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1、4.1 Basic Properties of Reservoir Rocks and Fluids 4.2 Natural Drive Mechanisms 4.3 Improved Oil Recovery 4.4 Simulation Reservoir Management Lesson 44.1 Basic Properties of Reservoir Rocks and Fluids In order for a geological formation to form a commercial reservoir for hydrocarbons,the rock must e
2、xhibit two basic properties:-porosity and permeability.Porosity is the void space within the reservoir rock.which is filled with water and(hopefully)hydrocarbons.Permeability is the ability of the rock,a porous medium,to transmit fluids.地质组成地质组成4.1.1 Porosity Porosity is defined as the percentage or
3、 fraction of void space to bulk volume of rock.If the sedimentary particles of a rock were of uniform size and packing,as shown in figure 4.1,the calculation of porosity would be a simple exercise in solid geometry.Of course,actual reservoir rock is a much more complicated mixture of particles,and i
4、ts porosity must be measured directly from core samples or estimated by well log analysis.4.1.2 Permeability Permeability,like porosity,can also vary throughout the reservoir,depending on the type of formation and the method of its deposition.The reservoir engineer usually works with the geologist t
5、o define such permeability distributions before beginning major reservoir studies.It is also important to note that a permeability change can be imposed through drilling operations.The plugging of the pore spaces in the formation immediately adjacent to the wellbore can reduce permeability and creat
6、e a“damaged”zone.Just as the formation near a wellbore may be damaged and the permeability reduced,so too may be enhanced.The formation may be acidized and/or fractured thereby increasing the permeability near the wellbore and reversing the pressure-flow rate effects ofa damaged well Table 4.2 gives
7、 some examples of porosity and permeability values for productive fields around the world.4.1.2 Permeability 4.1.3 Reservoir Fluid Behavior The porous and permeable reservoir rock is normally saturated with at least two and sometimes three fluids:water,oil,and gas.The relative amounts of these fluid
8、s present,their composition.and physical of these fluids vary from field to field and often from reservoir to reservoir.It is even possible for the characteristics of a hydrocarbon to change with depth within a single reservoir The engineer must be concerned with the behavior of the oil or gas as it
9、 undergoes a constant temperature(isothermal),pressure reduction in the reservoir and a combined temperature and pressure reduction from the bottom of the well to the stock tank.A commonly referred-to property of natural gas is specific gravity,the ratio of the density of the gas to the density of d
10、ry air at standard conditions.Since natural gas is less dense than air,gas-specific gravity is normally 0.6 to 0.9 or so.Although the composition of reservoir gas is different for each reservoir(table 4.3),certain relationships may be applied to all gases.The volume of gas in a reservoir will expand
11、 as its pressure is reduced and contract as its temperature is reduced.4.1.3 Reservoir Fluid Behavior where =reservoir temperature(=+460)=reservoir pressure(psi)The API gravity normally used within the oil industry is related to specific gravity by the following:API=(141.5/S.G.at60.)-131.54.2 Natura
12、l Drive Mechanisms The production of oil and gas is possible only because of potential energy stored in the compressed fluids and rock of the reservoir or because of energy added to the reservoir.Reservoir energy is released when a pressure difference is imposed between the wellbore and the reservoi
13、r.While this pressure differential is maintained,fluids will flow from high to low-pressure.4.2 Natural Drive Mechanisms(1)Gasdissolvedinoil;(2)Free gas under pressure gas reservoiroil reservoir with free gas cap;(3)Fluid pressure hydrostatic-hydrodynamic compressedwater,gas,oil;(4)Elastically compr
14、essed rock;(5)Gravity;(6)Combinationsoftheabove.4.2.1 Solution-Gas Drive Natural gas dissolved in oil will come out of solution and form bubbles,which expand as the fluid pressure is reduced.This action,similar to that occurring in the uncorking of a champagne bottle,provides the driving force in a
15、solution-gas drive reservoir(also called dissolved gas drive,internal gas drive,and depletion drive).When production is first initiated,compressed oil expands in response to the pressure reduction at the wellbore.This continues until the bubble-point pressure is reached.At the bubble-point,gas 4.2.1
16、 Solution-Gas Drive bubbles begin to evolve from solution.With further pressure reduction,the expanding bubbles continue to support production.This occurs until they reach a critical saturation.the saturation where they join together and begin to flow as a single gas phase.Above this pressure,the ex
17、panding gas bubbles.tend to drive the oil out of the reservoir.Below this pressure,the gas phase,because gas has amuch lower viscosity tha;oil,begins to flow to.the wellbore much more rapidly than the oil.More and mere free gas is produced witR crude oil.4.2.1 Solution-Gas Drive This drives the pres
18、sure down more.rapidly and the finite energy source is rapidly depleted.Ultimately,the wells cease to flow.Depending on the geology and rock characteristics,the gas may alsomigi to the top Of the reservoir and form asecondary gas ca This gas cap expands as the pressure is reduced but doe not signifi
19、cantly add to the available energy.Some enei is also supplied by the expansion of connate water,gas dissolved in connate water,and the reservoir rock itself.4.2.1 Solution-Gas Drive Compressibilities of reservoir rock and fluids(106voltime/volume/psi)Formationrock 3-10Water 2-4Oil(above bubble point
20、)5-10GasatlOOOpsi(6894.8kPa)500-130Gas at 5000 psi(34,473.8 kPa)50-2004.2.2 Gas-Cap Drive As we can see,the reservoir pressure decline is not as steep as for a solution-gas drive reservoir.The peaks seen in the gas-oil ratio curve reflect the sequential shut-in or recompletion of wells as the expand
21、ing gas cap reaches their perforations.The oil rate will usually not decline as steeply as in the case of a solution-gas drive reservoir.Of course,the oil rate will be more highly dependent on the structural placement of wells than in the solution-gas drive case.Gas or water injection can be utilize
22、d to maintain pressure in a gas-cap drive溶解气驱气油比气顶气驱 reservoir,just as it is practiced in a sotution-gas drive reservoir.When an expanding gas cap is coupled with water influx from an aquifer or from injection,the combination of displacing fluids provides an efficient recovery mechanism.4.2.2 Gas-Ca
23、p Drive 溶解气驱气顶4.2.3 Gravity Drainage In addition to the reservoir drives mentioned,the force o,gravity will cause oil to move downdip relative to gas,and oil updip relative to water.Recovery efficiency is inreased if vertical permeability is great and withdrawal rates are low.Steeply dipping reservo
24、ir are classic examples of where gravity dramage provides an effective recovery mechanism.The East Texas field is a case in point.Usually,production is the result of some combination of all of the mechanisms mentioned.Often it is difficult to determine exactly how much each mechanism contributes to
25、the overall recovery.Table 4.7 shows some typical ranges for percentage recovery under different drive mechanisms.In general,water drive reservoirs have the best primary recoveries and solution-gas drive reservoirs the worst.Of course,the recovery in awater drive reservoir is greatly influenced by t
26、he positioning of wells and the sweep efficiency.4.2.3 Gravity Drainage 扫油效率4.2.4 Material Balance The reservoir can be thought of as a tank of pressurized fluids,in which reservoir pressure and producing characteristics change as fluids are produced.Observing these changes one can determine the typ
27、e(s)of drive mechanism(s),in effect,the original volumes available,and the expected recovery.The material balance equation is the foundation of the reservoir engineers analysis of a reservoir.Based on the law of conservation of mass converted to a volume relationship,it is simply expressed as:volume
28、 in-volume out=net change in volume This relationship can be restated in terms of reservoir quantities.For a given amount of production and the associated pressure change,the formula is as follows:Reservoir withdrawal=Expansion of oil and originally dissolved gas +Expansion of gas cap +Reduction in
29、hydrocarbon pore volume due to rock and water expansion +Water Influx4.2.4 Material Balance 4.2.4 Material Balance As a relationship between pressure drop and volume changes,the material balance equation is very valuable because it enables one to make an estimate of the original volume of hydrocarbo
30、ns basedonthe pressure-production performance.This estimate is relatively independent of the three-dimensional geological interpretation.Of course,in order to apply the equation to determine OOIP,we must have some production and pressure data.原始地质储量4.2.5 Gas Reservoirs If the reservoir hydrocarbon c
31、omposition is such that the accumulation exists as a gas at reservoir conditions,production of such a gas reservoir is usually a matter of simple gas expansion,sometimes assisted by water influx.Recovery is generally very good,particularly if the bottomhole flowing pressure can be reduced to a minim
32、um,by lowering the backpressure at the wellhead with compressors.In such cases,recovery can be as high as 85%to 90%.A gas condensate reservoir can be described as one that produces light colored,or colorless,liquids with gravities above 45.API,at GORs in the range of 5000 to 100,000 SCF/B(900 to 18,
33、000 m3/m3.)(Craft and Hawkins 1959).However,these parameters vary with producing conditions and measurement facilities,so the more technical definition described earlier and displayed in figure 4.9(b)should be used.4.2.5 Gas Reservoirs 4.2.5 Gas Reservoirs Oil and gas accumulations are complex hydro
34、carbon mixtures,the composition of which determines the specific behavior of the phases under changing conditions of pressure and temperature.The sources of energy that drive the production system include pressurized gas dissolved in the oil or free in the gas cap,compressed oil and water,compressed
35、 rock,active aquifer,and-gravity segregation.4.3 Improved Oil Recovery The use of reservoir energy to produce oil and gas generally results in arecovery of less than 50 of the original oil in place.Table 4.7 shows the median percentage recovery for each of the primary recovery mechanisms.These mecha
36、nisms,or a combination of one or more of these,and gravity drainage account for most of the worlds oil production.Secondary recovery techniques,in which external energy is added to a reservoir to improve the efficiency of the primary recovery mechanisms,have been in use for many years.The injection
37、of water to supplement natural water influx has become an economical and predictable recovery method and is applied worldwide.Less commonly,gas injection has been used to displace oil downdip in“attic”oil recovery projects or to maintain gas cap pressure.Still,both primary and secondary recovery tec
38、hniques have only been effective in producing roughly one third of the oil discovered.The remaining two thirds,more than 300 billion barrels(4.7696 x W10 m3.)in the United States alone,is a target for more sophisticated processes.Such processes,developed to increase recovery from reservoirs consider
39、ed depleted by primary mechanisms and secondary methods of water or gas injection.were historically termed tertiary recovery techniques.However,because some of these processes may be applied earlier in the life of a reservoir,perhaps even in the first day of production,the“tertiary”term is no longer
40、,appropriate here,and as a result,the term enhanced oil recovery methods has been introduced as the term to be used for all processes that attempt to alter the physical forces that control the movement of oil within the reservoir.4.3.1 Waterflooding and Recovery Efficiency All improved rec.,methods
41、involve the injection of fluids into the reservoir via one or more wells,and the production of oil(and perhaps ultimately the injected fluid)from one or more other wells.The methods differ in the nature of the fluids used and the physical changes they bring about in the reservoir,but water usually p
42、lays a part inhelping to displace the oil.The amount of oil re covered(and obviously the success of the project)is dependent upon the percentage of oil in place that is con tacted and moved by the displacing fluid.This concept is represented by the equation:where:The oil recovered(Np)is the product
43、of the volume of the oil in place(N),the fraction that is contacted(Ey),and the fraction of the oil contacted that is displaced(ED).The volumetric sweep efficiency(Ey),given as a fraction,is the product of the areal sweep efficiency(EAs),and the vertical sweep efficiency(Evs).Usually all of these va
44、lues(except N)increase during the life of an improved recovery project,until an economic limit is reached.Enhanced recovery methods differ in the manner in which they attempt to improve either Ev or ED.Volumetric Sweep Efficiency:The volumetric sweep efficiency(Ey),at a given point in time,is the fr
45、action of the total reservoir volume contacted by the injected fluid during an improved recovery project.It is the corn posite of the areal sweep efficiency(EAS),and the vertical sweep efficiency(Evs).4.3.2 Waterflooding and Recovery Efficiency reservoir permeability information is available in term
46、s of relative-permeability,the mobility is expressed as:The mobility ratio is defined as the mobility of the displacing phase in the portion of the reservoir contacted by the injected fluid,divided by the mobility of the displaced case of water displacing oil(waterflooding):If M is less than or equa
47、l to one,it means that the oil is capable of traveling at the same or greater velocity than the water,under the same conditions.The water,therefore,will not bypass the oil and will instead push it ahead.lfAfis greater than one,the water is capable of moving faster than the oil and will bypass some o
48、f the oil,leaving unswept areas behind.An increase in the viscosity of the oil will cause the mobility ratio to increase.This is logical,as one can imagine attempting to push a viscous,heavy oil through a pore system and having the less viscous water“finger”through or around the slow moving oil.An o
49、bvious approach to improving the mobility ratio would be to decrease the difference in oil and water viscosities,by increasing the water viscosity and/or decreasing the oil viscosity.The areal sweep of water through an oil reservoir depends upon where the water is injected relative to where the oil
50、is produced.Laboratory models have enabled researchers to measure the areal sweep efficiencies for different mobility ratio flood pattern combinations.For example,if wells are spaced in a five-spot pattern and are producing from a homogeneous uniform reservoir,the areal sweep efficiency at the point