Page 16 - Acoustic Fluid Level Measurements

Fluid Level Measurement Applications for Gas Wells 9-1

Petroleum Extension-The University of Texas at Austin
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Fluid Level Measurement Applications for Gas Wells

In this chapter:
• Monitoring gas well performance with fluid level measurements in tubing and casing
• Determination and analysis of liquid loading
• Gas well troubleshooting
• Tubing and casing integrity testing

The principal objective when performing acoustic additional back-pressure acting on the formation when
measurements in a flowing gas well is to determine the there is liquid loading in the tubing. When gas velocity
quantity of liquid inside the tubing (or the annulus, when drops below the critical rate, production rates are reduced
the tubing is used for removing liquid from the wellbore by liquid accumulation in the tubing. Removing this
by means of a pump) and whether the produced liquid liquid requires applying a deliquifying technique, such
(1) is uniformly distributed over the length of the well as installing plungers or pumps, adding surfactants, or
as a mist or annular flow pattern or (2) has fallen back, redesigning the flow string to increase gas velocity .
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accumulating near the bottom of the well. The acoustic test in flowing gas wells is designed
In the first case, the gas flow rate is above a value to determine which flowing gradient conditions exist
defined as the critical rate, and the liquid is uniformly in a well. The test involves performing a series of fluid
distributed. The gas velocity is sufficient to continuously level and surface pressure measurements while the flow
carry liquid as a fine mist or small droplets to the surface, at the surface is stopped for a length of time sufficient
establishing a relatively low and fairly uniform flowing to identify the behavior and distribution of the fluids in
pressure gradient throughout the tubing. the tubing or tubing/casing annulus. The advantages of
In the second case, when the gas flow rate is below the acoustic test over wireline flowing pressure surveys
the critical rate, the gas velocity is not sufficient to carry include lower costs, because equipment is very portable,
all the produced liquid to the surface, and most of the and lower risks, because measurement tools are not
liquid accumulates and stays in the lower part of the introduced into a flowing well. An important byproduct
well. A flowing pressure traverse in the wellbore will of acoustic testing is determining the condition of the
show two different gradients: a light gas gradient above downhole hardware and integrity of the tubulars.
the gas/liquid interface and a heavier gradient in the
lower section of the well below the gas/liquid interface.
The gradient of the fluid below the gas/liquid interface DETERMINING LIQUID LOADING
reflects the liquid concentration, which is controlled OF A GAS WELL
by the gas flow rate. The liquid in this section of the The acoustic fluid level test is used to determine the tubing
wellbore recirculates in place, with zero net liquid flow, (or annular) pressure distribution in a flowing gas well
as the gas bubbles or slugs of gas percolate through the by momentarily shutting in the flow for the duration of
liquid, and only the gas flows to the surface. the test. An analysis of the acoustic fluid levels acquired
Knowledge of the flowing gradient and fluid dis- on gas wells can be used to determine the:
tribution in the well is important in determining the • Amount of liquid loading on the formation

For distribution by Petroleum Extension-The University of Texas at Austin
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