Investigation into descaling of production tubing in oil and gas wells.
El Kamkhi, M
One of the most common production problems in oil and gas fields is scale deposition within oil and gas wellbores. Scale formation in surface and subsurface oil and gas production equipment has been recognised to be a major operational problem. The effect of scale formation is a reduction in the well productivity and the damage to safety valves and gas-lift mandrels. This investigation proposes a new and novel technique to address the problem and lay the foundations for a methodology for descaling in-situ production tubing. This problem currently requires either aggressive chemicals or extraction and replacement tubing both of which are very expensive. \ud The overall aim of this investigation is to study the fundamentals of the decaling process using flat fan sprays at high water pressure ( < 6 MPa ) and high impact force (< 0.657MPa).The spray was characterised quantitatively and qualitatively under ambient conditions utilising single flat fan atomiser or a combination of two or three atomisers with an overlapping configurations. The volume (or mass) flux (VLf) was measured using a patternator which was designed during this investigation. Liquid volume (or mass) flux was found to be symmetrical at different downstream distances (25, 50 and 75 mm). Impact force (IF) was also experimentally measured at different downstream distances (25, 50 and 75 mm) and at various water supply pressure of 3.7, 4.8 and 6 MPa for the corresponding sprays. The result showed that impact force can lie between 0.146 to 0.657MPa. Sprays were also characterised using Phase-Doppler Anemometry (PDA). The range of velocity (U) was found to be between 62.48 to 81.85 m/s at 25 mm downstream distance whilst at 50 mm downstream, it was in the range of 55.96 to 73.13 m/s and at 75 mm was between 48.89 to 66.98 m/s. However, the Sauter mean drop size diameter (D^ at 25 downstream distances was found to be in the range of 62.45 μm to 75.80 μm and at 50 mm downstream the drop size was between 67.30 μm to 81.43 μm whereas at 75 mm it was 73.56 μm to 86.42 μm. Also, from the results, the liquid volume flux (ULf) obtained using PDA was measured to be between 0.063 and 1.2 (cm3/s)/cm2 at 25 mm downstream and 0.016 to 0.250 (cm3/s)/cm2 at 50 mm and 0.010 to 0.120 (cm3/s)/cm2 at 75 mm downstream distance. Comparing these results with those found via patternator measurements showed in some instances, that there is significant (i.e. up to 90%) difference between each values which were obtained by the PDA, measured at the centre of the sprays. This was mainly to lack of capturing the certain drop sizes by PDA and also the design geometry of the patternator together with the nature of flat shape of the spray being not truly flat. Simulated laboratory scale removal rig was subsequently designed and built to demonstrate the effects of using overlapping flat sprays atomisers to remove scales that normally found in oil and gas. Four scale samples (candle wax, soft oil wax, soft gas and oil hard) were tested. The quantity of scale removed using soft candle wax was (53 cm3) at 30 degree atomiser angle to the vertical axis. The scales samples which were obtained from both oil and gas fields were also tested as follows: - a total of 11.688 cm3 of soft scale was removed using three high pressure and high impact atomisers whilst the volume of the scale removed from the oil wax was within 13.750 cm3 for the hard oil scale was found to be approximately 0.989 cm3 .The structures of the flat sprays were also mathematically analysed using the Computational Fluid Dynamics (CFD) package which allowed validating the PDA data. The velocity of the drops compared well with those obtained from PDA. The liquid volume flux, however, was found, generally, between 26% < ULf < 90% compared with those of PDA data.