The Basic Physics of Water Penetration into Hot Rock

Abstract

There are a number of rival ideas about how heat is transferred to the hot fluids of major geothermal areas. One of these is the theory of water penetration into hot rock, where thermal shrinkage of the rocks permits them to crack on a relatively small scale and then transport heat away from the hot boundary thus formed. The theory has the advantage of requiring a relatively small area of contact between water and hot rock to produce the high hermal outputs of large geothermal areas: of the order of 1 km². This is because the thermal boundary layer is established by the advance of a cracking front into the rock itself, and is a function of front velocity rather than preexisting geological structure. A critical review of the physics of the process shows that the weakest areas of the theory are in the understanding of the mechanics of the cracking process and in the structure of the porous-medium convection that discharges the heat. The problems in both these areas stem from a basic lack of knowledge of the physics, and not from weaknesses in the theory itself. The one-dimensionality of the treatment is not a serious limitation because the predicted crack spacing, of the order of a meter, is much smaller than the kilometer scale of three-dimensionality in the geometry.

The theory predicts correctly the order-of-magnitude of the hot water temperature even when data not appropriate to mafic crustal rocks has to be used. Examination of exposed regions of ophiolite suites that should have undergone the cracking process is at too primitive a stage to confirm or disprove the theory. A partially-controlled experiment where large volumes of water were pumped onto an advancing lava flow confirms that the cracking process does take place, and the numbers are in general agreement with the theory in spite of conditions being substantially different from those treated in the calculations.