Elastic Characteristics of Overpressure in Shale Based on Micro-Mechanism Analysis

Characterizing the overpressure of shale relies on a good understanding of its mechanism and has been of necessity and interest in pore pressure prediction for drilling safety. I provide a solution to cluster hundreds of wells by using an unsupervised algorithm (k-means) to recognize the overpressure mechanisms. Combining the model-driven feature and data-driven method delivers robust results of four clusters, corresponding to normal compaction or disequilibrium compaction, fluid expansion, fluid loss, and a mixture of fluid expansion and loss. The procedures developed can assist people to rapidly determine which model to use while performing pore pressure prediction based on the neighboring wells.

Two end-member trends of smectite-to-illite transition are further categorized. The smectite-to-illite transition with fluid expansion is characterized by a decrease of sonic velocity and little change in the bulk density. In contrast, the transformation with fluid loss is characterized by an increase of density and little change in the sonic velocity. The fluid expansion typically gives rise to high magnitude overpressure and tends to happen when the overlying formations have more shaly contents and low permeability. For the fluid loss case, it has relatively deeper overpressure onsets, and its overlying formations have more sandy contents and relatively high permeability.

I develop a modeling framework to capture the elastic and pore pressure evolution characteristics in shale during the smectite-to-illite transition. With proper bulk volume models, the velocity, density, and pore pressure increase of shale can be computed in the fluid expansion, fluid loss, and a mixture of these two scenarios. After calibration with logging data, the modeling result can reasonably interpret the rock property evolution within the smectite-to-illite transition zone. I extend the model strategy to the case of overpressure caused by kerogen maturation in organic shale, which can benefit drilling safety and unconventional reservoir characterization.