Thermal drilling for application in gas and oil wells: Efficiency enhancement by acoustic inspection

Abstract

Thermal drilling has recently attracted the attention of researchers due to its high efficiency in breaking hard rocks. However, the advent of low-efficiency processes, such as melting and evaporation, is challenging thermal drilling performance. The implementation of realistic guidelines to monitor spallation may help enhance the performance of this technology. In this article, the results of the continuous and intermittent application of butane oxygen flame on dry and wet limestone were compared with the results of diamond drilling based on the resulting rate of penetration and specific energies. A high-quality camera was operated as a device for visual inspection of the processes to determine any obvious macroscopic physical and chemical changes. In addition, acoustic monitoring of the processes involved during the thermal heating experiments was carried out using a microphone with a frequency response region below 20 kHz. The background noise in the recorded acoustic signals was eliminated by using noise reduction methods in MATLAB. The software performed Fast Fourier Transform (FFT) on the obtained clean signals to reveal the related frequency spectrograms. According to our experimental observations, the frequency response for spallation occurrence in wet and dry samples appears to be around 7 kHz and 15 kHz, respectively. However, the frequency response for fracture formation did not reveal a sensible dependence on moisture content in the limestone samples. Under continuous heating, the rate of penetration and the specific energy for wet samples were respectively 131% higher and 68% lower than those for the dry samples. In the dry samples, wetting between the two heating processes led to a considerable increase in the penetration rate and a dramatic decrease in the specific energy due to keeping the local temperature gradient high in the rock texture.