Measuring Solvent Production in Miscible Phase Oil Recovery Operations
Authors: Dean M.R. , Brinkley T.W. 
Published: December 2007

A method is presented for determining quantitatively the history and extent of solvent production during miscible-phase type oil recovery operations in the field. The data required can be obtained during normal field operations using conventional production equipment. The necessary analyses of reservoir oil, separator oil and gas can be obtained with conventional laboratory procedures. An example calculation is presented using data from the LPG-slug type project under way in Central Bisti unit, New Mexico. The results showed that significant quantities of propane from the previously injected LPG slug were being produced even though the propane concentration in the separator gas was less than before injection of solvent was started into the injection wells.

Introduction

Several field tests of the miscible-slug process of recovering oil from a petroleum reservoir are under way. The analysis of these field tests would be more complete if the history and the amount of the eventual production of the solvent slug could be determined. Determining when the eventual breakthrough of the solvent slug occurs has usually consisted of analyzing field separator gas at intervals to determine when components of the slug appear in larger than normal concentration in the gas. While such a method may sometimes be useful as a qualitative method, it does not allow a quantitative determination of slug production, and may indeed fail entirely to disclose the breakthrough Of the solvent slug. Presented here is a method for determining quantitatively the extent of the solvent slug production. The necessary field measurements can be made with conventional field equipment. These data are: rates Of separator oil and gas production, and rates of tank oil production. In addition the compositions of the injected solvent slug and all injected gases must be known. During the period of application of the miscible-slug process to the reservoir, the compositions of the separator oil and gas must be determined on a suitable schedule. The reservoir oil physical properties which are usually required are: relative volumes under field separation conditions, viscosity and solution gas-oil ratio. When the reservoir is produced after the pressure has fallen below the reservoir oil saturation pressure, a gas-oil relative permeability curve will be needed.

Development of Equations

The equations are developed from two volumetric balances:

a balance around the gas-oil separation system on one of the components of the injected slug, and a balance on gas around the same separation system. For convenience, the equations are derived from a volumetric balance on propane, although, in principal, the derivation can be made on any component of the injected slug.

A liquid-volume balance on propane around the separator gives

Ns + No = Nr + Nf + Ni +N3 . . . . (1)

where: N3 = liquid volume of propane from injected slug, Ns = liquid volume of propane in separator gas, Ni = liquid volume of propane carried toproducing well by the injected gas, Nr = liquid volume of propane in producedreservoir oil, Nf = liquid volume of propane in producedreservoir gas, No = liquid volume of propane in separator oil.

A gas-volume balance on the various gas streams around the separator provides Eq. 2.

Vs = Vr + Vi + V + Vf . . . . . . . . . (2)

where: V = gaseous volume of produced slug, Vs = volume of separator gas, Vr = volume of gas evolved in separator fromproduced reservoir oil, Vf = volume of produced gas that was "free"gas in reservoir resulting from producingreservoir below saturation pressure, Vi = volume of injected gas which has brokenthrough to producing well.

Eqs. 1 and 2 are combined and the quantities expressed in terms of field measurements in the following equations. The liquid volume equivalent of propane in the separator gas Ns is obtained from Eq. 3.

JPT

P. 519ˆ