Page 10 - Acoustic Fluid Level Measurements

Fundamentals of Acoustic Fluid Level Surveys 3-1

oil and gas well production studies. For pumping wells, University of Texas at Austin

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Fundamentals of Acoustic Fluid Level Surveys

In this chapter:
• Information required for understanding acoustic fluid level records and analyzing surveys
• Propagation of sound and sound pressure waves in pipes and annuli
• Effect of composition, pressure, and temperature on acoustic velocity in gases and other fluids
• Reflection, attenuation, resonance, and interference
• Correlations for acoustic velocity calculations

The bottomhole pressure (BHP) corresponding to vari- SOUND PULSE GENERATION
ous rates of production allows for the determination of a AND WAVE PROPAGATION
well’s productivity potential (as discussed in chapter 1). A wave is a disturbance or change from a preexisting
Thus, it is one of the most important measurements in
Petroleum Extension-The
condition that moves in space from one point to another,
carrying the deviation information at a certain finite
especially rod-pumping wells, direct measurement us- speed depending on the medium’s properties.
ing downhole pressure sensors is impractical and costly Acoustic or sonic waves are generally caused by
because the rods must be pulled prior to installation of pressure changes in a gas or liquid and propagate through
the sensor, which disrupts production and alters the pres- the fluid at a speed defined as acoustic velocity, also
sure response. Permanently installed pressure sensors known as sonic velocity. Propagation of a sonic wave
with surface readouts are not economically justifiable requires the presence of a material medium: solid, liq-
for routine monitoring of pressure in rod-pumped wells, uid, or gas. Sound cannot propagate in a vacuum and is
since most of these wells produce at low oil rates. greatly attenuated when the pressure in the gas is lower
For these reasons, acoustic fluid level measurements than atmospheric pressure. The shape or character of the
were introduced long ago with two objectives: wave is arbitrary; it does not have to be oscillatory or
• Determining the distribution of fluids present sinusoidal. It can be triangular, rectangular, bell-shaped,
in the wellbore (particularly the amount of or spike-shaped, depending on how it is generated.
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liquid above the pump intake, defined as pump For many types of waves, their motion is described
submergence) mathematically by the wave equation, which can be
• Estimating the dynamic and static pressures at written as:
the depth of the producing zone without the need ∂ u
2
2
2
to introduce any tools into the well 2 c ∇ u – —– = 0 Eq. 3.1
∂ t
2
Over the years, this technology has been refined so where u is the physical property (for example, pressure in
that regulatory agencies in many states and countries a gas or strain in a solid) of the disturbance, the operator
accept the results of acoustic surveys for calculating is defined as the partial second derivatives with respect
well potentials and BHPs . to rectangular xyz coordinates, t is time (in seconds),
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For distribution by Petroleum Extension-The University of Texas at Austin
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