6 MBE Applications

7/31/2019 6 MBE Applications

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These are all pressure related func5ons and can be obtained from PVT data.

Which gives

Dake1 proposes structuring a table with the following steps: Steps:(i) (ii) (iii) (iv) (v) (vi) (vii) (viii)

Pressure ,Y,Z(p) S o k rg/k ro So So Np/N GO(psia) /psi PV PV PV scf/stb

(i) Pressure in steps below bubble point(ii) Table of ,Y &Z(p) values, calculated at the average pressures between the

steps in I.(iii) So prior to the pressure drop p.(iv) ela5ve permeability ra5o at last value of S

o.

(v) So determined using eqn. 77.(vi) The lower value of S o at reduced pressure.(vii) The frac5onal recovery from bubble point. Equa5on 82(viii) The GO obtained from GO eqn using k rg/k ro value obtained from new So

So =oil remaining

one PV=

N N p( )BoNB oi

1 Swc( )

N pN

= 1 So

1 Swc

BoiBo


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All the procedures are similar , and are very dependant on reservoir, uidand rock data. The quality of a material balance study of a reservoir isrelated to the quality of the data. There clearly should be sufficient data

both with respect to quan5ty and quality as in any simula5on study thequality of the output is directly related to the quality of the input. Anotherchallenge is the deni5on of the average reservoir pressure. We willbriey look at these two data perspec5ves.

Data for Solu/on Gas Drive Predic/onsPrior to carrying out the MB procedure it is important to test the data, ifthere is past produc5on data available. The methods can then be used toexamine if they are capable of predic5ng past performance. Clearly if thecold data does not enable past performance predic5ons to be matched,then there is an opportunity to adjust some of the data to obtain a historymatch. The data can then be used with be^er condence to predictfuture performance. Listed below are the data which are used in thevarious solu5on gas drive predic5on procedures. Some of them could beadjusted in history matching.


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Gas Drive ReservoirsAs discussed in Drive Mechanisms Chapter, Gas Drive is also a deple5ontype reservoir. From the nature of the pressure gradients within the oilcolumn it is also likely that solu5on gas drive is also ac5ve when deple5nga gas cap drive reservoir. Cole has pointed out that gas drive is essen5ally afrontal drive displacing mechanism. In this respect the high mobility of thedisplacing gas to that of the displaced oil is such that in deple5ng the oilreservoir it is important to minimise the rate , to reduce bypassing of oilby the advancing gas oil contact. The density differences of gas and oil

clearly help to offset the advancing mobility ra5o effect. Tarner's methodcan be used for Gas Cap drive reservoir predic5ons. The equa5on howevermay need altera5on to account for gas coming out of solu5on migra5nginto the gas cap. In Tarner's method the equa5on for Gas Produc5on, G ,becomes:

N pR p =N B o + R si R s( )Bg Bob( )+ mB ob Bg BgiBgi

N p Bo R sBg( )

Bg


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Average Reservoir PressureThe material balance approach is some5mes considered as the tankmodel. In the applica5on of the MB equa5on we are assuming that thepressure is uniformly distributed across the reservoir. If there is uniform

pressure decline in all the wells in the reservoir then this pressure declinegives condence for applica5on of the MB tool. Dake pointed out that ifthis equilibrium is not achieved, the MB approach can s5ll be used. Hesuggested that an average pressure can be determined to represent areservoir where there are large differen5al pressures across the reservoir.He presents an averaging procedure for reservoirs where pressure

equilibrium has not been achieved. In the gure below from Dake arepresented the pressures for equilibrium condi5ons and the well posi5onsand boundaries for a non equilibrium condi5on.


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Well Posi5ons and

DrainageBoundaries

Well pressure for Non

Equilibrium Wells


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Having worked through this chapter the Student will be able to:

Given the MB equa5on be able to present it in a short hand form asa basis for use in linear forms.

Using the various linear forms with sketches illustrate the MBequa5on for use for: eservoir with no water drive or gas cap. No water drive but with known gas cap.

Comment with the aid of sketches the impact of water drive on theapplica5on of MB equa5on in linear and other forms.

Derive and use a simplied MB equa5on for applica5on to an oilreservoir above the bubble point, in terms of recovery, and oil, rockand water compressibility.

Derive the instantaneous gasoil ra5o equa5on and use to explainthe producing GO of a solu5on gas drive reservoir.


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6 MBE Applications