Chapter 2: Reservoir Fluids

1 Formation Evaluation MSc Course Notes Reservoir FluidsDr. Paul Glover Page 6 Chapter 2: Reservoir Fluids fall into three broad categories; (i) aqueous solutions with dissolved salts, (ii)liquid hydrocarbons, and (iii) gases (hydrocarbon and non-hydrocarbon). In all cases theircompositions depend upon their source, history, and present thermodynamic conditions. Theirdistribution within a given Reservoir depends upon the thermodynamic conditions of thereservoir as well as the petrophysical properties of the rocks and the physical and chemicalproperties of the Fluids themselves.

2 This Chapter briefly examines these Reservoir DistributionThe distribution of a particular set of Reservoir Fluids depends not only on the characteristicsof the rock-fluid system now, but also the history of the Fluids , and ultimately their source. Alist of factors affecting fluid distribution would be manifold. However, the most importantare:Depth The difference in the density of the Fluids results in their separation over time due togravity (differential buoyancy).Fluid Composition The composition of the Reservoir fluid has an extremely importantcontrol on its pressure-volume-temperature properties, which define the relative volumes ofeach fluid in a Reservoir .

3 This subject is a major theme of this Chapter . It also affectsdistribution through the wettability of the Reservoir rocks ( Chapter 7). Reservoir Temperature Exerts a major control on the relative volumes of each fluid in Pressure Exerts a major control on the relative volumes of each fluid in a Migration Different Fluids migrate in different ways depending on their density,viscosity, and the wettability of the rock. The mode of migration helps define the distributionof the Fluids in the Clearly, the effectiveness of the hydrocarbon trap also has a control on fluiddistribution ( , cap rocks may be permeable to gas but not to oil).Rock structure The microstructure of the rock can preferentially accept some Fluids and notothers through the operation of wettability contrasts and capillary pressure.

4 In addition, thecommon heterogeneity of rock properties results in preferential fluid distributions throughoutthe Reservoir in all three spatial fundamental forces that drive, stabilise, or limit fluid movement are: Gravity ( causing separation of gas, oil and water in the Reservoir column) Capillary ( responsible for the retention of water in micro-porosity) Molecular diffusion ( small scale flow acting to homogenise fluid compositions withina given phase) Thermal convection (convective movement of all mobile Fluids , especially gases) Fluid pressure gradients (the major force operating during primary production)Formation Evaluation MSc Course Notes Reservoir FluidsDr.

5 Paul Glover Page 7 Although each of these forces and factors vary from Reservoir to Reservoir , and betweenlithologies within a Reservoir , certain forces are of seminal importance. For example, it isgravity that ensures, that when all three basic Fluids types are present in anuncompartmentalised Reservoir , the order of Fluids with increasing depth isGAS:OIL: water , in exact analogy to a bottle of french dressing that has been left to FluidsAccumulations of hydrocarbons are invariably associated with aqueous Fluids (formationwaters), which may occur as extensive aquifers underlying or interdigitated with hydrocarbonbearing layers, but always occur within the hydrocarbon bearing layers as connate Fluids are commonly saline , with a wide range of compositions and concentrations.

6 Table shows an example of a Reservoir brine. Usually the most common dissolved salt isNaCl, but many others occur in varying smaller quantities. The specific gravity of pure wateris defined as unity, and the specific gravity of formation waters increases with salinity at a rateof about per 100 parts per thousand of dissolved solids. When SCAL measurements aremade with brine, it is usual to make up a simulated formation brine to a recipe such as thatgiven in Table , and then deaerate it prior to Composition of Draugen 6407/9-4 Formation WaterComponentConcentration, g dm-3 Pure pH = 7 Why a connate water phase is invariably present in hydrocarbon bearing Reservoir rock iseasily explained.

7 The Reservoir rocks were initially fully or partially saturated with aqueousfluids before the migration of the oil from source rocks below them. The oil migratesupwards from the source rocks, driven by the differential buoyancy of the oil and the water . Inthis process most of the water swaps places with the oil since no Fluids can escape from thecap rock above the Reservoir . However, the water is not completely displaced as the initialreservoir rock is invariably water -wet, leaving the water -wet grains covered in a thin layer ofwater, with the remainder of the pore space full of oil. water also remains in the micro-porosity where gravity segregation forces are insufficient to overcome the water -rock aqueous Fluids , whether as connate water or in aquifers, commonly contain dissolvedgases at Reservoir temperatures and pressures.

8 Different gases dissolve in aqueous Fluids todifferent extents, and this gas solubility also varies with temperature and pressure. Table a selection of gases. If gas saturated water at Reservoir pressure is subjected to lowerFormation Evaluation MSc Course Notes Reservoir FluidsDr. Paul Glover Page 8pressures, the gas will be liberated, in exactly the same way that a lemonade bottle fizzeswhen opened. In reservoirs the dissolved gas is mainly methane (from 10 SCF/STB at 1000psi to 35 SCF/STB at 10 000 psi for gas- water systems, and slightly less for water -oilsystems).

9 Higher salinity formation waters tend to contain less dissolved Dissolution of Gases in water (dissolved mole fraction) at 1 barGas104 Xgas @ 1 = mole fraction of gas dissolved at 1 bar pressure, Fluids are relatively incompressible compared to oils, and extremely so compared togases ( 10-6 to 5 10-6 per psi decreasing with increasing salinity). Consequently, if a unitvolume of formation water with no dissolved gases at Reservoir pressure conditions istransported to surface pressure condition, it will expand only slightly compared to the sameinitial volume of oil or gas. It should be noted that formation waters containing a significantproportion of dissolved gases are more compressible than those that are not gas waters expand slightly more on being brought to the surface.

10 However the reduction intemperature on being brought to the surface causes the formation water to shrink and there isalso a certain shrinkage associated with the release of gas as pressure is lowered. The overallresult is that brines experience a slight shrinkage (< 5%) on being brought from reservoirconditions to the waters generally have densities that are greater than those of oils, and dynamicviscosities that are a little lower (Table ). The viscosity at high Reservoir temperatures(>250oC) can be as low as cP, rises to above 1 cP at ambient conditions, and increaseswith increasing Evaluation MSc Course Notes Reservoir FluidsDr.