Gelling

Abstract

Today wells are drilled in deeper and colder water. If the circulation is stopped and the drilling fluid is at rest, most drilling fluids will develop gel strength. In colder water the gel strength becomes higher. To resume circulation higher energy is required to break the gel strength. To break the gel inside the drill pipe the pump has to be applied to break the gel. This can lead to an increase of the bottom hole pressure and if it is high enough the formation can be damaged. The objective of this thesis is to investigate the gel breaking pressure with respect to pipe length. Our hypothesis is that the pressure will increase linearly with increasing pipe length up to certain length. From this length the pressure slope will flatten out. This is because the gel will than break in the top of the pipe first instead of over the whole pipe length. In present thesis a mathematical model for the height (Δh) needed to lift the pipe to break the gel is made. The model is based on theory that there are only shear resistance between the inner pipe wall and the drilling fluid that we need to overcome to break the gel. To overcome the shear resistance we are, using the gravity force by, lifting the pipe up. A laboratory experiment measuring Δh in 5 different length is being done. Water based mud with three different weight% of Bentonite is used as drilling fluid. A Fann viscometer is used to measure the gel strength and from these measurements the gel strength is used in the theoretical model to calculate the Δh. The laboratory investigation gave as a result that the Δh needed to break the gel is linear with respect to length for our chosen pipe lengths. The mathematical model gives us a linear increase in pressure with increasing pipe length, just like the laboratory investigation. But the Δh from the mathematical model is higher than the measured Δh from laboratory. This is because the standard API gel strength measurements are a poor procedure. The Fann viscometer is inaccurate in measuring the gel strength.