Page 20 - Introduction to Petroleum Engineering

4 INTRODUCTION

1.1.3 Production Performance Ratios

The ratio of one produced fluid phase to another provides useful information for
understanding the dynamic behavior of a reservoir. Let q , q , q be oil, water, and
g
o
w
gas production rates, respectively. These production rates are used to calculate the
following produced fluid ratios:
Gas–oil ratio (GOR)
q
GOR g (1.1)
q o
Gas–water ratio (GWR)
q
GWR g (1.2)
q w
Water–oil ratio (WOR)
q
WOR w (1.3)
q o
One more produced fluid ratio is water cut, which is water production rate divided by
the sum of oil and water production rates:

q
WCT w (1.4)
q o q w
Water cut (WCT) is a fraction, while WOR can be greater than 1.
Separator GOR is the ratio of gas rate to oil rate. It can be used to indicate fluid
type. A separator is a piece of equipment that is used to separate fluid from the well
into oil, water, and gas phases. Separator GOR is often expressed as MSCFG/STBO
where MSCFG refers to one thousand standard cubic feet of gas and STBO refers to
a stock tank barrel of oil. A stock tank is a tank that is used to store produced oil.

Example 1.1 Gas–oil Ratio
A well produces 500 MSCF gas/day and 400 STB oil/day. What is the GOR in
MSCFG/STBO?

Answer
500 MSCFG/day
.
GOR 125 MSCFG/STBO
400 STBO/day

1.1.4 Classification of Oil and Gas

Surface temperature and pressure are usually less than reservoir temperature and
pressure. Hydrocarbon fluids that exist in a single phase at reservoir temperature
and pressure often transition to two phases when they are produced to the surface