MB for Oil Reservoirs

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1 Koya University Faculty of Engineering Petroleum Engineering Department MB for Oil Reservoirs Lecture 8 Prepared by: Haval Hawez haval.hawez@koyauniversity.org 1

2 Reservoir Engineering Tasks Be able to make dependable estimates of initial hydrocarbons in place. Predict the future reservoir performance. Ultimate hydrocarbon recovery. MB for Oil Reservoirs 2

3 Material Balance Equation Basic tool in reservoir engineering. Many reservoir engineering techniques involve some application of the material balance. Principle of conservation of mass underlies the MB equation. It is however written on a volumetric basis. Mass of fluids originally in place = fluids produced + remaining reserves MB for Oil Reservoirs 3

4 Material Balance Equation A general MBE can be applied to all reservoir types. First presented by Schilthuis in Relates volumes to pressures. Limited in application since no time dependent terms. Provides relationship with reservoir cumulative production cell and its average pressure. MB for Oil Reservoirs 4

5 Material Balance Equation Scope of the analysis depends on the Reservoir simulators apply material balance approach within each cell MB equation enables one to get a feel of the reservoir and the contribution of various processes.. MB for Oil Reservoirs 5

6 Material Balance Equation Basic material balance equation The reservoir volume of original fluids in place= reservoir volume of fluids produced + volume of remaining reserves MB for Oil Reservoirs 6

7 Material Balance Equation As a consequence of pressure depletion in a reservoir a number of things will happen. The pore volume of reservoir will decrease Connate water will expand Undersaturated oil will expand Saturated oil will shrink as gas comes out of solution. Free gas will expand. Water may start flowing into reservoir. MB for Oil Reservoirs 7

8 MB for Oil Reservoirs above Pb: Np Pi P Above the bubble point, the undersaturated condition, production is due to expansion of liquids, oil and water and reduction in pore volume. Assuming oil production only due to oil expansion. Then: MB for Oil Reservoirs 8

9 MB for Oil Reservoirs below Pb: Below bubble point gas liberated in the reservoir. The mechanism of Solution Gas Drive. Produced fluids: oil plus its dissolved gas, gas which has come out of solution in reservoir and produced and free gas which has come out of solution in reservoir and remains here MB for Oil Reservoirs 9

10 MB for Oil Reservoirs below Pb: Free gas in reservoir = original gas in solution remaining gas in solution produced gas Gps Original oil volume = remaining oil volume + volume of free gas MB for Oil Reservoirs 10

11 MB for Oil Reservoirs below Pb: Equation in terms of original stock-tank volume in reservoir Np Gps Pi P MB for Oil Reservoirs 11

12 MB with gas cap and water drive: So far no volume change in reservoir considered. If gas cap expands or water encroaches there will be a loss to reservoir volume Change in volume due to gas cap expansion: Change in volume due to water encroachment: Total change in volume = original oil volume (remaining oil volume + free solution gas) MB for Oil Reservoirs 12

13 MB with gas cap and water drive: Np Gpc Gps Wp Pi P We MB for Oil Reservoirs 13

14 MB with gas cap and water drive: Np Gpc Gps Wp Pi P We MB for Oil Reservoirs 14

15 MB with gas cap and water drive: Np Gpc Gps Wp Pi P We MB for Oil Reservoirs 15

16 Effect of Pore Volume Changes: Water band rock pore compressibility although low can contribute to overall pore volume changes. Impact of pore volume changes due to rock. As pressure falls bulk volume reduces ( increased net overburden stress) and increase in volume of grains. Net effect reduction in porosity. Compressibility of rock c f. MB for Oil Reservoirs 16

17 Effect of Pore Volume Changes: Impact of pore volume changes due to connate water. Expansion of water can contribute to reduction in pore volume for hydrocarbons. Compressibility of water: Total Pore Volume Change due to rock & water: MB for Oil Reservoirs 17

18 Effect of Pore Volume Changes: This term can be added to MB equation and expressed in terms of oil (and gas) in place. If we neglect a gas cap then the pore volume = Compressibility of water and rock If we also include gas cap then m is ratio of gas to oil in place If free gas present ten errors in gas compressibility effects greater than absolute pore compressibility effects, so M ignored. MB for Oil Reservoirs 18

19 General Material Balance Equation Net water influx + gas cap expansion + pore volume reduction = Original oil volume volume of remaining oil and free solution gas. MB for Oil Reservoirs 19

20 General Material Balance Equation Figure. Elements of the MB Equation MB for Oil Reservoirs 20

21 Other forms of the MB Equation: Equation sometimes presented using total formation volume factor. Bt. Using m, where Using Gp where MB for Oil Reservoirs 21

22 Modifications to the General MB Eqn.: All of the parameters not significant over the life of a reservoir. Above bubble point some terms go to zero. Above Pb, Rs is constant. Gp Np Rs =0. only solution gas produced. Above Pb no gas cap, G or m = 0. Below Pb, gas related terms have significance. Some consider pore & water compressibility terms can be neglected when compared to the errors associated with the free gas terms. As well as water influx, We, the equation can be used for artificial drive, e.g. gas injection, Gi and water injection, Wi. MB for Oil Reservoirs 22

23 Alternative method for d:eriving MB eq. According to Dake, Underground withdrawal= Expansion of the system+ Cumulative water influx Reservoir volume at pressure P Of the produced fluids = expansion of primary gas cap + expansion of oil plus originally dissolved gas + expansion of connate water + water influx + reduction of total pore volume MB for Oil Reservoirs 23

24 Assumptions in MB Equation: Pressure the MBE equation is tank model. Pressure constant throughout the reservoir at any time. An average pressure has to be selected to be represent fluid properties. Temperature changes in a reservoir take place at constant temperature, isothermal. Production rate time has no part within MBE. Representative PVT data PVT measurements should be made or calculated to reflect behavior in the reservoir. Good production data essential. MB for Oil Reservoirs 24

25 Significance and use of MBE: MBE is a relation between: Oil & gas in place, N & G Production, Np, Gp, & Wp Water influx, We Average reservoir pressure, PVT parameters and in compressibility terms If three of these are known the fourth can be calculated. If production and pressure data available and oil & gas in place known, then water influx can be determined. If no water drive then can history match reserves. For a known oil in place, the pressure at future dates can be determined for a proposed production plan. MB for Oil Reservoirs 25

26 Significance and use of MBE: Should be known Np Rp Wp Cw Swc Bw Potential unknown N We P Bo, Bg, Rs m Cf 6 known and 8 unknowns need more independent equations. MB for Oil Reservoirs 26

27 Significance and use of MBE: 6 known and 8 unknowns need more independent equations. In reservoir simulation more unknowns are reservoir description, porosity, relative permeabilities etc. Np & Rp generally best known except when good productions records not available.. Petrophysical data is generally good. MB for Oil Reservoirs 27

28 Significance and use of MBE: Unknowns Once production starts MB provides useful route to upgrade STOIIP estimate, N. MB provides opportunity to determine water drive, We. Size of gas cap if not drilled may be difficult to determine. Important to determine rock & water compressibility. MB zero dimensional. Requires average pressure. Can be obtained from range of pressures from wells in drainage area. MB for Oil Reservoirs 28

29 Sources of Data for use in MBE: PVT data From PVT records Production data Well and reservoir records Oil & Gas in place From volumetric estimates Connate Water Saturation From petrophysics Water Compressibility Should be measured Pore Compressibility Should be measured Reservoir Pressures From pressure surveys Water Influx Calculation or history match MB for Oil Reservoirs 29

30 Limitations of MBE: Zero dimensional fluid properties averaged over entire reservoir. Saturations distributions can not be determined No time parameter. It will calculate what will happen but not when. MB for Oil Reservoirs 30

Primary Recovery Mechanisms

Primary Recovery Mechanisms The recovery of oil by any of the natural drive mechanisms is called primary recovery. The term refers to the production of hydrocarbons from a reservoir without the use of