Petroleum and Reservoirs. Origin, Migration, and Accumulation of Petroleum

Transcription

1 Petroleum and Reservoirs Origin, Migration, and Accumulation of Petroleum This is one of the more problematical subjects in the discussion of petroleum geology since there is only limited agreement about how petroleum forms, how it migrates, and how it accumulates. Since there is oil structures that include reservoirs far away from what appear to be potential source beds, the fact that oil does form, migrate, and accumulate is indeed a reality. Theories concerning oil and formation (cata genesis) involve organic and inorganic considerations. Present evidence brings most weight to bear on the side of the organic origin of petroleum because hydrocarbon compounds are present in organic material derived from plant animal life. For us the origin of petroleum will be considered to be from organic sources. Source rock analysts and geochemists are not in complete agreement on the types of organic material. Some suggest that only plant material involved. Others conclude that animal and plant material contribute to petroleum generation. Additional problems concern theories of how petroleum migrates, the driving mechanisms of migration, and the distances that it migrates. Accumulations of petroleum occur in places from which migrating fluids cannot escape. ow migrating petroleum responds to changes in reservoir and trap considerations in the site of accumulation is difficult to assess. The time required for accumulation is propably variable to perclude establishment of definite parameters.

2 Origion of petroleum: Organic origion of petroleum indicates petroleum is the product of altered organic material derived from microscopic plant and animal life. Microscopic plants and animals are carried in great volume by streams and rivers to lakes or the sea, where they are deposited under deltaic, lacustrine and marine conditions with finely divided clastic sediments. These environments produce their own microscopic plants and animals, which are deposited with organic materials introduced by the streams and rivers. As deposition of the organic material takes in the marine, deltaic and lacustrine environments, burial and protection by clay and silt accompany it. This prevents decomposition of the organic material and allows it to accumulate. The amount of burial is a function of how much sediment is discharged by streams and rivers into lakes and the sea as well as the amount of time involved in the depositional process. Thick accumulations of silt, clay, and organic material can produce large volumes of petroleum if there is enough time for the alteration process to occur. Some research indicates that terrestrial organic material generates coal and gas, and marineorganic material forms oil. onversion of the organic material is called catagenesis. It is assisted by pressure caused by burial, temperature and thermal alteration and degeradation, these factors result from depth, some bacterial action in a closed non oxidizing chemical system, radio activity and catalysis.

3 1- Normal heat flow within the earths crust produces an average geothermal gradient of approximately 1.5 F o for each 100 feet of depth. Maturation studies on various crude oils types indicate that temperature required to produce oil occur between the approximates of 5,000 and 20,000 feet. Temperature below 20,000 feet are excessive for oil generation and produce gas. Above 5000 feet is too cool to generate oil or gas. 2- Like temperature, pressure is a function of depth and increases 1 psi for one feet of depth. Pressure caused by sedimentary overburden. 3- Bacterial action is important in conversion of organic material to petroleum in shallow depths. auses of breaking down the original material into hydrocarbon compounds which become biogenic gas. 4- The best source rocks are considered to be organically rich, black-colored shale deposited in a non oxidizing, marine environment. 5- Most petroleum is generated and occurs in sedimentary rocks. Some petroleum occurances in iguous and metamorphic rocks may have inorganic or altered organic origions. Migration:

4 The concept of petroleum migration remains a reality not with standing limited agreement about mechanism and distances. When petroleum moves from source beds to reservoir rocks, it does so by primary migration. If it moves within the reservoir after it has accumulated, it does so by secondary migration. ow far petroleum migrates is not kown. It is probable that some small amounts of petroleum generate in reservoir rocks in which they accumulate. owever, the bulk of it comes from source beds external to the reservoir. Accumulation: Petroleum accumulates and is stratified according to its fluid phases and the amounts of formation water. Gas is lightest and accumulates above oil, which overlies water. The quality of gas dissolved in oil depends on pressure, temperature, and hydrocarbon characteristics. Petroleum accumulates in the highest permeable portions of the reservoir because of hydrodynamics, this is why the highest area of an anticline is usually the best place to drill an exploratory well. Petroleum accumulations probably require long period of time to form, particularly in reservoirs of low permeability. Mobility of fluids within a reservoir is enhanced by increasing permeability. (1)The Nature of petroleum

5 There is probably no other technical field in which so many major questions remain unanswered and yet which functions as efficiently as the oil industry. For example, geologists have yet to agree completely on the origion and accumulation of petroleum, geophysicists have no tool which searchs directly for oil, petroleum engineers are still leaving unrecoverable oil in the ground, and chemists and chemical engineers must still evaluate crude oil on the basis of empirical tests rather than by precise analyses. In short, we don t know what it is, how it origionates and accumulates, how to find it, or how to get it all out of the ground hemical composition Petroleum may be defined as a naturally occurring, complex mixture of hydrocarbons which may be either gas, liquid, or solid, depending upon it is own unique composition and the pressure and temperature at which it is confined. The principal hydrocarbon series found in petroleum are 1. Paraffins(also called saturated hydrocarbons or alkanes) which have the general formula n 2n+2.These compounds are chemically stable and have either straight or branched chains. The branched chain members are called isomersand exibit somewhat different properties than their straight chain counterparts, all crude oils contain some paraffins particularly as the more volatile (low boiling point) constituents. The first few members of this series are: Abbreviation Formula hemical structure Name 1 4 Methane

6 2 2 6 Ethane Propane Normal Butan i Iso-Butan Note that iso-butan differs from normal by the manner in which the carbon atoms are arranged. This is the only isomer possible with 4 ; the possible atomic

7 combinations increase, however, with the length of the chain, as illustrated in table below POSSIBLE ISOMERS OF VARIOUS PARAFFINS arbon content Number of isomers , ,491,178,805,831 etc. 2. ycloparaffins (naphthenes) having the general formula n 2n. These compounds have a ring structure, the simplest member being cyclopropane. Typical members of this group are: 3 6 yclopropane 4 8 yclobutane

8 3. Aromatics (benzene series) having the general formula n 2n-6. These compounds are chemically active and contain the benzene ring. The simplest member is 6 6 Benzene In addition to hydrocarbons, petroleum may contain numerous impurities such as carbon dioxide (O 2 ), hydrogen sulfide ( 2 S) and other complex of nitrogen, sulfur and oxygen. Any particular crude oil may contain a portion of each of the listed hydrocarbon series plus numerous impurities, therefore is unique, precise chemical analysis is impossible. Petroleum is often classified by a base designation- as either a paraffin base, asphalt base, or mixed base crude. A paraffin base crude is an oil whose chief components are paraffins, when completely distilled leaves a solid residue of wax. An asphalt base crude is an oil composed of cyclic compounds (mostly naphtheness), when distilled leaves a solid residue of asphalt. Oils fall in the middle of these categories classified as mixed base. rude petroleum yields a large number of products the simplest refining process is fractional distillation whereby the constituents in the oil are separated by utilizing their differences in boiling points. A few of these products are shown in figure below.

9 1.2. Properties of liquid petroleum: The most widely used indicator of a crude oil's worth to the producer is its API (American petroleum institute) gravity. This value is actually a measure of an oil's density, and related to specific gravity by the following formula. API Gravity (degrees) = Note that an API gravity of is equivalent to a specific gravity of 1. The price which a producer receives for his oil depends on its gravity, the less dense oils (higher API gravity) being the most valuable. The price schedule is based on the premise that the lighter oils contain higher percentage of the more valuable products such as gasoline. It is possible that a particular oil may be more valuable than some oil, due to high yield of desirable products. The refiners feel that these differences average out, probably that crude oil will always be sold on a gravity basis. The surface or tank oil as by a producer is not the same liquid which existed underground. The differences between tank oil and reservoir oil are very important. A reservoir oil always contains in solution components which would be gases at standard temperature and pressure. Their solubility is due to elevated pressure and temperature existing underground. As oil is produced (brought form underground to the surface), the pressure is decreased until it reaches atmospheric conditions in stock tanks. This pressure reduction causer changes in reservoir fluid properties: a) some of the volatile fraction vaporize, causing

10 b) the liquid volume shrink, and c) the liquid viscosity to increase. These effects are shown in figures 1.2 and 1.3 also a number of fundamental ideas must defined 1- Bubble point pressure: The pressure shown as P b is the bubble point or saturation pressure. It is the pressure at which the first gas is liberated from the reservoir oil upon isothermal pressure reduction at reservoir temperature. 2- Formation volume factor: This the quantity denoted by B o the reservoir volume occupied per volume of tank oil (oil reduced to standard conditions 14.7 psia and F) and its dissolved gas. This quantity is always greater than 1.0 because of (a) thermal expansion [shown as V T in figure 1.2 (D) and (b) swelling asgas is dissolved at higher pressures [ shown by increasing values of V ro in figure 1.2 (A),(B),(),(D). Note that Bo increases as the pressure is decreased from P i to P b due to liquids expansion. 3- The solution gas-oil ratio. Denoted R s, is the number of standard cubic feet of gas dissolved per barrel of tank oil. 4- The oil viscosity ( µ o ) behavior shown in figure 1.3() is typical and is found by step wise determination in a high pressure viscosimeter. This behavior explained as follows. (a) Viscosity decreases as pressure is reduce from P i to P b due to liquids expansion; greater intermolecular freedom of motion is possible, and internal friction is reduced. (b) Viscosity increases with pressure reduction below P b because the low viscosity fractions are lost.

11 The process explained in figure 1.2 is defined as flash vaporization, since the composition of the system remained. If the cell pressure had been reduced by removing liberated gas while holding volume constant, the process is differential vaporization. The actual process taking place in underground petroleum reservoir nearly fits the differential vaporization. It is commonly used in laboratory analyses. The above fluid properties are called PVT (pressure - volume - temperature) and are very important in solution of many petroleum engineering problems. These values are normally obtained from analyses of subsurface or recombined surface fluid samples. There are correlations in literature enable to obtain these factors from available field data Gaseous petroleum (Natural Gas): Recently natural gas has come into its own as a highly valuable product. Gas produced with oil was sold primarily on a local scale and any excess was flared. As the natural gasoline and liquefied petroleum gas (butane and propane) industry developed, the use of the residue gas (dry gas remaining after liquid removed) also

12 increased. Today natural gas and its associated liquid products are virtually as much in demand as oil. Natural gas is produced from three classes of wells: (1) From wells where the dominate products oil (oil wells). (2) From wells where the gas itself is the principal product (gas wells). (3) As gas from condensate wells. ondensate wells produce from reservoir in which the hydrocarbons (gas and liquid) originally existed as a single fluid (or phase), the reservoir temperature and pressure being above the critical point of the hydrocarbon mixture. Each natural gas, like each crude oil, is a unique mixture of hydrocarbons. All gases composed mostly of light members of the paraffin series and mostly methane. Numerous impurities found in petroleum gases, mostly carbon dioxide (o 2 ), hydrogen sulfide ( 2 S), water vapor, nitrogen, and helium. These impurities decrease the value of natural gas by raising the costs of processing it to pipe line and consumer standards. There are a number of basic definitions which presented here below. 1- Wet gas: A natural gas is said to be wet if it contains an appreciable natural gasoline content. 2- GPM: The natural gasoline content of a gas expressed in gallons per thousand standard cubic feet (MF). Gases having a GPM of 1 to 2 are wet, while gas with a GPM of 0.2 is considered somewhat dry. 3- Sour gas: Natural gas containing hydrogen sulfide. 4- Sweet gas: Natural gas containing no hydrogen sulfide.

13 5- Gas gravity: The ratio of the density of a gas to the density of air at standard conditions (a specific gravity scale based on air). 6- Standard conditions: 14.7 psia and F The gas law: The gas law as applied to the behavior of natural gas is most commonly stated as PV = z n R T Where P = pressure, absolute V = volume N = number of moles R = gas constant T = absolute temperature

14 Z = deviation (or compressibility) factor to account for the difference between actual and ideal gas volumes. The value of R is dependent on the system of units used as given in table below: Table Values of the gas constant R for different values of P,V, and T P V T R Atmospheres c-c 82.1 Atmospheres Liters mm mercury cc gm per sq. cm cc Ib per sq. in cuft 10.7 Ib per sq. f cuft 1545 Atmospheres cuft 0.73 Equation (1.1) may be rewritten: (1.2) Where W/M = n W = total wt. of gas M = molecular wt. of gas ence

15 (1.3) Or. (1.3a) Where = specific volume of the gas Also,.. (1.4) Where = gas density Another useful expression relating the PVT behavior of a constant number of mols of gas is (1.5) Determination of Z: The values of Z for natural gas mixture have been experimentally correlated as functions of pressure, temperature, and composition. The ratio of the volume of a particular substance to its volume at its critical point is the same for all substances at the same ratio of absolute pressure to critical pressure, and absolute temperature to critical temperature. This theorem may be applied to compounds of similar molecular structure such as the light paraffins and natural gases.

16 In preparing a correlation for hydrocarbon mixtures, the ratios of actual pressure and temperature to the molal average critical or pseudo-critical pressure and temperature used. These ratios called reduced pressure and reduced temperature. Figure 1.4 is a correlation of z as a function of these quantities. For determination of the pseudo-critical properties a correlation of as function of gas gravity have been found accurate.figure 1.5 shows this correlation. (2) oncepts of petroleum Geology and Basic rock properties 2.1. Requirements for commercial oil accumulations: ertain requirements must be fulfilled for a commercial petroleum deposit to be present. These are 1. A source: material from which oil is formed.

17 2. Porous and permeable beds (reservoir rocks) in which the petroleum may migrate and accumulate after being formed. 3. A trap: subsurface condition restricting further movement of oil such that it may accumulate in commercial quantities Source of petroleum: Many theories on the origion of petroleum have been proposed and classified into two groups: 1. Inorganic theories. These are of historical interest only. 2. Organic theories. At present most authorities favor organic approach, principle reasons follows below: 1) No inorganic theory can account for necessary quantities of carbon and hydrogen needed to form large petroleum deposits. 2) Many crude oils contains porphyrins and contain nitrogen. Presence of these materials suggests organic origion and these are present in all organic matter. 3) Petroleum rotates the plane of polarized light. This property restricted to organic materials known as optical isomers this conclude organic source of petroleum. Studies of thousands oil fields has led most geologists to the following conclusions: 1- Petroleum origin from organic material mostly vegetable altered by heat, bacterial action, pressure, and other agents over long periods of time. 2- onditions favoring petroleum formation found only in sedimentary rocks.

18 3- The principle sediments considered probable source rocks are shale's and limestones that were originally muds under saline water Porous and permeable beds (Reservoir Rocks): After petroleum formation it migrates from source rock into porous permeable beds were it accumulates and continues its migration until finally trapped. Forces causing this migration are: 1. ompaction of sediments as depth of burial increases. 2. Diastrophism: crustal movement causing pressure differentials and subsurface fluid movements. 3. apillary forces causing oil to be expelled from fine pores by preferential entry of water. 4. Gravity causing fluid segregation due to density differences. The term porous and permeable denote two distinct rock properties whose measures and quantitative definitions have comprised most technical literature of the oil industry. Porosity: Porosity is a measure of the void space within a rock expressed as a fraction (or percentage) of the bulk volume of that rock. Where = porosity

19 = bulk volume of the rock. = net volume occupied by solids (grain volume). = pore volume = the difference between bulk and solid volumes. In actual rocks porosity is classified as A) Absolute porosity: total porosity of a rock, regardless of whether or not individual voids are connected, and B) Effective porosity: only that porosity due to voids which are connected. It is the effective porosity which is of interest in the oil industry. Geologically, porosity is classified in two types, according to the time of formation. 1- Primary porosity (inter granular): porosity formed at the time the sediment was deposited. The voids contributing to this type are the spaces between individual grains of sediment. 2- Secondary porosity: voids formed after the sediment was deposited. Primary porosity: The sedimentary rocks which exhibit primary porosity are the clastic (fragmental or detrital) rocks which are composed of erosional fragments from older beds. These are classified by grain size. Typical clastic rocks which are common reservoir rocks are sandstones, conglomerates, and ooliticlimestones. Secondary porosity:

20 Porosity of this type subdivided into three classes based on the mechanism of formation. (1) Solution porosity: Voids formed by the solution of the more Soluble portions of the rock in percolating surface and subsurface waters containing carbonic and other organic acids. This is also called vagular porosity and individual holes are called vugs. Voids of this origion ranges from small vags to cavernous openings. (2) Fractures, fissures and joints: These voids are common in many sedimentary rocks and they are formed by structural failure of rocks under loads caused by folding and faulting. This type of porosity is very difficult to be quantitatively evaluated due to irregularity. (3) Dolomitization: This process is transformation of limestone (ao 3 ) to dolomite amg(o 3 ) 2 2ao 3 + Mgl 2 amg(o 3 ) 2 + al 2 It is should be noted that primary and secondary porosity can occur in the same reservoir rock. Quantitative use of porosity data: As defined previously, is a measure of the void space within a rock, and may be used to determine the quantity of fluidstored in that rock. onsider a bulk volume of rock with a surface area of one acre and a thickness of one foot. This constitutes the basic rock volume measurement used in oil field calculations, an acre foot. All liquid volume expressed in barrels. 1 acre = 34,560 ft 2 1 acre ft = 43,560 ft 3 1 bb = 42 gal = 5.61 cuft 1 acre ft = = 7758 bb1

21 It is obvious that pore space of a rock is equal to 7758 x = (bb1/acre ft) where is rock porosity. See fig. 2.5 which shows that Volumetric Equation of oil in place: = 2.2 Where N = tank oil in place, bbl / acre ft So = fraction of pore space occupied by oil (oil saturation) Sw = the water saturation Bo = the formation volume factor for oil at the reservoir pressure, barrels reservoir space, barrel tank oil. Determining proper values of Sw in Equation 2.2 is more difficult than obtaining. Some water will always exist in reservoir rock and its volume must be subtracted from squce available for oil. This water is called connate water. Note that the pore space is assumed to be occupied either oil or water, and that no free gas is present. Equation above must be applied to the reservoir at or above bubble point and used to compute the initial oil in place. A similar expression may be derived for the amount of gas stored in a particular sand. Gas volume will be expressed in terms of SF or in MF (thousands of standard cubic feet). From gas law.. (2.3) Where subscript, s, denotes standard conditions, (not shown), then (2.4)

22 Where G = standard gas volume contained in at conditions P,T,z. But: = 3,560 (1 Sw) cuft/ acre ft = = R = 14.7 pasia Substitution of these values in Eq.2.4 gives G = 43,560 (1 Sw) X.. (2.5) Or G = MF/Acre ft (2.5a) Permeability: Darcy's Equations In addition to being porous, a reservoir rock must be permeable; that is, it must allow fluids to flow through its pore network at practical rate under reasonable pressure differentials. Permeability is defined as a measure of a rocks ability to transmit fluid. Permeability was first given in an empirical relationship developed by the French hydrologist enry D'Arcy who studied the flow of water through unconsolidated sounds. This law in its differential form is:.. (2.6)

23 Where v = apparent flaw velocity M = viscosity of the flowing fluid dp/dl = pressure gradient in the direction of flow k = permeability of the porous media q 1 = q 2 P 1 dp P 2 A q 1 q 2 dl onsider the liner system of figure above. The following assumptions are necessary to the development of the basic flow equations: 1- Steady state flow conditions exist. 2- The pore space of the rock is 100% saturated with the flowing fluid. Under this restriction k is absolute permeability.

24 3- The viscosity of the flowing fluid is constant. For all real fluids. owever, this effect is negligible if M at the average pressure is used, and if conditions 4-6 hold. 4- Iso thermal conditions prevail. 5- Flow is horizontal and liner. 6- Flow is laminar. With these conditions, let.. (2-7) Where q = volumetric rate of fluid flow A = total cross sectional area perpendicular to flow direction. This is further assumption since only the pores, and not the full area, conduct fluid. ence, v = an apparent velocity. The actual velocity, assuming uniform medium =. ase 1: Liner Incompressible Fluid Flow Substitution of (2.7) in (2.6) gives.. (2.8) Separation of variable and insertion of the limits shown in figure above

25 . (2-9) By integration. (2.10) Or Note that the negative sign used to denote a negative pressure gradient in the direction of integration was removed by reversing the pressure limits equation (2.10) is basic and following units server to define the Darcy. If q = 1 cc/sec A = 1 M = 1 centipoise Atmosphere/cm Then K = 1 Darcy which is much higher than that commonly found in reservoir rocks. Therefore common unit is millidarcy, where 1 Darcy = 1000 millidarcy ase II, Linear compressible fluid flow: onsider the same linear system of figure above, except that flowing fluid is compressible; then constant, but ois a f(p)

26 Assuming that Boyles law is valid (z=1) Then = constant. (2-11) (2-12). (2-13) From which (2-14) Expressing Eq (2-14) in terms of the system is the rate of gas flow at the average pressure in.. (2-15) But Therefore. (2-16) Which is the same as Eq. (2-10) An expression for the standard flow rate,, is from charle's law:. (2-17)

27 Where = (520 ) = 1 atm = flowing temperature Thus =. (2-18) Note: for standard units, = 1 atm and is often omitted from eq (2-18). The use of liner equations is limited to laboratory testing. Radial flow (linear) equations are used in fields.