Pressure Prediction with Flowline Temperature Gradients
Authors: Wilson G.J. , Bush R.E. 
Published: December 2007

The procedure described here is applicable anywhere in the world, from the Louisiana Gulf Coast to the South China Sea. However, in using this method to predict the existence of overpressured shale and the first overpressured sand below a shale interval, one must carefully consider many factors affecting flowline temperature response to changes in geotemperature.

Introduction
Abnormal pore pressure is found at very shallow depths in several areas of the world and very short pressure transition zones are often encountered. At pressure transition zones are often encountered. At one time, all methods of determining the presence of abnormal pressure depended upon drilling data accumulated after the bit had reached the transition zone or upon after-the-fact analysis of wireline logs. All methods have inherent limitations that create additional difficulties in detecting higher pressure either as the depth of abnormal pressure decreases, or as the length of the transition zone decreases, or both. A complementary procedure was needed that would not be dependent upon the amount of mud weight overbalance, upon penetration far into the transition zone, upon the nature of the rock matrix, or upon correlations of log parameters with pressure values. A method satisfying these requirements was suggested by variations observed in surface mud temperature. Detailed studies resulted in the development of a pressure prediction and detection procedure that pressure prediction and detection procedure that utilizes relationships between pore pressure and mud flowline temperature. This paper reviews theory presently associated with the flowline temperature presently associated with the flowline temperature method, discusses data collection and interpretation, and suggests further exploitation.

Theory
The geothermal gradient or the rate at which subsurface temperature increases with depth can be defined by
...........(1)
where the gradient is expressed in F per 100 ft of depth. For any given area, the geotemperature gradient is usually assumed to be constant. While the average gradient across normal pressured horizons may be constant, higher-than-normal geothermal gradients have been found to be associated with pore-pressure gradients greater than normal. Lewis and Rose presented a theory to explain how high-pressure zones presented a theory to explain how high-pressure zones relate to departures of the geothermal gradient from the normal value. Practically all heat flowing within the earth moves outward from the molten core and is radiated into space. The total heat flowing across all depth increments is constant, but the temperature differential across any increment depends upon the ability of materials in the increment to conduct beat. Below a formation whose conductivity is relatively low, heat builds up until the temperature differential across the formation allows heat flow through the insulator to equal the flow of incoming heat. Accordingly, higher geothermal gradients must exist across insulating formations and relatively lower temperature gradients are present in formations with relatively higher conductivities.
JPT
P. 135