Adjustment Of Differential Liberation Data To Separator Conditions

Abstract

The actual reservoir process is neither flash nor differential.

Thus, regardless of the testing procedure, some adjustment

needs to be made to the resultant data to approximate the fluid

behavior in the oil production process. The conventional

method of adjustment extrapolates to negative values of

solution gas-oil ratio and formation volume factor leads to

values less than one at low pressure. Both extrapolations do

not conform to the physical behavior. This is due to the fact

that the conventional approach does not observe that the oil

relative density at reservoir conditions is the same regardless

of the process.

This paper presents a new approach to the adjustment of

differential liberation data to separator conditions. This

approach is based on the fact that both flash and differential

data should give the same value for the oil relative density at

the reservoir conditions. This is achieved by correcting all the

properties, i.e. solution gas-oil ratio, formation volume factor,

gas relative density and oil relative density. The new method

overcomes the disadvantages and limitation of the

conventional approach. This method is tested on 400 PVT files

from all over the world and the result is consistent with

physical behavior. A sample calculation is presented to outline

the new method.

Introduction

The differential solution gas-oil ratio is not the same as the

flash solution gas-oil ratio as shown in Fig. 1. Similarly, the

differential and flash oil formation volume factors are not the

same as depicted in Fig. 2. Thus, regardless of the testing

procedures - flash or differential, some correction needs to be

made on the resultant data to approximate the fluid behavior in

the oil production process.

The oil that leaves the reservoir is flashed to the separator,

necessitating that the solution gas oil ratio and the formation

volume factor should be determined by a flash process.

In order to calculate the combination fluid properties from

standard data analysis, several assumptions were stipulated,

but these assumptions limit the range of application.

This paper describes a new method to adjust the

differential liberation data to separator conditions. This

method overcomes the disadvantages and limitations of the

current method and come up with a correction procedure that

results in a consistent physical trend.

Reservoir Process

In differential liberation process, gas is removed from oil as it

is released from solution. While, in flash liberation process,

gas remains in contact with oil.

Generally, petroleum engineers consider that the liberation

process in the reservoir more closely approaches a differential

process. The fluid produced from the reservoir to the surface

is considered to undergo a flash process.

The actual liberation process in the reservoir is neither

flash nor differential. In certain localities, the process is flash,

and in others, the process is differential. In some other

localities the process does not match either of them. A

combination test proposed by Dodson2 is the closest to the

reservoir process. At each step of differential liberation test, a

sample is taken and flash liberated to obtain Rs, γo, Bo, and γg.

Here it can be seen that all properties including the γapi are

different at different pressures. Although this combination test

or composite liberation is an improvement and closest to

reservoir behavior, it does not match the actual reservoir

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