Nathaly L. Archilha

Permeability and acoustic velocity controlling factors determined from x-ray tomography images of carbonate rocks

Carbonate reservoir rocks exhibit a great variability in texture that directly impacts petrophysical parameters. Many exhibit bi- and multimodal pore networks, with pores ranging from less than 1 μm to several millimeters in diameter. Furthermore, many pore systems are too large to be captured by routine core analysis, and well logs average total porosity over different volumes. Consequently, prediction of carbonate properties from seismic data and log interpretation is still a challenge. In particular, amplitude versus offset classification systems developed for clastic rocks, which are dominated by connected, intergranular, unimodal pore networks, are not applicable to carbonate rocks. Pore geometrical parameters derived from digital image analysis (DIA) of thin sections were recently used to improve the coefficient of determination of velocity and permeability versus porosity. Although this substantially improved the coefficient of determination, no spatial information of the pore space was considered, because DIA parameters were obtained from two-dimensional analyses. Here, we propose a methodology to link local and global pore-space parameters, obtained from three-dimensional (3-D) images, to experimental physical properties of carbonate rocks to improve P-wave velocity and permeability predictions. Results show that applying a combination of porosity, microporosity, and 3-D geometrical parameters to P-wave velocity significantly improves the adjusted coefficient of determination from 0.490 to 0.962. A substantial improvement is also observed in permeability prediction (from 0.668 to 0.948). Both results can be interpreted to reflect a pore geometrical control and pore size control on P-wave velocity and permeability.

3D Pore Structure Investigation of Albian Carbonates from Campos Basin

The pore structure of three samples from two neighbouring wells in the Campos Basin, offshore Brazil, were investigated by high resolution X-ray tomography. The samples show an inverse relationship between velocity and porosity and velocity and permeability, but there are important deviations on those trends. In order to explain this behavior, the sample pore structure was investigated using a high resolution µCT. This technique provided reliable information about 3D geometry of macropores and allowed estimation of the volume of microporosity. The analysis showed an expressive difference between the pore size of samples with different textures, but not enough information to explain differences in RQI values, suggesting that other parameters, like tortuosity, are controlling permeability. The aspect ratio (ratio between width and length) does not offer a clear control on velocity trend deviation, suggesting that small differences in rock texture or cement distribution might be in control of acoustic properties of these rocks.

High-resolution synchrotron-based X-ray microtomography as a tool to unveil the three-dimensional neuronal architecture of the brain

The assessment of neuronal number, spatial organization and connectivity is fundamental for a complete understanding of brain function. However, the evaluation of the three-dimensional (3D) brain cytoarchitecture at cellular resolution persists as a great challenge in the field of neuroscience. In this context, X-ray microtomography has shown to be a valuable non-destructive tool for imaging a broad range of samples, from dense materials to soft biological specimens, arisen as a new method for deciphering the cytoarchitecture and connectivity of the brain. In this work we present a method for imaging whole neurons in the brain, combining synchrotron-based X-ray microtomography with the Golgi-Cox mercury-based impregnation protocol. In contrast to optical 3D techniques, the approach shown here does neither require tissue slicing or clearing, and allows the investigation of several cells within a 3D region of the brain.

Carbonate Pore System Evaluation Based on Prediction of Microporosity and S-wave Velocity under Pressure Effects

This work evaluates carbonate pore system under texture control by velocity-porosity-pressure relationships and digital image analysis, minimizing the uncertainties of microporosity aspect ratio estimative, which are difficult to be detected in laboratory. The data set which integrates different carbonates from literature and cores from Albian age, totalling 366 samples, was arranged according to the texture and two groups of effective pressure (at low: 5-7.5 MPa and high: 40-50 MPa) for elastic velocity analysis. The rock is characterized by macro-, meso-, and micropore systems, which are stated in the physical properties as Vp and Vs, correlated with elastic moduli of interest. Thus, the main method considers three pore-space scales in two representative inclusion scenarios: 1) the macro-mesopore aspect ratio, and 2) the microporosity aspect ratio predicted by the measured Vp. Differential Effective Medium model was employed to invert the microporosity aspect ratio using the input parameters as representative macro-mesopore aspect ratio, porosity, density and mineral matrix to express measured Vp in laboratory at a minimum error. Thereafter, Vs was calculated and compared to the measurements. Microporosity aspect ratio and Vs were predicted satisfactorily. Polynomial curves were fitted for carbonate textures using velocity crossplots, and impacts on pore system were evaluated.

Relationship between the Consolidation Parameter and Aspect Ratio in Microporous Carbonate Rocks

The estimation of dry bulk modulus is determinant for the success of the application of Biot-Gassmann theory to forecast fluid changes within a reservoir. The Pride model is one of the various models described in literature for predicting the dry elastic moduli of rocks. However, that model depends on the consolidation parameter and its appropriate choice implies in the accuracy of that model. In this paper, the consolidation parameter is estimated in microporous carbonate rocks. Mean aspect ratios were estimated using Kuster-Toksoz methodology and aided to establish a relationship between the consolidation parameter, the unconfined porosity and the aspect ratio. Those studies can advise the estimation of the consolidation parameter in carbonate rocks, especially when compressional and shear velocities are unknown.

Microporosity Prediction Using Velocity-porosity Relationship, DIA and DEM theory for Carbonate Pore System Evaluation

In this work, we propose a methodology to predict amount of microporosity and representative aspect ratio of geometric inclusion. Thus, we consider three pore-spaces onto two representative inclusion scenarios: 1) macro-mesopore median aspect ratio, from thin-section digital image analysis (DIA); and, 2) microporosity aspect ratio to be predicted in agreement with measured P-wave velocity. By laboratorial analysis of 10 grainstone core samples from Albian age, P- and S-wave are evaluated at 3.5 MPa of effective pressure. The analytical theories in proposed methodology are functions of the macro-mesopore system recognized from DIA, amount of microporosity supposed at difference between porosities estimated from laboratorial helium-gas and thin-section petrographic images, and P-wave velocity at dry and effective pressure conditions. DIA procedure is applied to estimate local and global parameters and textural implications concerning ultrasonic velocities. Macro-mesopore inclusion contributes for rock stiffer and high velocity, although the microporosity inclusion contributes for rock softer and low velocity. We observe high potential of methodology to use microporosity aspect ratio inverted from Vp to predict Vs with good agreement. The results are acceptable to characterize the Albian grainstone carbonate samples, the representative macro-mesopore aspect ratio is 0.5 and microporosity aspect ratio inverted ranges from 0.01 to 0.07.

Elastic Properties Characterization and Pore System Evaluation Using Kuster-Toksös and Gassmann Models

Carbonates represent a significant portion of the Brazil’s deepwater oil production, whose importance has increased with recent discoveries in the post-salt and pre-salt oil deposits, justifying studies on the subject, the economic potential they represent. The study of the physical properties of rocks is important for reservoir characterization and monitoring. It correlates the seismic response of rock properties. Reservoirs carbonate rocks might be a complicated system pores, resulting in different compressibility that affect the velocities of seismic waves directly. The goal is to estimate elastic moduli considering interparticle porosity expected in limestones using Effective Elastic Media (EEM) theories as Kuster-Toksoz model (KT). This work consists in interpretive analysis of the elastic properties of ultrasonic data measured by triaxial rock deformation and physics system, installed in integrated reservoir modelling laboratory at LENEP/UENF in Macae, Rio de Janeiro, Brazil.