THE RESERVOIR PETROLEUM RESERVOIR • ROCK PROPERTIES • FLUID PROPERTIES • PRESSURE • RESERVOIR DRIVE ROCK PROPERTIES Rocks are described by three properties: – Porosity - quantity of pore space – Permeability - ability of a formation to flow – Matrix - major constituent of the rock note: porosity & permeability has been discussed partially in “Chapter I Introduction” PERMEABILITY • Permeability is a property of the porous medium and is a measure of the capacity of the medium to transmit fluids • Absolute Perm: When the medium is completely saturated with one fluid, then the permeability measurement is often referred to as specific or absolute permeability • Effective Perm: When the rock pore spaces contain more than one fluid, then the permeability to a particular fluid is called the effective permeability Effective permeability is a measure of the fluid conductance capacity of a porous medium to a particular fluid when the medium is saturated with more than one fluid • Relative Perm: Defined as the ratio of the effective permeability to a fluid at a given saturation to the effective permeability to that fluid at 100% saturation DARCY’S LAW p2 p1 L q Direction of flow qμ L • k= A ( p1 − p ) k = permeability (measured in darcies) k/μ = kh/μ = A L = length q = flow rate p1, p2 = pressures A = area perpendicular to flow μ = viscosity DARCY’S LAW: RADIAL FLOW rw r 2πkh( P − Pw ) q= μ ln r / rw h = height of the cylinder (zone) P = pressure at r Pw = pressure at the wellbore PERMEABILITY – POROSITY CROSSPLOT Sandstone A1 Limestone A1 Permeability (md) 100 1000 100 10 10 1 0.1 0.1 0.01 0.01 10 14 Porosity (%) 10 14 18 CALCULATING RELATIVE PERMEABILITIES • Oil • Water • Gas k k k ro rw rg = k eo k = k = k ew k eg k Bottom Water Drive Oil producing well Oil Zone Water Cross Section • Water moves up to fill the "space" vacated by the oil as it is produced Water Drive • • • This type of drive usually keeps the reservoir pressure fairly constant After the initial “dry” oil production, water may be produced The amount of produced water increases as the volume of oil in the reservoir decreases Dissolved gas in the oil is released to form produced gas Gas Invasion • Gas is more mobile than oil and takes the path of least resistance along the centre of the larger channels • As a result, oil is left behind in the smaller, less permeable, channels Gas Cap Drive Gas from the gas cap expands to fill the space vacated by the produced oil Gas Cap Drive • As oil production declines, gas production increases • Rapid pressure drop at the start of production Solution Gas Drive After some time the oil in the reservoir is below the bubble point Solution Gas Drive • • • An initial high oil production is followed by a rapid decline The Gas/Oil ratio has a peak corresponding to the higher permeability to gas The reservoir pressure exhibits a fast decline GRAVITY DRAINAGE Gas Gas Oil Gas Point C Oil Point B Oil Point A Recovery = to 60% of OOIP Drives General • A water drive can recover up to 60% of the oil in place • A gas cap drive can recover only 40% with a greater reduction in pressure • A solution gas drive has a low recovery Gas/oil Ratio Trends Solutiongas drive Gas/oil ratio, MSCF/STB Gas-cap drive Water drive 20 40 60 80 100 Cumulative oil produced, percent of original oil in place Drive Mechanism Solution-gas drive Gas-cap drive Water drive Gravity-drainage drive Drive Mechanism Volumetric reservoir (Gas expansion drive) Water drive Average Oil Recovery Factors, % of OOIP Range Average - 30 15 15 - 50 30 30 - 60 40 16 - 85 50 Average Gas Recovery Factors, % of OGIP Range Average 70 - 90 80 35 - 65 50 Drive Problems Water Drive: • Water can cone upwards and be produced through the lower perforations Gas Cap Drive: • • Gas can cone downwards and be produced through the upper perforations Pressure is rapidly lost as the gas expands Gas Solution Drive: • • Gas production can occur in the reservoir, skin damage Very short-lived Secondary Recovery • • • Secondary recovery covers a range of techniques used to augment the natural drive of a reservoir or boost production at a later stage in the life of a reservoir A field often needs enhanced oil recovery (EOR) techniques to maximise its production Common recovery methods are: – Water injection – Gas injection • In difficult reservoirs, such as those containing heavy oil, more advanced recovery methods are used: – – – – Steam flood Polymer injection CO2 injection In-situ combustion Secondary Recovery water injection gas injection [...]... path in reservoir 60 50 40 30 % Liquid 790 20 80 B ub bl ep oi nt lin e 90 e % Liquid 50 10 33 30 le bb Bu lin Volatile oil Pressure Black Oil 80 9 0 7 60 0 Dewpoint line Critical point int po THE FIVE RESERVOIR FLUIDS 2 40 Pressure, psia Pressure path in reservoir 5 10 20 Separator Separator Dew ne Temperature Temperature, °F Pressure path in reservoir Pressure path in reservoir Pressure path in reservoir. .. 2 Separator Temperature Dry Gas • Dry gas - gas at surface is same as gas in reservoir • Wet gas - recombined surface gas and condensate represents gas in reservoir • Retrograde gas - recombined surface gas and condensate represents the gas in the reservoir but not the total reservoir fluid (retrograde condensate stays in reservoir) FIELD IDENTIFICATION Initial Producing Gas/Liquid Ratio, scf/STB Initial... the sum of the lithostatic and fluid pressures • Reservoir Pressures are normally controlled by the gradient in the aquifer • High pressures exist in some reservoirs The chart shows three possible temperature gradients The temperature can be determined if the depth is known High temperatures exist in some places Local knowledge is important • A reservoir normally contains either water or hydrocarbon... Liquid Dark Colored Lightly Colored Water White No Liquid *For Engineering Purposes LABORATORY ANALYSIS Phase Change in Reservoir Heptanes Plus, Mole Percent Oil Formation Volume Factor at Bubblepoint Black Volatile Retrograde Oil Oil Gas Bubblepoint Bubblepoint Dewpoint > 20% 20 to 12.5 < 12.5 Wet Gas No Phase Change < 4* < 2.0 > 2.0 - - *For Engineering Purposes Dry Gas No Phase Change < 0.8* - PRIMARY... density of the gas to that of air at the same temperature and pressure" • A fluid phase is a physically distinct state, e.g.: gas or oil • In a reservoir oil and gas exist together at equilibrium, depending on the pressure and temperature • The behaviour of a reservoir fluid is analyzed using the properties; Pressure, Temperature and Volume (PVT) • There are two simple ways of showing this: – Pressure... or hydrocarbon or a mixture • The hydrocarbon may be in the form of oil or gas • The specific hydrocarbon produced depends on the reservoir pressure and temperature • The formation water may be fresh or salty • The amount and type of fluid produced depends on the initial reservoir pressure, rock properties and the drive mechanism • Typical hydrocarbons have the following composition in Mol Fraction... have a low irreducible water saturation compared to small-grained formations because the capillary pressure is smaller • • • The phenomenon of capillary pressure gives rise to the transition zone in a reservoir between the water zone and the oil zone The rock can be thought of as a bundle of capillary tubes The length of the zone depends on the pore size and the density difference between the two fluids... Definitions Sw = water saturation So = oil saturation Sg = gas saturation Sh = hydrocarbon saturation = So + Sg • Saturations are expressed as percentages or fractions, e.g – Water saturation of 75% in a reservoir with porosity of 20% contains water equivalent to 15% of its volume • Amount of water per unit volume = φ Sw • Amount of hydrocarbon per unit volume = φ (1 - Sw) = φ Sh φ (1-Sw) Sw Hydrocarbon ...THE RESERVOIR PETROLEUM RESERVOIR • ROCK PROPERTIES • FLUID PROPERTIES • PRESSURE • RESERVOIR DRIVE ROCK PROPERTIES Rocks are described by three... Pressure, psia Pressure path in reservoir 10 20 Separator Separator Dew ne Temperature Temperature, °F Pressure path in reservoir Pressure path in reservoir Pressure path in reservoir Retrograde gas... as gas in reservoir • Wet gas - recombined surface gas and condensate represents gas in reservoir • Retrograde gas - recombined surface gas and condensate represents the gas in the reservoir