A study of annulus lubrication for oil well completion using scale model tests

When an oil reservoir reaches the end of its production life the well which has been drilled to extract the oil must undergo completion or ldquowell abandonmentrdquo. Abandonment is the process whereby all the productive zones within the well are isolated with cement, removing some or all of the production tubing and setting a surface plug in the well with the top of the plug between 30 and 50 m below the mudline. The process of abandonment can be hampered if it is not possible to remove pipes from within the well casing. It is proposed that by lubricating the annulus, the space between the casing and pipe, it may be possible to extract problematic pipe work by reaching pressure equilibrium between the reservoir and annulus. Experiments have been carried out to investigate the mixing time of a heavy brine with water. This mixing is known as lubrication. Lubrication can be used to increase the pressure (or fluid weight) at the bottom of a well. When heavy brine is added at the top of a column of water the fluids will mix and the resulting fluid has a greater density than that of water which increases the pressure at the bottom of the well. The experiments were carried out in test rigs comprising of a clear plastic tube sealed at the lower end and connected to a Perspex tank by a ball valve at the top of the tube. The dimensions of the test rigs were chosen to be a scale representation of a typical oil well. The well geometry was simplified with the upper part of the tube vertical and the lower part of the tube inclined at an angle to the vertical with the joint occurring at roughly 60 % of the vertical distance from the top. The diameter of the tubing could not be scaled using the same length scale and so instead a number of tube diameters were tested. The ratio of fluid volumes above and below the circulation point was conserved with the brine volume being equal to 75 % of the water volume; i.e. a 3:4 ratio. The pressure at the lower end of the tube was measured usi- ng a piezoelectric sensor inserted through the centre of the plug sealing the end of the tube. Tests were carried out for 10 mm, 20 mm, 40 mm, 60 mm and 100 mm diameter tubes. The aim of these tests was to demonstrate the relationship between tube diameter and time taken to reach equilibrium. For all of these tests the total vertical distance from the top of the tube to the pressure sensor was 3.0 m. Additional experiments were carried out to establish if the relationship between tube length and time taken to reach equilibrium is linear. Straight lengths of 1.0 m, 1.5 m, 2.0 m, 2.5 m, and 3.0 m of 40 mm diameter tube were tested. Initially the mixing of the brine and the water is rapid but as the time from valve opening increases the rate of change of pressure with time reduces approaching the final value asymptotically. This makes it unlikely that the lubrication process will be carried out in order to reach the final pressure and so results are presented in terms of the time taken to reach 75 % of the final pressure. Increasing the tube diameter was shown to reduce the time required to reach 75 % of the final pressure. It was also established that there is not a linear relationship between tube length and time taken to reach equilibrium. This demonstrates that whilst lubrication can be used to increase the pressure at the bottom of an oil well the time scale needed to produce a significant increase in pressure is very large.