Abstract: Mechanics of Secondary Migration and Entrapment of Hydrocarbons

Tim T. Schowalter

The mechanics of secondary migration and entrapment of hydrocarbons are well-understood physical processes that can be considered quantitatively in hydrocarbon exploration. The main driving force for secondary migration of hydrocarbons is buoyancy. If the density of the hydrocarbon phase and the water phase is known, then the magnitude of the buoyant force can be determined for any hydrocarbon column in the subsurface. Hydrocarbon and water densities vary significantly. Subsurface oil densities rangefrom 0.5 to 1.0 g/cc; subsurface gas densities range from 0.001 to 0.5 g/cc; and subsurface water densities range from 1.0 to 1.2 g/cc. When a hydrodynamic condition exists in the subsurface, the buoyant force of any hydrocarbon column will be different from that for the hydro tatic case. This effect can be quantified if the potentiometric gradient and dip of the formation are known.

The main resistant force to secondary hydrocarbon migration is capillary pressure. The factors determining the magnitude of the resistant force are the radius of the pore throats of the rock, hydrocarbon-water interfacial tension, and wettability. For cylindrical pores, the resistant force can be quantified by the simple relation: Pd = 2^ggrcos ^THgr/R, where Pd is the hydrocarbon-water replacement pressure or the resistant force, ^ggr is interfacial tension, cos ^THgr is the wettability term, and R is radius of the largest connected pore throats. The radius of the largest connected pore throats can be measured indirectly by mercury-capillary techniques utilizing cores or drill cuttings. Subsurface hydrocarbon-water interfacial tensions range from 5 to 35 dynes/cm for oil-water system and from 70 to 30 dynes/cm for gas-water systems. Migrating hydrocarbon slugs are believed to encounter water-wet rocks. The contact angle, of hydrocarbon and water against the solid rock surface as measured through the water phase, ^THgr, is thus assumed to be 0°, and the wettability term, cos ^THgr, is assumed to be 1.

A thorough understanding of these principles can aid both quantitatively and qualitatively in the exploration and development of petroleum reserves. When subsurface values for the variables affecting secondary hydrocarbon migration and entrapment can be obtained, we can, for example, estimate: (1) the distance downdip to the hydrocarbon-water contact from a producing well in a developing field or from a hydrocarbon show adjacent to a stratigraphic trap; (2) the vertical or lateral seal capacity of any potential seal expressed in terms of vertical hydrocarbon column that the seal can contain; and (3) the hydrocarbon column needed to migrate through a given reservoir carrier bed.

An understanding of the mechanics of secondary migration also can be used to construct a model for petroleum migration from source rocks to a trapped accumulation. Petroleum expelled from a fine-grained source rock will accumulate at the source rock-carrier bed boundary until a large enough oil or gas column is present to migrate laterally through the reservoir. Residual gas or oil will be left behind along any migration path. Petroleum will accumulate wherever there is a structural trap or a lateral-displacement pressure barrier along the migration path. Structural traps will spill petroleum updip when full, and stratigraphic traps will leak oil or gas laterally when the hydrocarbon column is sufficient to create a buoyant force exceeding the displacement pressure of the confining la eral seal. If the stratigraphic spill point is reached prior to attaining a column sufficient to cause lateral (breakthrough) leakage, the stratigraphic trap will spill oil updip in a manner exactly analogous to a structural trap. A stratigraphic trap that leaks petroleum updip through a displacement pressure barrier will leak out approximately one half of the trapped oil or gas column; then it will reseal and can be refilled to near capacity. Traps filled to spillpoint will cause differential entrapment of oil updip from gas. Traps that leak petroleum updip through a displacement-pressure barrier will differentially entrap gas updip from oil. Trapped petroleum can remigrate if the structural, sealing, or hydrodynamic conditions of the trap are changed.

AAPG Search and Discovery Article #90969©1977 AAPG-SEPM Rocky Mountain Sections Meeting, Denver, Colorado