Material Balance Method

Transcription

1 Material alance Method Dr. Mahbubur Rahman Deartment of Petroleum & Mineral Resources Enineerin anladesh University of Enineerin & Technoloy Short Course on Petroleum Reserve Estimation, Production and Production Sharin Contract PSC) Directorate of Continuin Education DCE) UET Aril, 2008

2 Presentation Outline Introduction Material alance Method- asic Princile Material alance Equation ME) General Form Linear Form Alications Oil Reservoirs Gas Reservoirs Flowin Material alance Drive Mechanisms Conditions for Alication Conclusions

3 Introduction Reserve Estimation Methods: More than one available. Different methods alicable at different staes of develoment. Data requirement different for each method, with some common redominant methods: 1.Volumetric Method Early stae of reservoir develoment Geoloy, Geohysics, Rock and Fluid roerties Recovery Factor RF) assined arbitrarily No time deendency, No Production data 2.Material alance Later stae of develoment after 20% of initial oil/as is roduced, or 10% of initial reservoir ressure has declined) Geoloical data, Rock and Fluid roerties, Production data RF is calculated Time deendant

4 Introduction contd.) 3.Decline Curve Analysis Later stae of develoment, when roduction rate underoes natural decline Mostly Production data RF is calculated Time deendant 4.Reservoir Simulation Can be alied at any stae but more useful and reliable for matured reservoirs Geoloical data, Rock and Fluid roerties, Production data More useful as reservoir manaement tool Uncertainties associated with each method More than one method should be used when alicable

5 Material alance Method- asic Princile A = Increase in HCPV due to the exansion of the oil hase oil dissolved as). = Increase in HCPV due to the exansion of the as hase free as in the as ca). C = decrease in HCPV due to the combined effects of the exansion of the connate water and the reduction in reservoir ore volume. D = decrease in HCPV due to water encroachment from aquifer) Volume chanes in the reservoir associated with a finite ressure dro ;a) volumes at initial ressure i b) at the reduced ressure. Underround withdrawal oil as water) = Exansion of oil dissolved as A) Exansion of as-ca as ) Reduction in HCPV C) Cumulative water influx D) 1)

6 Material alance Equation ME) [ ] N [ o R R s ) ] W w = N ) R R ) o oi si m N oi / i 1) 1 m) N ) c S c ) oi S w wc f 1 wc W e w 15) s N G We HCPV m = oil oriinally in lace STOIIP), stb) = Initial free as in lace in the as ca GIIP), scf) = Cumulative water influx into the reservoir stb) = total hydrocarbon ore volume oil zone as ca) rb) = Initial as ca ratio Initial hydrocarbon volume of the as ca m = = Initial hydrocarbon volume of the oil G N i oi

7 ME- Definitions of Variables Production data N = Cumulative oil roduced stb) G = cumulative as roduced scf) W = Cumulative water roduced stb) R = G/N = Cumulative roduced as-oil ratio scf/stb) Reservoir Data i = Initial mean ressure in the reservoir si) = current mean ressure in the reservoir, si) Swc = connate water saturation, fraction) cf = Comressibility of formation si-1) Fluid PVT Data i = Initial as volume factor at i ft3/scf) = Gas volume factor at current ressure ft3/scf) oi = Initial oil volume factor at i rb/stb) o = Oil volume factor at current ressure rb/stb) cw = Comressibility of water si-1) w = Formation volume factor of water at current ressure rb/stb) Rsi = solution as-oil ratio at initial ressure i scf/stb) Rs = solution as-oil ratio at current ressure scf/stb)

8 ME in Linear Form F = summation of roduction terms E o = Oil and Dissolved as exansion terms E = Gas ca exansion term E f,w = rock and water comression/exansion terms N [o R Rs) ] W w [ ) R R ) ] o oi si s = oi / i 1) rb/stb) 1 m ) ) ) oi S c S c w wc f 1 wc rb) The comlete material balance equation ME) F= N Eo m E Ef,w) We w 16) Equation 16 can be modified as equations of straiht lines, which can be alied to different tyes of reservoirs. Some of the alications are illustrated next.

9 ME Alications: Saturated Oil Reservoirs F Sloe = N E o Case 1 ME for under saturated volumetric oil reservoirs Volumetric deletion, W e = 0 No Gas Ca, m = 0 E fw = neliible F = N Eo Case 2 ME for under saturated oil reservoirs with stron water drive Stron water drive, W e 0 No Gas Ca, m = 0 E fw = neliible F/Eo = N We / Eo

10 ME Alications: Under-saturated Oil Reservoirs Case 3 Volumetric under saturated oil Reservoir Volumetric deletion, W e = 0 Gas Ca resent, m 0 E fw = neliible F/Eo = N m N E /Eo Case 4 under saturated oil reservoirs with stron water drive Stron water drive, W e 0 Gas Ca resent, m 0 E fw = neliible F W e = N E m E ) E m E o o )

11 ME for Gas Reservoirs For GAS RESERVOIRS- ME equation 15) can be reduced to: G W w = G i) We w 19) For volumetric as reservoir, We = 0 Assumin water roduction is neliible, W = 0 Equation 19 becomes: G = G i) 20) Alyin the definitions of as volume factor G z T sc sc T G z i sc T i sc T i = G z T sc sc T Assumin Isothermal chanes in the reservoir T = Ti) z i i = G z i G z 22) i

12 ME Alications: Gas Reservoirs Case 5 ME for Volumetric Gas reservoirs Case 6 influx ME for Gas reservoirs with water Volumetric deletion, We = 0 Efw = neliible z i = G z G i z i i Stron water drive, W e 0 E fw = neliible G W e i G = W i w

13 Flowin Material alance Develoed for Gas reservoir later extended for oil reservoirs Does not account for water drive Requires seudo-steady state flow reime: Reservoir boundaries are felt ressures at all locations in the reservoir declines at the same rate Requires constant flow rate- later extended for variable rate

14 Flowin Material alance contd.) Classical ME /z lot requires averae reservoir ressure Requires lenthy shut in tests to determine averae reservoir ressure Flowin ME Flowin bottom hole ressures is used wf/z vs G is lotted Well head ressures can also be used

15 ME Plottin Technique Detectin Water Drive For some water drive as reservoirs, deviation from /z lot is not detected until much later Error in GIIP- lare difference between volumetric and M estimates Error in Drive mechanism wronly assumed deletion tye Alternative lottin technique is more sensitive

16 Drive Mechanisms & Drive Indices A reservoir can have a redominant drive mechanism, or can have a combination of mechanisms. Identifyin the drive mechanism is imortant for develoment stratey and ultimate recovery. The drive indices show the relative manitude of each drive mechanism contributin to total roduction.

17 Drive Mechanisms & Drive Indices contd.) [ ] ) ) ) ) s o s si oi o R R N R R N ) ) 1 s o i oi R R N m N ) ) ) ) R R N c S c S m )N s o f wc w wc oi 1 1 ) ) ) s o w e R R N W W Deletion Drive Index DDI) Sereation Drive Index SDI) Comaction Drive Index CDI) Water Drive Index WDI) DDI SDI CDI WDI = 1 Dividin throuh equation 15 by LHS:

18 ME: Conditions for Alication Pressure Pressure Time Fiure 10: Individual well ressure declines dislayin equilibrium in the reservoir Time Fiure 11: Non-equilibrium ressure decline in a reservoir 1. There should be adequate data collection on roduction, ressure and PVT roerties. 2. It must be ossible to define an averae ressure decline trend for the system under study.

19 ME: Conditions for Alication contd.) j, q j, V j The averae ressure decline can be determined by the volume weihtin of ressures within drainae area of each well av) = Σ j Vj / Σ Vj fiure 12 It is ossible to verify the 2nd condition by lottin the individual well ressures as a function of time fiure 10). It is not really necessary to have raid ressure equilibrium across the reservoir. Averae ressure decline trend can be defined even if there are lare ressure differences across the reservoir under normal roducin conditions. Fiure 11 shows such a reservoir, where each well has a distinct drainae area and ressure decline fiure 12).

20 Conclusions Volumetric method is alied at early stae of a reservoir, with mostly eoloical and fluid roerties data. No roduction or time deendency is incororated in volumetric estimates. As roduction continues, other methods become alicable. Material balance can be alied when about 20% of the initial estimated reserve is roduced, or when 10% of initial reservoir ressure has declined. ME is a owerful tool that hels determine the reserves, recovery factor, and drive mechanism. ME can be alied to a variety of reservoirs, either with or without water influx.

21 Conclusions contd.) Unlike volumetric method, RF can actually be calculated by ME. Volumetric method enerally ives the absolute, theoretically maximum ossible hydrocarbon in lace. ME ives an indication about the volumes that will actually flow. Flowin material balance technique can rovide reasonably ood aroximation of GIIP. Flowin material balance technique method removes the need for determinin static averae reservoir ressure by lon and exensive shut in tests.

22 THANK YOU! Questions?