Pore fluids

The main component of sandstone reservoirs ( siliciclastic reservoirs ) is quartz (Si02). Chemically it is a fairly stable mineral which is not easily altered by changes in pressure, temperature or acidity of pore fluids. Sandstone reservoirs form after the sand grains have been transported over large distances and have deposited in particular environments of deposition. 

Nearly all reservoirs are water bearing prior to hydrocarbon charge. As hydrocarbons migrate into a trap they displace the water from the reservoir, but not completely. Water remains trapped in small pore throats and pore spaces. In 1942 Arch/ e developed an equation describing the relationship between the electrical conductivity of reservoir rock and the properties of its pore system and pore fluids. 

It is often important to quantify the contamination of pore fluid in the unsaturated soil 2one, where monitoring wells are ineffective. In this region, suction cup lysimeters are useful (7). These samplers consist of a porous cup, typically ceramic, having two access tubes which are usually Teflon. One access tube provides a pressure-vacuum, the other discharges the sampled fluid to the surface. The porous cup, typically between 2 and 5 cm in diameter, is attached to a PVC sample accumulation chamber. 

At a microscale, a sorbable component exists at three locations—in a sorbed phase, in pore fluid, and in fluid outside particles. As a consequence, in material balances time derivatives must be included of terms involving tij, (the pore concentration), and Cj (the extraparticle concentration). Let tij represent tij averaged over particle volume, and let Cp represent averaged over pore fuiid volume. 

Eath. In other cases, attachment can be essentially permanent, but in oth cases, only detached molecules undergo transport. In contrast, the following four instances illustrate cases where diffusion of adsorbate molecules occurs in their adsorbed state within phases that are distinct from the pore fluid ... 

Combined Pore and Solid Diffusion In porous adsorbents and ion-exchange resins, intraparticle transport can occur with pore and solid diffusion in parallel. The dominant transport process is the faster one, and this depends on the relative diffusivities and concentrations in the pore fluid and in the adsorbed phase. Often, equilibrium between the pore fluid and the solid phase can be assumed to exist locally at each point within a particle. In this case, the mass-transfer flux is expressed by ... 

For particles that have no macropores, such as gel-type ion-exchange resins, or when the solute holdup in the pore fluid is small, 0 may Be... 

The rate equation is given by item A in Table 16-11. With pore fluid and adsorbent at equihbrium at each point within the particle and for a constant dlffusivity, the rate equation can be written as ... 

Parallel Pore and Solid Diffusion Control With a linear isotherm, assuming equilibrium between the pore fluid and the solid adsorbent, batch adsorption can be represented in terms of an equivalent solid diffusivity = ( pD i + ppD, )/( p + pp Q). Thus, Eqs. (16-96) and (16-99) can be used for this case with D, replaced by D. ... 

FIG. 19 Scheme of a simple fluid confined by a chemically heterogeneous model pore. Fluid modecules (grey spheres) are spherically symmetric. Each substrate consists of a sequence of crystallographic planes separated by a distance 8 along the z axis. The surface planes of the two opposite substrates are separated by a distance s,. Periodic boundary conditions are imposed in the x and y directions (see text) (from Ref. 77). 

To compare molecular theoretical and molecular dynamics results, we have chosen the same wall-particle potential but have used the 6 - oo fluid particle potential. Equation 14, Instead of the truncated 6-12 LJ potential. This Is done because the molecular theory Is developed In terms of attractive particles with hard sphere cores. The parameter fi n Equation 8 Is chosen so that the density of the bulk fluid In equilibrium with the pore fluid Is the same, n a = 0.5925, as that In the MD simulations. 

Iv) Shear stress and viscosity. As explained In Section 1 three Independent estimates of the shear stress can be made for this particular type of flow. For both systems they all agree within the limits of statistical uncertainty as shown In Table II. The shear stress In the micro pore fluid Is significantly lower than the bulk fluid, which shows that strong density inhomogeneities can induce large changes of the shear stress. 

They considered that cement formation was the result of an acid-base reaction leading to the formation of hydrates by a through-solution mechanism, by nucleation and precipitation from pore fluids. Two phases were found in the matrix, one amorphous and the other crystalline. The crystalline phase was newberyite. Finch Sharp concluded that the amorphous phase was a hydrated form of aluminium orthophosphate, AIPO4, which almost certainly contained magnesiiun. They ruled out a pure AlP04.nH20, for they considered that the reaction could not be represented by the equation... 

NMR signals are highly sensitive to the unusual behavior of pore fluids because of the characteristic effect of pore confinement on surface adsorption and molecular motion. Increased surface adsorption leads to modifications of the spin-lattice (T,) and spin-spin (T2) relaxation times, enhances NMR signal intensities and produces distinct chemical shifts for gaseous versus adsorbed phases [17-22]. Changes in molecular motions due to molecular collision frequencies and altered adsorbate residence times again modify the relaxation times [26], and also result in a time-dependence of the NMR measured molecular diffusion coefficient [26-27]. 

For the same quantity of electricity, twice as many water molecules are electrolyzed at the cathode than at the anode, producing a chemical gradient of molecular water. As a consequence of the reactions, two supplemental ionic species, H+ and OH (in addition to the migration of existing anions and cations in the pore fluid under the electrical field), are generated, and can have a significant influence on local conductance. 

The fifth factor is the water content of the soil. Electroosmotic flow is promoted at higher water contents. Therefore, high moisture content, and in particular saturated conditions are favored. However, the technique can be used in partially saturated deposits by supplying a pore fluid at... 

The sixth factor is conditioning. Similar to the changes in current density, the pore fluid at the anode and cathode compartments can be conditioned to a specific pH or chemistry to increase the efficiency of the process. 

Swolf, H.S., Chemical effects of pore fluids on rock properties, in Symposium on Underground Waste Management and Environmental Implications, Houston, TX, Cook, T.D., Ed., American Association of Petroleum Geologists 18, 1972, pp. 224-234. 

When the adsorption isotherm is nonlinear, a numerical solution is generally required. For a Langmuir system with negligible solute holdup in the pore fluid, item A in Table 16-11 gives ... 

I shall apply the new method of solution to a problem of early diagenesis in carbonate sediments. I calculate the properties of the pore fluid in the sediment as a function of depth and time. The different reservoirs are... 

Fig. 2.3. Configuration of a reaction path as a titration model. One or more reactants are gradually added to the equilibrium system, as might occur as the grains in a rock gradually react with a pore fluid.

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