Experiment study of pore structure effects on velocities in synthetic carbonate rocks

Abstract

Carbonate rocks have a complex pore structure, show strong heterogeneity, and have a wide range of velocities that lead to more complicated velocity-porosity relationships compared with sandstones. We designed and prepared 72 carbonate synthetic cores with known pore structures according to the control variate principle. We measured the P- and S-wave velocities of these cores by an ultrasonic pulse transmission method, analyzed the effects of the pore aspect ratio (AR) and pore size [Formula: see text] on velocities, and compared the experimental results with predictions of effective medium theories (EMTs). The matrix of our synthetic cores was consolidated mixture of carbonate cuttings and epoxy. We randomly imbedded predesigned penny-shaped silicone disks or expandable polystyrene balls into the matrix during the core preparation process to simulate secondary pores. The experimental results indicated that Han’s empirical linear velocity-porosity relation was a good prediction for cores with only interparticle pores. Secondary pores played an important role in the velocity variation of carbonates. Cores with a larger AR had faster velocities. Different ARs could lead to velocity variations as high as [Formula: see text] at a given porosity. When the wavelengths [Formula: see text] were larger than the pore size, cores with larger secondary pores found higher velocities under the same pore shape, pore fluid, and porosity condition. Different pore sizes could contribute to nearly 15% velocity variation at a given porosity. The comparison between our measurements and EMT predictions indicated that for carbonate rocks with a complicated pore structure, the self-consistent model gave more reliable predictions when the secondary pore size was relatively small ([Formula: see text]) and Kuster and Toksoz formulations as well as the differential effective medium model gave more satisfactory results when the secondary pore size was relatively large ([Formula: see text], or even smaller).

Mineral Alteration and Fracture Influence on the Elastic Properties of Volcaniclastic Rocks

“…Small volumes of microfractures significantly decrease the wave velocities in low‐porosity rocks (e.g., crystalline rocks; Nur & Simmons, ). Spherical pores highly contribute to rock porosity but have a lower effect on waves speed than microfractures when effective pressure varies (Wang et al, ). Mineral alteration can affect wave speeds: Clay minerals commonly decrease the wave speeds due to their low elasticity and affinity to water (Han et al, ; Planke et al, ), but alteration to stiffer minerals (e.g., chlorite and carbonates) could increase wave speeds, stiffening the rock (Adam et al, ; Frolova et al, ; Wyering et al, ).…”

Joint Elastic‐Electrical Properties of Artificial Porous Sandstone With Aligned Fractures

“…When studying the physical properties of anisotropic rocks, instead of using natural fractured samples (Sarout & Guéguen, 2008;Valcke et al, 2006), where the fracture parameters (e.g., fracture density and geometry) cannot be controlled, experimentalists usually employ synthetic materials to construct physical models that contain aligned penny-shaped fractures (Ass'ad et al, 1992;Ding et al, 2017;Rathore et al, 1995;Tillotson et al, 2012;Wang et al, 2015), although such geometries are not readily available in natural rocks. The idealization of fractures as penny-shape spheroids is recognized as a good assumption, because they (1) can capture some essential properties of the subsurface voids, (2) can provide intuitively simple parameterization of enormous complexity of the real fracture space, and (3) are relatively easily amenable to theoretical analysis (Gurevich et al, 2009).…”

Pore pressure prediction using geophysical methods in carbonate reservoirs: Current status, challenges and way ahead

“…Rock physics models considering the pore structure effects More powerful rock physics models are the key to solve the PP prediction problem in carbonates. Many recent experiment studies show that the mineral composition (Scotellaro et al, 2007;Kenter et al, 2007), pore structure (Anselmetti and Eberli, 1993;Assefa et al, 2003;Weger et al, 2009;Wang et al, 2015), and frame texture (Verwer et al, 2008) can be important factors of carbonate velocity variation. But researchers presently have no consistent findings.…”

“…Therefore, if we do not consider the pore structure effects on velocities, we may get results with high uncertainty using conventional PP prediction methods, and even fail to identify abnormal PP formations without apparent velocity deviations. For carbonate rocks with well-developed secondary pores, the pore structure effects on velocities should be as important as porosity (Eberli et al, 2003;Wang et al, 2015).…”