Geometric Models of Porosity Reduction Mechanisms in Tight Gas Sands

摘要

Pore volume reduction of sediments by plastic deformation during compaction and by cementation of grains has been evaluated for different proportions of ductile and hard grains. We attempted to produce the compaction behavior of grains with a purely geometric model, which uses the cooperative rearrangement algorithm to produce dense, random packings. To account for deformation of lithic grains, we used the softshell model, in which grains can interpenetrate until their inner rigid cores come into contact. We varied the fraction of grains assumed to be ductile and the radius of the rigid core of the ductile grains. By changing these two variables, we can have very small values of porosity after compaction of our packing, which is the character of tight sands. The predicted relation between the fraction of ductile grains in the sediment and the porosity after compaction agrees well with experimental data from Pittman and Larese (1991)1. The radius of the rigid core of the ductile grains proves to be a good proxy for different kinds of ductile material, ranging from brittle to extremely ductile. We simulated diagenesis in our compacted rock by considering quartz precipitation. The overgrowth or rim cement was modeled by uniformly increasing the radius of all the grains, while holding their centers fixed. These simulations yield detailed descriptions of pore scale geometry resulting from processes common in forming tight gas sandstones. Such models in turn can provide insight into two-phase flow properties of these reservoirs, particularly the sensitivity of gas permeability to water saturation2.