innovation, page-33

Viscosity Effects.

While Murphy's Law would suggest that in any action that you attempt to do the laws of nature will always try to thwart you, there are in fact some effects that work in your favour. One of those was discussed here a few weeks ago on how the thermosiphen effect gave us a minor but still significant increase in flow efficiency.

For some time I have been musing over the effects of the viscosity of the downhole brine solution. Viscosity is of course the 'thickness' or resistance to flow of a liquid. Honey flows sluggishly and is difficult to suck up through a straw but it helps if it is heated somewhat. (Glass is also a high viscosity liquid.) The viscosity of liquids generally varies as a rapid function of temperature. We tend to think water as being always just water, but it also turns out to be very temperature dependent. Around room temperature (20 C) and a bit higher the viscosity does not change much but by 100 C it has reduced to 1/4. It gets more interesting down the well at about 300 C where the value has dropped to 1/10 of that at room temperature. This helps to understand how it is possible to get reasonable flows through micro fissures between wells 1000m apart. The higher the temperature the greater the flow efficiency. Lower viscosity means fluid movement in finer pores and turbulence in smaller depressions and around edges thus achieving greater heat transfer.

An even more dramatic action occurs at 300 C where the viscosity passes through an inflection point and it begins to fall even more rapidly so that at 400 C it has collapsed to a value of just 3% of that at room temperature. There are lots of interesting implications from this. What are they? How can GDY benefit from this?


1. It becomes less surprising that rapid flows can be sustained around 12-15 km of fluid circuit in an 8 inch pipe.

2. There should be a considerable greater impedance to flow on the injection (cooler, 95 C) side of the circuit than on the production side.

3. While one might intuitively expect a slower flow on the cooler side gradually increasing as the brine heats up, this cannot be the case, as outflow must equal inflow.

4. Minor fracture zones may become operable under extremely low coefficients of viscosity.

5. Over years, all other parameters being the same, constant flow rates will not be maintained. Estimates from modelling for GDY suggest a depletion rate of 40 C over 10 years. Were changing viscosity effects taken into account in their modelling? Note that this is not a dramatic problem at 5000 m as further stimulation can make a compensatory increase in the gap width to restore the desired flow rate.

6. Does artificial stimulation always work? i.e. Do the natural micro fractures always lead to an increase in the gap width? What happens at greater depth (~7000 + m ) where some thermoplastic yielding effects might become evident? If the gap width cannot be maintained after the usual fraccing pressure release we may be able to rely on the natural rapid decrease in viscosity with increasing depth (read temperature).

Juke