Analysis of Melt Water Freezing in the Ice Borehole

The remote sites and difficult environment cause ice drilling in cold, deep ice to be very time consuming and costly. One of the potential threats to the drill is freezing in the bottom of an ice borehole due to melt water. Two basic problems to be solved in studying the rate of melt water freezing are: 1) What is the drill temperature change as it is lowered into the borehole? and 2) How does the melt water freeze around the drill?

The heat exchange between the drill and the surrounding drill fluid is dominated by convection as the drill descends in the borehole. An analytical solution of the drill temperature change during descent was obtained by assuming the temperature varies only with time because the drill pipes (stainless steel) are very thin and have relatively high thermal conductivity. In the upper length of a borehole, it would take two minutes for the drill to change temperature by 20 degrees Celsius. During descent in the lower part of the borehole, the drill has a temperature lag behind that of the surrounding drill fluid, but only by a very small amount. For a descent speed of 0.53 m/s, the drill temperature change can be assumed to be the same as the surrounding fluid.

The melt water freezing around the drill in the bottom of a borehole is an axisymmetric, multiple phase-change problem. A finite element model has been developed to handle this special problem. An isoparametric element was used. The latent heat effects were taken into account through a Dirac delta function in the heat capacity. The Crank-Nicolson method (Θ = 0.5) was used in the transient process modeling. Modeling results show that the freezing fronts close up first in the space between the outer pipe and the ice wall. For ice temperature of -5 degrees Celsius, it would take 75 minutes to freeze up. Freezing of slush with a different percentage (in volume) of ice chips in the melt water was also modeled. The results show that the freezing-up time was reduced 0.44 times, from 75 minutes for pure water to 35 minutes for 75% ice chips in the water, at ice temperature of -5 degrees Celsius. One may reduce the possibility of the drill freezing in the bottom of a borehole by slowly lowering the drill to the bottom of the borehole and/or moving the drill up and down when it reaches the bottom of a borehole.