Buoyancy migration

Once the well is drilled, the oil is either released under natural pressure or pumped out. Normally crude oil is under pressure (were it not trapped by impermeable rock it would have continued to migrate upward), because of the pressure differential caused by its buoyancy. When a well bore is drilled into a pressured accumulation of oil, the oil expands into the low-pressure sink created by the well bore in communication with the earth s surface. As the well fills up with fluid, a back pressure is exerted on the reservoir, and the flow of additional fluid into the well bore would soon stop, were no other conditions involved. Most crude oils, however, contain a significant amount of natural gas in solution, and this gas is kept in solution by the high pressure in the reservoir. The gas comes out of solution when the low pressure in the well bore is encountered and the gas, once liberated, immediately begins to expand. This expansion, together with the dilution of the column of oil by the less dense gas, results in the propulsion of oil up to the earth s surface As fluid withdrawal continues from the reservoir, the pressure within the reservoir gradually decreases, and the amount of gas in solution decreases. As a result, the flow rate of fluid into the well bore decreases, and less gas is liberated. The fluid may not reach the surface, so that a pump (artificial lift) must... 

Most of the features of the theoretical treatment of bubble motion are present in the treatment that considers the water incompressible and neglects gravity effects. We quote from Cole (Ref 1, Chapt 8) The simplest approximation to the true motion of the bas bubble is the one in which it is assumed that the motion of the surrounding water is entirely radial and there is no vertical migration. In this approximation, which has been discussed by a number of writers, the hydrostatic buoyance resulting from differences in hydrostatic pressure at different depths is neglected. It is thus assumed that at an infinite distance from the bubble in any direction the pressure has the same value as the initial hydrostatic pressure P0 at the depth of the charge... 

A common structural trap, the anticline, is an upward bulge in the rock layers which forms an arch capable of holding oil under its apex. The buoyancy of oil and gas carries them upward through porous rock layers into the apex until they are trapped by an impermeable layer. Anticlinal type of folded structure is shown in Fig. 2. Reservoirs formed by folding of the rock layers or strata usually have the shapes shown in Fig. 2(a) and (b). These traps were filled by upward migration of oil and/or gas through the porous strata or beds to the location of Ihe trap. Further movement was arrested by a combination of the forms of the structure and the seal or cap rock provided by the formation covering the structure. 

Shook, G.M., Kostarelos, K., and Pope, G.A. (1997). Minimization of vertical migration of DNAPLs using surfactant enhanced aquifer remediation at neutral buoyancy. Presented at SPE Annual Tech. Conf., Soc. Petrol. Eng., 39294, 1-8. 

Villareal, T. A., and Carpenter, E. J. (2003). Buoyancy regulation and the potential for vertical migration in the oceanic cyanobacterium Trichodesmium. Microh. Ecol. 45, 1—10. 

Scott D. R. and Stevenson D. J. (1989) A self-consistent model of melting, magma migration, and buoyancy-driven circulation beneath mid-ocean ridges. J. Geophys. Res. 94,... 

Nunn J. A. and Meulbroek P. (2002) Kilometer-scale upward migration of hydrocarbons in geopressured sediments by buoyancy-driven propagation of methane-filled fractures. Am. Assoc. Petrol. Geologists Bull. 86, 907-918. 

Two types of local groundwater flow systems may develop in sedimentary basins at depth ranges of interest for studies of hydrocarbon migration and accumulation flow systems driven by buoyancy (Section 2.4.1) and those driven by osmosis (Section 2.4.2). 

These upward directed forces are generally called buoyancy forces and are the driving forces for separate phase oil or gas migration through water under hydrostatic conditions. The magnitude of the buoyancy force for a vertical length Zq of a body of oil or gas immersed in water can be expressed by... 

The displacement pressure is a rock property and is defined as the force required to replace water from a cylindrical pore with oil or gas. Hence the displacement pressure determines the minimum buoyancy pressure needed for migration. 

Initially, the hydrocarbons entering at the base of a horizontal carrier rock are very finely dispersed and the buoyancy forces are still too small to initiate hydrocarbon migration. 

Continued supply of hydrocarbons from the source rock increases the vertical height of the hydrocarbon column (Zo). As soon as Zq is large enough, i e as soon as the buoyancy force of the hydrocarbon column is greater than the resistant force of the carrier rock, vertical upward migration through the carrier rock will start. 

These estimated specific discharges for the buoyancy-driven migration of oil through a carrier rock are about 5 orders of magnitude greater than the estimated expulsion rates for oil from source rocks as given in Chapter 3. 

As outlined in Section 4.1, the migration of very finely dispersed oil droplets or gas bubbles with diameters smaller than those of the smallest pore throats of the carrier rock will not be influenced by capillary forces. In addition, according to Tissot and Welte (1984) the very finely dispersed oil droplets will not strictly follow the law of buoyancy. In the initial stages of secondary migration, the hydrocarbons in separate phase may occur as droplets or... 

Hydrostatic secondary hydrocarbon migration systems, in which the dominant forces influencing hydrocarbon migration are the buoyancy forces and the capillary pressure gradients (Section 4.3.3). 

After an initial upward migration of the hydrocarbons as induced by other hydrocarbon migration mechanisms prevailing at greater depths (e.g. buoyancy relict or actual conditions of burial-induced flow) or after initial lateral migration from e.g. a subsiding part of the basin ... 

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