity have used rocks with pore-size dominantly in the meso-or larger scale. This paper extends the line of inquiry started by Raj et al (2019) and fills the knowledge gap of examining how nanopores affect dryframe V P pressure sensitivity. Raj et al (2019) demonstrated the effect of pore-size distribution on V P with samples from the Mississippian age mixed carbonate-siliciclastic reservoir rocks from the mid-continent, United States, commonly known as the "Miss Lime" formation.…”

“…This reduced the number of samples for further analysis to five in Facies A (Figure 5), five in Facies B (Figure 6), and four in Facies C (Figure 7). The second set of plots were along the lines of Figure 3D, which is a crossplots between log 10 A r and log 10 P e of the individual pores, binned to best preserve the continuity of the distribution, e.g., Raj et al (2019). Visualizing data in this manner provided an overview of the pore-size distribution without much regards to the shape.…”

“…In the meso-to-micro range (4 mm-1 µm), Weger et al (2009) found that ultrasonic V P increased as the pores became larger and less complex. On the other hand, in the microto-nano range (62.5 µm-1 nm), Raj et al (2019) discovered the opposite; they noted that the ultrasonic V P increased as the dominant pore size decreased. A key difference between the two experiments was that Weger et al (2009) samples were saturated while Raj et al (2019) samples were dry.…”

“…On the other hand, in the microto-nano range (62.5 µm-1 nm), Raj et al (2019) discovered the opposite; they noted that the ultrasonic V P increased as the dominant pore size decreased. A key difference between the two experiments was that Weger et al (2009) samples were saturated while Raj et al (2019) samples were dry. Another difference was that while Weger et al (2009) compared samples of vastly different pore architectures, Raj et al (2019) first classified their sample facies-wise and then compositionwise, ensuring that samples of similar pore architecture are compared.…”

“…A key difference between the two experiments was that Weger et al (2009) samples were saturated while Raj et al (2019) samples were dry. Another difference was that while Weger et al (2009) compared samples of vastly different pore architectures, Raj et al (2019) first classified their sample facies-wise and then compositionwise, ensuring that samples of similar pore architecture are compared. Regardless, both studies showed that, at least in the carbonate rocks, the pore-size distribution might play an equally important role in determining the V P as does Φ.…”