A Comprehensive Dynamic Theory of Hydrocarbon Migration and Trapping
Authors: Siddiqui Fareed I. , Lake Larry W. 
Published: December 2007

Understanding hydrocarbon migration and trapping is important since it can mean the difference between success and failure in exploration projects. The current understanding is that a capillary pressure change between a seal and a carrier bed (or reservoir) is the main factor responsible for the trapping. The current theory uses capillary pressure gradients under static (no flow) conditions to define the maximum amount of hydrocarbon that can be trapped under a particular seal. It assumes that at the very low flow rates encountered in secondary migration viscous pressure drops are negligible. Using numerical simulation and theoretical analysis, we show that, even at very low flow rates, viscous pressure drops are not negligible and that pressure gradients within phases can be substantially different from the static gradients. We present a theory that includes the effect of both the capillary and viscous forces. An innovative way of including the effect of capillary pressures in the method of characteristics is used to solve the migration and trapping problem. Migration and trapping are explained as a result of reflection and refraction of non-linear saturation waves from the heterogeneity boundaries. When viscous forces are included the seals can trap substantially more hydrocarbons than those predicted by the current theory. It is possible to classify seals into static and dynamic seals based on their capillary pressure curves and on the petrophysical properties of the carrier bed. In both cases, we are able to associate a time scale to the accumulation and indicate explanations for several other features commonly observed in secondary migration. The results from the proposed theory are confirmed using numerical simulations.

Introduction

Hydrocarbons (oil and gas) are formed by the decomposition of organic solids deposited in fine grained sediments, mostly shales. With subsequent burial, the pressure and temperature in these rocks increase and some of the bonds in the kerogen are broken to produce oil or gas. After their production, these hydrocarbons must be transported and concentrated into more porous and permeable regions to form hydrocarbon reservoirs.

The movement of hydrocarbons just after their formation in the source rocks until they reach the more permeable rocks is called primary migration (Fig. 1). Primary migration finishes when hydrocarbons are expelled from the fine grained rocks into the large permeability rocks called carrier beds. The subsequent movement of hydrocarbons after they emerge from the source rock is called secondary migration (Fig. 1). There are different ways in which secondary migration can occur: in solution, as micelles or as a separate hydrocarbon phase. Most of the evidence points to separate phase migration. Even in the case when the migration occurs in other forms, the hydrocarbons must come out of solution to form traps. Thus, the final stage of secondary migration will always be a separate hydrocarbon phase migration. This paper assumes that all the migration occurs in a separate hydrocarbon phase, either an oil or a gas phase, depending on the temperature and pressure conditions and the composition of the fluids. As hydrocarbons enter the large pores of a carrier bed they may coalesce to form larger globules. These large globules will move up by buoyancy. Hydrocarbons move in these carrier beds until they reach locations where further movement is partially or totally stopped. The obstacles to the further movement of hydrocarbons are called seals. The region beneath the seal that contains the trapped hydrocarbons at high concentrations is called a hydrocarbon trap or a hydrocarbon reservoir (Fig. 1).

No seal is perfect. They all fail under certain conditions, allowing the hydrocarbons to leak from the trap. Leakage is in effect a continuation of secondary migration although it is sometimes also referred as tertiary migration. After leaking from a trap, the hydrocarbons may trap under another seal or may ultimately seep to the surface.

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