Migration rate

From a linear rate standpoint, the least efficient process along the migration path is the rate-limiting step that controls the overall rate of the process. In a sequence of sands and shales, the rate limiter is the least permeable shale and the expulsion rate of hydrocarbons in the source. However, the migration rate of a nonwetting separate phase through barriers like shales is self-adjusting. This is accomplished by enlarging the area through which the process operates. For example, in traps, hydrocarbons are accumulated and spread laterally until the accumulation size causes the rate of migration into the structure to equal that going out of the structure. This accumulation process increases the hydrocarbon flux rate through the overlying shale by increasing the contact area of the hydrocarbons with the shale. Any weaknesses in the shale, such as fractures, will eventually be reached by the accumulating hydrocarbons, increasing the leakage from the trap. Accumulation size is also limited by the spill point.

Parallel and serial processes[edit]

The flux rate of hydrocarbon transport in the subsurface is viewed as consisting of both parallel and serial processes. Parallel processes occur simultaneously. They are

Diffusive transport
Aqueous transport in solution and as micelles
Separate phase transport

Serial transport processes are dominant. They occur sequentially along the most effective migration path. Examples of serial processes are

Expulsion from the source rock
Capillary restrictions along the migration route to the trap
Leakage through the seal

Rates for different mechanisms[edit]

Hydrocarbons migrate by different mechanisms; each has its own rate. The table below lists the mechanisms and rates.

Migration mechanism	Migration rate
Hydrodynamic	0.1 and 100 m/year
Compaction	0.001 and 1 m/year
Buoyancy	Meters per day for gas (oil not measured)
Diffusion	1 to 10 m/m.y.

Hydrodynamics or compaction transport rate[edit]

The rate of water movement through pore systems places an upper limit on the rate of hydrocarbon transport by hydrodynamics or compaction. If the hydrocarbons are present as a free phase, buoyant forces may be added to the rate. In practice, however, the additional force supplied by hydrodynamics or compaction is largely counterbalanced by capillary forces and relative permeability effects. Rates vary for hydrodynamic transport, depending on permeability and elevation head. Rates from compaction depend primarily on permeability since pressure can only vary between hydrostatic and geostatic pressure.

Buoyancy transport rate[edit]

The rate of transport of hydrocarbons by buoyancy depends on the density contrast of the hydrocarbons with water and hydrocarbon column height. The rate of transport of large hydrocarbon masses is limited by the time it takes the mass to grow to a column height that can overcome capillary forces of barriers to migration. Once a continuous thread of hydrocarbons connects two coarse-grained units through an intermediate fine-grained unit, the transfer of hydrocarbons from the lower unit to the upper is only limited by the permeability of the pathway.

Diffusion transport rate[edit]

Hydrocarbon transport by diffusion is very slow. Rates depend on the concentration at the location from which diffusion proceeds. For a free phase this is always a concentration of one; the diffusion coefficient is between 10^–10 and 10^–12 m²/sec.

Rate measurements[edit]

Rate measurements of migration are seldom made because of the uncertainty associated with migration length, cross-sectional area, and time interval. Linear rate estimates of gas-phase migration in the upper length::200 m of sedimentary basins are as high as tens of meters per day, based on known times of injection of gas into storage reservoirs and subsurface coal burns. Estimates of vertical seepage velocities over larger areas are between 75 and 300 m/year. Oil volume rate estimates of 50 m³ (300 bbl) per year have been made in the marine environment by collecting bubbles. These rates clearly indicate separate phase migration along multiple narrow migration pathways.

Maximum rates[edit]

Maximum rates of separate phase migration are estimated to be much faster than commonly envisioned. Many old fields, particularly gas fields, have produced more hydrocarbons than their original estimates of reserves in place. Initial production rates often decline to a low steady-state value. Discounting the uncertainties involved in these estimates, it appears production may decline until it is balanced by the area integrated charge rate of the field. Many shut-in wells show pressure buildup, indicating transfer of fluids into the field at relatively rapid rates. It is, however, uncertain what portion of the recharge is hydrocarbons and what portion is water.

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