Digital Rock Studies of Tight Porous Media

Abstract

Abstract. This technical report summarizes some recently developed approaches to studies of rock properties at a pore scale. Digital rock approach is complementary to laboratory and field studies. It can be especially helpful in situations where experimental data are uncertain, or are difficult or impossible to obtain. Digitized binary images of the pore geometries of natural rocks obtained by different imaging techniques are the input data. Computer-generated models of natural rocks can be used instead of images in a case where microtomography data are unavailable, or the resolution of the tools is insufficient to adequately characterize the features of interest. Simulations of creeping viscous flow in pores produce estimates of Darcy permeability. Maximal Inscribed Spheres calculations estimate two-phase fluid distribution in capillary equilibrium. A combination of both produce relative permeability curves. Computer-generated rock models were employed to study two-phase properties of fractured rocks, or tight sands with slit-like pores, too narrow to be characterized with micro-tomography. Various scenarios can simulate different fluid displacement mechanisms, from piston-like drainage to liquid dropout at the dew point.A finite differences discretization of Stokes equation is developed to simulate flow in the pore space of natural rocks. The numerical schemes are capable to handle both no-slip and slippage flows. An upscaling procedure estimates the permeability by subsampling a large data set. Capillary equilibrium and capillary pressure curves are efficiently estimated with the method of maximal inscribed spheres both an arbitrary contact angle. The algorithms can handle gigobytes of data on a desktop workstation. Customized QuickHull algorithms model natural rocks. Capillary pressure curves evaluated from computer-generated images mimic those obtained for microtomography data.

Influence of Pore Structure on Gas Flow and Recovery in Ultradeep Carbonate Gas Reservoirs at Multiple Scales

“…Studying gas flow in pore structures is beneficial to describing reservoir production characteristics. − 2D visualization models are widely used to observe fluid flow in porous media, but the 2D structure cannot fully reflect the complex pore structure of carbonate reservoirs, and it is not suitable for analyzing single-phase gas flow and pressure distributions. , Under certain conditions, core surface characteristics cannot effectively represent internal pore structures, which makes it difficult to fully reveal the influence of core experimental results. Developing 3D digital cores is one of the most direct and accurate methods to reflect internal pore characteristics. − The 3D pore network models (PNMs) based on CT scanning can not only match the real pore structure but also extract and establish the representative elementary volume (REV) of rare samples to simulate and analyze the flow characteristics to compensate for the deficiency of experiments, theoretical modeling, and other methods. , However, there are few studies on the influence of complex pore structures on gas flow in ultradeep carbonate gas reservoirs by 3D PNM. In addition, 3D PNM can also be used to simulate gas–water two-phase (G-W) flow.…”

Supercritical CO2 flow through a layered silica sand/calcite sand system: Experiment and modified maximal inscribed spheres analysis

“…The method of maximal inscribed spheres technique (MIS) [Silin and Patzek, 2006, Silin et al, 2003, Silin, et al, 2010, and Section 5 of Silin, 2012] employs the one-to-one correspondence between the magnitude of the capillary pressure and the mean curvature of the interface between two immiscible fluids in equilibrium. Given a segmented image of the pore space, the MIS algorithms calculate the volume occupied by the nonwetting fluid through inscribing spheres of a given radius and testing the connectivity of the obtained structure.…”