Shale Rock Properties, Keys for Successful Dynamic Modeling of Hydrocarbon Migration

Okui, Akihiko ¹
(1)Technical Evaluation Department, Idemitsu Oil and Gas Co., Ltd., Tokyo, Japan.

In nature, slowest process controls the rate of whole process. In case of hydrocarbon migration, permeability for fine-grained rocks such as shale is much less than course-grained rock such as sandstone. Therefore, the permeability for shale should be main controlling factor on hydrocarbon migration in basins. Since hydrocarbon migration occurs as multi-phase fluid flow, relative permeability as well as capillary pressure is also important to evaluate and model the dynamic nature of hydrocarbon migration.

Static migration modeling applying ray-trace and invasion percolation ignores time effect and generally give optimistic result than natural system. In natural system, the time required for the migration through shale is long, and therefore the timing to reach traps should be delayed. In addition, some of hydrocarbons should be lost during its migration. Leakage through cap rock (seal) is generally evaluated by the balance between capillary pressure for cap rock and buoyancy of migrating hydrocarbon. However, this is also an evaluation on specific static condition, and can not be applicable to dynamic system such as continuous hydrocarbon charge, overpressured reservoir and so on.

It was found that absolute permeability, relative permeability and capillary pressure have close relationships, since they are all controlled by pore-throat size distribution of rocks. Among them, capillary pressure is easiest to measure by mercury injection test, and therefore capillary pressure curve is useful way to characterize the nature of multi-phase flow through shales. Capillary pressures at certain saturation of non-wetting phase (ex. 5%) collected from shales over the world have wide range, but it was found that the plotting against porosity enables to group them by shale type (actually mineralogy). This is because especially chemical compaction affects the pore-throat size distribution of shales. Characterization of shales by above way and application of the results to multi-dimensional basin modeling enables to model existing hydrocarbon accumulations and long distance migration in a Southeast Asian basin.

AAPG Search and Discovery Article #90135©2011 AAPG International Conference and Exhibition, Milan, Italy, 23-26 October 2011.