Process of porosity loss and predicted porosity loss in high effective stress sandstones with grain crushing and packing texture transformation

Abstract

Grain crushing is common, and porosity loss due to mechanical compaction dominates in cool, young, rapidly subsiding and deep basins where cementation do not have enough time to significantly reduce porosity. However, the application of existing porosity predicted model during compaction in deep buried sandstone may be doubt when the grain crushing is intense. Based on compaction experiments, the grain crushing, grain size distribution and packing texture were analyzed, and a quantitative evaluation method for predicting compaction porosity loss (COPL) were proposed, which consider grain breakage and packing texture evolution stages. The results of compaction experiments confirmed that grain crushing, grain size, grain packing texture and effective stress in sandstone all influence porosity reduction during compaction in cool, young, rapidly subsiding basins. The sandstones experienced grain sliding, grain crushing, and grain rearrangement after crushing during compaction. The grain packing texture transforms from one-component packing to binary packing texture due to intense grain crushing in well-sorted coarse sandstone. Parameter ‘S’, which is named as grain-crushing factor is significantly linear with effective stress. Analysis of our results leads to a dynamic equilibrium that describes the grain crushing in sandstone during compaction. The relationship between σ*(MZ)2 and COPL reflects the dynamic equilibrium during the compaction of sandstone with grain crushing. When mean grain size of sandstone under different stress can be obtained from experiment, the function between parameter σ*(Mz)2 and COPL are well applied in porosity prediction. The partial derivative (∂COPL/∂σ*) calculated by existing logarithmic porosity predicted model agree with the measured data better than that of values calculated by existing exponential porosity predicted model in sandstone with intense grain crushing. For underground sandstone which mean grain size under different stress cannot be obtained, the logarithmic porosity predicted model can be applied in sandstone which grain crushing is intense, the constant parameter in logarithmic porosity predicted model can be predicted by mean grain size. The exponential porosity predicted model were well applied in sandstone without grain crushing. The selection of porosity predicted model of sandstone during compaction should be based on the intense of grain crushing and the stage of compaction.