Petrophysical Correlations for the Permeability of Coquinas (Carbonate Rocks)

Abstract

The pore structure of many carbonate formations is known to be very complex and heterogeneous. Heterogeneity is manifested by the presence of different types, sizes, and shapes of pores resulting from sedimentation and diagenetic actions. These complexities greatly increase uncertainties in estimated rock hydraulic properties in that different permeability values may occur for samples having similar porosities. In order to understand the effects of pore structure and heterogeneity, petrophysical analyses were performed on coquina samples from the Morro do Chaves Formation (Barremian, Sergipe-Alagoas Basin), which is an analogue of Brazilian Pre-salt oil reservoirs of Itapema Formation in the Santos Basin. Routine core analyses, and NMR and MICP measurements were carried out to obtain pore body and pore throat distributions. Obtained T2 relaxation times were converted to pore size radii by matching the NMR and MICP curves. Pore-scale imaging and pore network modelling were performed using microCT scans and the PoreFlow software, respectively. Calculated permeabilities using PoreFlow showed excellent agreement with the routine laboratory measurements. Samples having pore bodies with a higher coordination number showed much larger permeabilities at similar porosities. This study includes a statistical analysis of various features that caused the observed differences in permeability of the coquinas, including the role of connectivity of the entire porous system. Limitations and challenges of the various techniques, and the imaging and pore-scale flow simulations, are discussed.

Effects of carbonated water injection on the pore system of a carbonate rock (coquina)

“…7 could be confirmed by the main percolation clusters in the coquina (Fig. 9) before and after CSW injection (Godoy et al, 2019;Hoerlle et al, 2020;Lima et al, 2020). A more detailed evaluation is presented in the following section based on the generated skeleton of the sample.…”

“…The non-destructive microCT experiments were used to investigate changes in the internal pore structure of the samples to better understand permeability changes that may occur after carbonated water injection. X-ray microtomography is now widely used for both qualitative and quantitative porescale analyses, including for pore-scale flow modeling (Godoy et al, 2019;Lima et al, 2020;Saxena et al, 2017), determination of wettability through studies of the contact angle in images (Al Ratrout et al, 2018;Andrew et al, 2014;Armstrong et al, 2021;Prodanović et al, 2004), and analyses of the distribution of immiscible fluids inside pores (Alvarado et al, 2004;Schnaar and Brusseau, 2006), among other soil and rock hydrologic applications. MicroCT results provide input for digital pore and pore network modeling (PNM) analyses to highlight the pore-scale basis of macro-scale (or continuum-scale) multiphase fluid flow and contaminant transport processes (de Vries et al, 2017;Rabbani and Babaei, 2019;Raoof et al, 2013).…”

“…We similarly used NMR techniques to indirectly estimate the PSDs through the excitation of nuclear spins, mainly 1 H contained in the saturating fluid molecules (e.g., Chi and Heidari, 2016;Lima et al, 2020). Typically, the pore distribution of a rock sample drives the distribution of transverse relaxation times (referred to as T2), determined by mathematical inversion of relaxation measurements (Buttler et al, 1981).…”

“…We note here that a constraint of the NMR approach (and resulting PSDs) concerns the bulk effect of the larger pores. Above 2.5 s (the saturating fluid relaxation time of the samples) the spins no longer completely collide with the pore walls of the pore network inside the sample, which may cause the measured relaxation times to be less reliable (Lima et al, 2020). This causes uncertainty in the size of especially the larger pores.…”

Study of empirical correlation between permeability and porosity with application for permeability upscaling

“…Porosity throughout the field tends to present a normal distribution of values [3], while the permeability tends to have a log-normal distribution [4]. Because of that, several authors present correlations of porosity with the logarithm of permeability [5][6][7][8]. As a simplification, the log (k) can be linearly correlated with , as shown in Eq.…”