A Study on Acoustic Logging Techniques in Oil and Gas producing Wells

Abstract

Gas lift is an artificial lift technique used in reservoirs with an inadequate pressure. The main idea behind the gas lift technique is to inject gas with low density into the production pipe so that the fluid mixture becomes lighter. By doing this, a higher production rate can be obtained. However, this also introduces high-pressure combustible gas into the annulus, which again constitutes a major safety risk. In relation to this, acoustic surveys can be used to either verify barrier integrity or reveal any potential weaknesses in the barriers. Since the operation is carried out from the surface, this reduces the overall risk since there is no need for well intervention. Additionally, the operation can be performed multiple times which allows for monitoring the current state of the well.In this thesis, a mathematical explanation on pressure wave propagation is provided. When an acoustic survey is performed, a pressure pulse is introduced into the well by utilizing a differential pressure between the well and the chamber on the acoustic equipment which contains atmospheric pressure. The introduced pressure pulse will result in reflected waves whenever it reaches a discontinuity in form of a cross sectional area change or a sudden change in density between two media. These waves will then travel in the opposite direction and get recorded by the microphone on the surface. The dynamics of wave propagation causes the reflected waves to be either positive or negative in magnitude. The latter is exploited when analyzing the acoustic signal afterwards by looking in the well barrier schematic and link the corresponding reflections and elements together. The positive and negative pressure waves are referred to as upkicks and downkicks in this thesis. An upkick means a reflected wave propagating with a higher pressure than the ambient pressure whilst a downkick means a reflected pressure wave propagating with a lower pressure than the ambient pressure. A frequency analysis is also performed on the acoustic signals from wells with an annular safety valve installed. A representation where the frequency domain is combined with the time domain is suggested and the method is used to analyze shots containing strong reflections. The active frequencies are linked together with the physical layout of the well and a thorough explanation is provided. The problem regarding resonance frequencies and strong harmonic multiples is also addressed and a proposal on how these can be attenuated is presented.Furthermore, a small scale version of a gas lift well is designed and a detailed description on how this can be related to a real sized gas lift well is given. Different scenarios are investigated, and the goal is to address the challenges operators face on a daily basis. The results serves as a good overview on how different factors affect the acoustic signal and they can also help to get a better understanding on the acoustic principle.