3-D Imaging and Analysis of Grain Compaction

Reservoir compaction is of significant concern to the oil and gas industry. While compaction may aid in production by squeezing oil from the rock it can also reduce permeability. Understanding the interplay of these effects is essential and can greatly benefit from 3D imaging studies.

We use 3D micro-CT analysis to image a set of unconsolidated samples which have undergone undergone triaxial testing at 1200 psi confining stress at a resolution of 3 microns. The samples used in this study are a 70:30 unconsolidated mixture of quartz:feldspar, including an undeformed (control) sample, and three samples at various points along the stress/strain curve. The porosity, quartz and feldspar grains are differentiated in the image data. Grain size distributions for both minerals, the degree of fragmentation, and variation in grain shape, particle aspect ratio, grain contact connectivity and size with compaction regime are quantified. Shapes of the quartz grains exhibit little deviation during compaction, while feldspar grains show distinct differences. Pores and constrictions are identified from the image data and illustrate the strong effect of compaction on the overall pore structure. Statistical parameters including pore coordination, aspect ratio and pore shape are given. Formation factor and permeability simulations are undertaken on the samples. Permeability exhibits an order of magnitude decrease associated with a small drop (~2%) in sample porosity. Three dimensional imaging and analysis of compacted core material at the local grain/pore scale may assist in understanding the micromechanics of compaction and its consequences to production.