Descaling of petroleum production tubing utilising aerated high pressure flat fan water sprays
, PhD thesis, University of Salford.
Recent attempts to utilise solid particles in combination with high pressure water sprays has caused environmental and safety concern, in cleaning mineral and organic scale inside the Oil and Gas Production Tubing. To increase cleaning performance only high pressure aerated water sprays at high impact force instead should be used. Multi-nationals petroleum companies are facing immense challenges in removing the scale due to the decrease in cavitation bubbles along the production tubing when high pressure water sprays are applied. This has also resulted in high maintenance costs and low productivity of the ‘wells’ with multi billions pounds financial losses per annum. Currently scales are removed using either aggressive chemicals (acids), complete replacement of the tubing, or solid-liquid sprays which are both expensive and causes environmental concern. This research demonstrated that the application of air-water combination (aerated sprays) are the solution in complete removal of various scales in the production tubing without the use of solid particles and the cavitation bubbles.
This novel experimental technique of scale removal utilised air concentration (or aeration) in combination with high pressure flat fan sprays, of up to 10 MPa, at low flow rate (up to 12 l/min) with high impact pressure of approximately 0.15 MPa, in removing scale along production tubing using a simulated aeration chamber. It was found that varying air concentration from 3 to 12%, within the emulated chamber, improved scale erosion up to 28% higher than non-aerated technique. This enabled the mass of the scale to be removed at the ‘stand-off distance’ of 25 mm relative to scale samples, irrespective of cavitation bubble length suppression which is normally about 2 mm away from the atomiser orifice exit, compared to non-aerated techniques (solids and water). Scale erosion was found to be 12.80g, 7.31 g, and 65.80 g at aerated conditions compared to non-aerated provision which found to be 9.88g, 6.33g and 5.31 g, at the required liquid pressure 10 MPa, for the hard, medium and soft scale samples that are typically found in oil production tubing.
Prior to scale removal trials sprays were characterised qualitatively and quantitatively under the ambient conditions as well as inside the aerated simulated chamber. Air velocities were found to be approximately 18m/s towards the water spray centre which then decays to 3 m/s towards the spray periphery under ambient conditions using hot wire anemometer. Moreover, the flat fan sprays were also characterised utilising Phase Doppler Anemometry (PDA). It was found that the high pressure water liquid droplet velocities were in the range of 75 to 117 m/s with droplet diameters of 55 to 81 µm (SMD) at flow rates of 7.6 to 11.3 l/min at various stand-off distances of 25, 50 and 75mm, providing an impact pressure of 0.05, 0.10 and 0.15 MPa respectively.
Qualitatively cavitation bubble length was also estimated using high resolution imaging techniques which were found to be between 1 to 2 mm from the atomiser exit orifice under submerged conditions, at the stand-off distance ≤ 25 mm where the scale is normally removed. Beyond this range (1-2 mm) where the cavitation bubbles are not present, that are normally the benefactors to scale removal process, requires air concentration up to 12%. This ensures that a complete removal of the mass of corresponding scales to be achieved with varying chemical scale compositions. The air concentration is the ratio of total mass of air within the simulated chamber to mass of the liquid sprays impacting directly onto the scale samples.
The results of the experimental trials were used to validate the available CFD fluent models with regards to spray dynamics, aerated air (velocities), cavitation bubble generations and scale erosion (removal). The sensitivity analysis using the CFD modelling gave close comparison with those obtained through experimental trials. Spray droplets size and their velocities were found to be within ±10% compared to those obtained via experimental findings. The aerated air velocities were also compared with the data generated from CFD which were found to be approximately ±9%. Furthermore, the cavitation bubble generation and the mass of the scale removed were found to vary within ±5% and ±7% respectively, when compared to the CFD data.
Finding emerged that the spray droplets especially at the centre undergoes acceleration after primary breakup, which due to higher velocities resulting from the acceleration has left the entrained-air particles behind, which is characterise with substantially low-pressure region, giving rise to utilisation of the air-water interaction model. This could be another approach in further understanding the break regions within the high pressure liquid sprays.
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