Connectivity of Pore Space: the Primary Control from Two-Phase Flow Properties of Tight-Gas Sands
We use quantitative grain-scale models to predict the variation of capillary pressure curves in tight gas sandstones. We model several depositional and diagenetic processes important for porosity and permeability reduction in tight gas sands, such as having various amounts of ductile grains and quartz cementation. The model is purely geometric and begins by applying a cooperative rearrangement algorithm to produce dense, random packings of spheres of different sizes. We simulate the evolution of this model sediment into a low-porosity sandstone by applying different amount of ductile grains and quartz precipitation. To account for deformation of ductile (lithic) grains, we used the soft-shell 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. The overgrowth or rim cement was modeled by uniformly increasing the radius of all the grains, while holding their centers fixed. In this way we produce model tight sandstones having porosities between 3% and 10%. A substantial fraction (tens of percent) of the original pore throats in the sediment are closed by the simulated diagenetic alteration. It is known that overgrowth cement causes the pore space to drop below the percolation threshold when half of the throats are closed. Thus the pore space in typical tight gas sandstones is poorly connected, and is often close to being completely disconnected.
The drainage curve for different model rocks was computed using invasion percolation in a network taken directly from the grain-scale geometry and topology of the model. Some general trends follow classical expectations and are confirmed by experimental measurements: increasing the amount of cement or decreasing the rigid radius of ductile grains while holding other parameters constant shift the drainage curve to larger pressures. Adding more cement or reducing the rigid radius of ductile grains causes the irreducible water saturations to increase. However, simulation of different packings show significant variability in drainage curves even for model rocks with similar porosities and similar pore throat size distributions. The variability is large enough to mask the general trends described above. We conclude that the connectivity of the matrix pore space is the most important factor for understanding flow properties of tight gas sandstones.
AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009