Page 80 - Intelligent Digital Oil And Gas Fields

Instrumentation and Measurement 51

• Calibration and maintenance: How often must the meter be calibrated?
• Gas void fraction: How much represent the fraction of gas or oil volume
in the pipe through time?
• Cost versus values.
• Government acceptance and health, safety and environmental issues.
• A meter’s position before or after a separation system.

2.1.3.4 Key Factors to Consider in Flowmeter Selection
Turndown ratio (TR). TR is the ratio of the maximum observed peak rate
divided by the minimum observed rate in a period of time, TR¼Q max /
Q min . It represents the rangeability of the instrument: the higher the num-
ber, the better range of sampling. If you have a well producing with slugging
patterns, showing intermittent high/low rate in <1h, then it requires a
meter with high TR, above 10:1.
Flowmeter location. Onshore, traditional meters (mass flow, flow rate, and
differential pressure meters) are used after the separation system (low pres-
sure). In this situation, it is very important to evaluate the flow regime and
pattern. Offshore, multiphase flowmeters (MPFM) might be used before the
separation systems.
Single-phase versus and multiphase flows. Gas is compressible and the density
changes significantly with changes in pressure, even when the change is low.
Liquids are considered incompressible and, in general, densities (oil and
water) do not change with pressure. If the pressure of a wet gas system
increases, the density of the gas will increase but liquid density remains
almost the same. Compositional fluids such as volatile O&G condensate,
change fluid composition with pressure. To distinguish the fraction of phases
on the flow rate (oil, water, and gas) requires flowmeter technologies to
quantify the volume fraction of each phase. The engineers need to know
if the fluids in a particular zone are flowing in single or multiple phases,
because it will change the type of meter needed.
Flow regimes and patterns. Flowmeters generally are placed between the
wellhead and the separation system or immediately after the separation sys-
tem. At this stage, fluids are governed by gravitational or kinetic forces. The
flow pattern depends on the volume of each phase, vapor and liquid prop-
erties, and the pressure and velocity of each phase. Therefore, the phases can
be distributed along the horizontal pipe in many sections and in many ways,
which is shown in Fig. 2.7 with a description below the figure.
• Stratified or wavily stratified flow takes place at low pressure, gas, and liquid
flow at different velocities; the surface between the liquid and the gas are