Novel Techniques for Monitoring and Enhancing Dissolution of Mineral Deposits in Petroleum Pipelines

Abstract

Abstract This paper presents a summary of work carried out and the results achieved in developing novel ultrasonic techniques for monitoring and enhancing dissolution of mineral scale deposits in petroleum pipelines. Problems of conventional ultrasonic techniques with respect to testing pipes are analysed. The development of mathematical and experimental models to extract the characteristic features from ultrasonic scans representing the acoustic properties of the mineral deposits beneath the pipe wall are described. These properties are compared with stored data to identify the type of deposits and hence assess their growth which aids performance optimisation of remedial scale dissolver and preventative scale inhibitor treatments. The process of de-scaling is further enhanced by ultrasonic irradiation and methods for monitoring the dissolution process in real-time are proposed. Introduction Mineral deposits in petroleum pipelines causes progressive flow restrictions leading to large production losses over a period of time. The composition and thickness of these deposits are variables and cannot be adequately predicted in advance. See Fig. 1. As a result, present de-scaling procedures tend to be based on guesswork, often causing expensive chemical wastage and costly production shut-downs. This paper describes the work carried out in developing novel techniques to monitor and enhance the dissolution of mineral deposits in petroleum pipelines. Apparently, there is no evidence of a reliable non-invasive detection method developed so far to address the above problems. Radiography may give an accurate assessment of the thickness of deposits but it is not a practical approach in a production environment. Similarly, calliper surveying requires production shutdowns and is costly. Among non invasive techniques, ultrasound offers many advantages, but conventional ultrasonic techniques such as time-of-flight and tomographic measurements are not directly applicable due to two main reasons. Firstly, the speed of sound in deposits are not initially known and are variable. Secondly, coupling ultrasound to the pipe results in the generation of strong reverberating signals which mask the echoes reflected off the lower boundary of the deposit, making it impossible to extract timing information. The novel approach taken in this work as illustrated in Fig. 2 is that the deposit is to be identified indirectly by analysing the characteristics of the reverberating signals, such as their decay patterns. These characteristics are compared with that obtained from test specimens and pipelines containing known deposits which are stored as acoustic templates. Laboratory tests were carried out using test assemblies simulating scale deposits as well as on a few ex-service scaled pipes. Promising results showing the representative characteristics of the reverberating signals were obtained which allowed the identification of the type of deposits tested. The recovery of the echoes reflected off the lower boundary of the deposit is attempted by signal thresholding and preferential filtering. It was possible to assess the thickness of the samples tested using the timing of the recovered signals under ideal laboratory conditions. Since these measurements are non-invasive, they may be carried out during normal production. When the type of deposit is identified the next step is to formulate optimum chemical scale dissolvers on the basis of available data and enhance the dissolution rate by artificial insonification. Very encouraging results have been obtained in this respect increasing the dissolution rates between 4 to 17 times for low to high ultrasonic energy levels. P. 507