The effect of pore geometry on seismic wave velocities in carbonate rocks from hydrocarbon reservoirs

The goal of this laboratory study is to investigate the effect of pore shapes on seismic wave velocities in carbonate rocks under reservoir conditions. In this research, 41 core plugs of carbonate rocks from oil fields of the southwest of Iran were prepared. The compressional and shear wave velocities were measured in both dry and brine saturated samples under several pressures especially reservoir pressure. The results from XRD and thin section studies were used to determine minerals, features and pore types of the samples. The cross plots of velocity versus porosity, density, pressure, mineralogy, and especially pore types have been investigated. These cross plots showed that the pore shapes are the main reason for variation in velocities and dispersion of the data points, so that for a constant porosity the variance of elastic wave velocity is about 1500 ms-1 and also the variance of porosity is about 20 percentage for a constant velocity. The velocity is higher in samples with vuggy porosity and lower in samples with small size vuggy porosity than the velocity determined from the time average equation.
Introduction: The parameters which have effect on velocity are divided into two classes. The first class are the parameters that are related to the natural character of the rock, lithology and rock physics such as porosity, pore type, grain size and a combination of them. The second class is the parameters which are affined to depositional environment and they are not physically related to rock structures. These parameters are such as depth of burial, confining stress and age of deposition.
The effect of rock properties, such as porosity, type of porosity, minerals and pressure on P and S wave velocities are investigated by laboratory measurements of compressional and shear wave velocities for both dry and brine saturated rock under different confining pressure.
The effect of pore shapes has been either overleaped or not suitably used in theoretical equations, therefore seismic inversion analysis, AVO and pore volume calculations, which are based on these equations are highly inconclusive.
Pore types were classified into five groups by Anselmity and Eberli (1993), which are inter crystalline and interparticle porosity, micro porosity, moldic porosity, inter grain porosity and low porosity with high cementation. They also studied the effect of pore types on velocity measurements.
Burial depth, compressibility, saturation, wettability, hysteresis of saturation and frequency of wave velocity are other factors which have an effect on velocity.
In this paper, first the factors affecting seismic wave velocity in carbonate rocks were reviewed and then flow work of this study consisting of preparation of samples, determination of pethrophysical properties (porosity, permeability) and compressional and shear wave velocities measurements were performed. Seismic wave velocity performed on 41 dry and brine saturated core plug samples under reservoir temperature and pressure. The diameters of the plugs were 3.7 centimeter for 34 samples and 5 centimeter for 7 samples.
Effective factors on seismic wave velocities in carbonate rocks Porosity: Porosity is one of the important parameters that has an effect on velocity, so that normally with an increase of porosity the velocity is decreased. Prediction of porosity just from seismic velocity is difficult, because in carbonate rocks seismic wave velocity is dependent on too many other parameters.
Minerals: Though the velocity differs in different minerals, the type of mineral is not the main factor that controls velocity in carbonate rocks.
Density: Generally a direct relation between density and velocity is found but there isn't any experimental equation for the relation between density and velocity. Therefore to increase certainty coefficient, laboratory measurements and determination of the relation between density and velocity are necessary.
Pressure: Pressure is one of the important factors that controls velocity in fractured rocks, which are brine saturated. Porosity reduction and better contact of grains in rock is the reason for the increase in velocity by the increase in pressure.
Pore geometry: The results of microscopic studies by Anselmiti and Eberli (1993) showed five different pore geometries in carbonate reservoir rock. They analyzed different types of pore geometries and compared data points in velocity-porosity cross plot with time average equation.
Flow work: Flow work consisted of plugging, cleaning, petrophysical properties and acoustic velocity measurements.
Conventional core properties such as porosity, permeability and grain density provide the fundamental data set for well bore and reservoir characterization.
The core plugs were cleaned to remove residual hydrocarbons, formation brine, salts and other contaminants by using toluene and then they were dried in a conventional oven.
Porosity and grain density of samples were measured under ambient conditions using helium expansion and the application of Boyles’s law to quantify grain volume by Ultraporosimeter 200A.
Air permeability was measured in ambient conditions by Ultrapermeameter, which uses the Darcy equation to calculate air permeability.
Acoustic velocity was measured in dry and brine saturated samples in reservoir temperature and pressure from 4400 psi to 800 psi by non equal steps.
Laboratory study of factors that control seismic wave velocity:The data points in the porosity- velocity cross plot, which resulted from laboratory measurements, were too scattered. Pore shape and cementation of pores are the reason for this scattering. The density-velocity cross plot shows that seismic wave velocity isn't controlled by the type of minerals. In all states mentioned above, the variability of velocity vs pressure is followed by V=a*Pb.

Results: * In carbonate rocks acoustic wave velocity is dependant on some parameters such as diageneous, mineralogy, pore structure, type of fluid, pressure, temperature and also in no dense carbonate the wave velocity is related to grain to matrix ratio, shape, size and sorting of grains.
* Cross plot of velocity versus porosity showed that for a constant porosity the variance of velocity is about 1500 ms-1 and also for a constant velocity the variance of porosity is about 20 percentage.
* The velocity is higher in samples with vuggy porosity and lower in samples with small size vug porosity than the velocity determined from the time average equation.
* The test results for non visible vuggy samples show negative deviation from the time average equation for calcite and dolomite, and they have lower velocity than vuggy and small size vuggy porosity samples.
* The density - velocity cross plot showed that the effect of the type of minerals to control elastic properties is negligible.
* Pore shape is the main factor which causes scattering of data points in velocity-porosity cross plot for carbonate rocks.