EFFECTS OF GRAIN PACKING TEXTURE AND DUCTILE GRAIN DEFORMATION ON SANDSTONE COMPACTION PROCESS: IMPLICATIONS FROM DEM NUMERICAL SIMULATION

Sandstone is one of the important reservoirs of underground fluids, such as oil and gas. The grain deformation and crushing are common, and will change the grain packing texture in sandstone under high effective stress.Grain packing texture and ductile grain deformation play an important role in porosity loss during compaction. However, few studies which focus on the effect of grain packing texture changing and ductile grain deformation on porosity reduction during compaction have been reported.

DEM (discrete element method) proposed by Cundall and Strack (1979) can be used to simulated the compaction process of grain media, and it has been applied to studied grain crushing and compaction of sandstone in petroleum geology. The force of grains and grain packing texture evolution in sandstone can be monitored during compaction in DEM, which is very difficult to observe during physical experiment. The aim of this study was to establish a mathematical model based on grain packing texture, which can be used to predicted porosity loss caused by grain packing texture and ductile grain deformation during compaction. The micromechanical properties of grains used in DEM simulation were determined by literature and physical experiment. The DEM simulation results of 2 sample were verified by physical experiment data. 112 DEM simulations were carried out with different ductile grain contents and initial grain packing texture.

The results show the ductile grain content and initial packing texture are the main factors controlling the porosity loss. There is a linear relationship between the content of ductile grains and porosity loss of sandstone with similar initial packing texture. A mathematical model for predicting porosity loss of sandstone based on grain packing texture was proposed, and the reliability is verified. When sandstone don’t have ductile grains, the correlation coefficient of model predicted results and DEM simulated results is as high as 0.999. When sandstone include some ductile grains (less than 50%), the correlation coefficient of model predicted results and DEM simulated results is 0.98. The maximum errors between model predicted results and simulated results are less than 4.5% of the average porosity loss during compaction. In addition, A set of physical experimental data from the literature was also used to verify this model, and the correlation coefficient of model predicted results and physical results are 0.88. This model is significant for predicting the porosity loss of sandstone during compaction.