A Theory Relating High Temperatures and Overpressures

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Abstract
The theory proposed here is that an overpressured zone, because it is uncompacted, constitutes a thermal barrier. That is, this uncompacted zone contains excess amounts of water, and water has three times the insulating value of the rock matrix. It follows that the greater the water content, the greater the insulating value of the zone. Introduction: For any drilling area, geothermal gradient maps have been made that attempt to show the "normal" temperature gradients that will be found. Along the Texas Gulf Coast these will range from 1.6 degrees to 2.2 degrees F/100 ft. Most of the time the temperature profile for a new well can be predicted from one of these maps. At times, however, we find that the temperature gradient changes with depth. Other authors have observed that in the Gulf Coast region the overpressured zones also have abnormal temperature gradients. The fact that abnormally high gradients seem always to occur with overpressured zones is both significant and unexplained. We advance a theory to relate high pressure zones and deviations from normal temperature profiles. profiles. The Theory It is generally agreed that almost all of the heat flowing within the earth comes from the core and moves outward to the surface where it is lost into space as radiant energy. Wherever flowing heat meets an obstacle (an insulator) there is a buildup of heat against its face. The temperature rises on this face until a higher temperature gradient exists across the obstacle. Enough heat will then flow through the insulator to balance the flow of incoming heat. It is clear that a higher temperature gradient must be imposed across a heat insulator than across a heat conductor to achieve equal heat-flow rates. Therefore, if we observe a very high gradient within the earth, we would normally conclude that the high gradient zone is an insulator. Our theory is that high pressure zones are heat insulators; therefore, we expect abnormal temperature gradients to exist across them. An analytical proof follows. proof follows. We must show that a high-pressure zone is in fact an insulator. A high-pressure zone differs structurally from the surrounding rock in that it is less compacted. There is much more pore space to be occupied by fluid. Thus, an overpressured, uncompacted rock has considerably more water than a normally pressured, compacted rock. The conduction of heat is expressed by where is the heat flow, kh is the coefficient of thermal conductivity of the substance, A is the cross-sectional area of the substance, measured perpendicular to the flow of beat, delta T is the perpendicular to the flow of beat, delta T is the temperature change across the substance, and L is the length (thickness) of the substance. It can be seen that varies directly with kh. Thus, if water has a lower value of kh than silica, water is the better insulator. JPT P. 11

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