The Use of Acoustic Data for Interference Testing
Authors: Laird A.D. , Bonner D.J. 
Published: December 2007

Abstract
This paper presents field examples of interference testing between pumping oil wells. The accuracy of the surface pressure data and acoustic fluid level measurements are demonstrated, along with the accuracy of the acoustic bottomhole pressure calculations.
The project work was performed in the Owl field in NE British Columbia, Canada. Based on the initial geological and engineering evaluation, it was thought that the field contained three separate Cecil oil pools. A pressure interference project was conducted to assess whether communication existed between the pools in order to demonstrate waterflood potential for a pending sale.
During the interference testing, four observation wells were used: two with acoustic measurements and two with subsurface recorders. In all four cases, definitive interference responses were measured. It is often thought that acoustic data cannot be used to measure the subtle pressure responses in interference tests. This project used modern temperature- compensated surface pressure recorders and classic cartridge-based, dual-channel fluid level recorders. The subsurface pressures were calculated using an equation of state (EOS) based density model that had been tuned to pool PVT data.
The resulting accuracy and sensitivity of the acoustic bottomhole pressures were in excellent agreement with the results from the downhole recorders.

Introduction and Background
To date, nine wells have been drilled into the Cecil formation in the Owl field in northeastern British Columbia. The discovery well was 15–09–86–18 W6M, which started production in 1987. Subsequent drilling resulted in seven additional wells in 1989. The ninth well was drilled later and began production in 1992.
At present, eight wells are on regular production while the well at 06–21 is not tied-in due to a high GOR. Only three of the eight producing wells are still flowing, with four wells converted to pump and one well on plunger lift. The wells are drilled on essentially one-quarter section spacing although some wells have apparently larger drainage areas.
Since the well 15–09 was first drilled, there had been speculation about the areal extent and degree of communication within the Cecil formation. The initial production test on each successive well indicated the same initial pressure, which in itself did not differentiate between separate pools. A major field pressure survey was conducted in 1991, and the initial interpretation of this data indicated a wide range in reservoir pressures. Based on this interpretation, the wells were grouped into three hypothetical pools:
A geological map was then drawn with internal boundaries or permeability barriers between the three groups of wells. Given that the interwell distances are about 500–800 meters, most of the boundaries were drawn within 200–300 meters of an existing well location.
The 1991 pressure survey data was reviewed and reanalyzed in 1995. The original interpretations of average reservoir pressure were found to be flawed and it appeared that problems with the well test analyses distorted and exaggerated the pressure differences that were observed in the field. For example, some of the calculated average reservoir pressures were lower than the last measured pressure during the pressure buildup surveys, which indicated problems with the test interpretations. P. 315^