Estimating Abnormal Pressures from Log Data in the North Sea
Authors: Herring E.A. 
Published: December 2007

This paper was prepared for the Second Annual European Meeting of the Society of Petroleum Engineers of AIME, to be held in London, England, April 2–3, 1973. Permission to copy is restricted to an abstract of not more than 300 words. Illustrations may not be copied. The abstract should contain conspicuous acknowledgment of where and by whom the paper is presented. Publication elsewhere after publication in the JOURNAL OF PETROLEUM TECHNOLOGY or the SOCIETY OF publication in the JOURNAL OF PETROLEUM TECHNOLOGY or the SOCIETY OF PETROLEUM ENGINEERS JOURNAL is usually granted upon request to the Editor PETROLEUM ENGINEERS JOURNAL is usually granted upon request to the Editor of the appropriate journal provided agreement to give proper credit is made.

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The use of log derived shale properties to predict pore pressures of drilled or about to be drilled formations is well known and widely practiced. The technique is also applied to seismic data to predict abnormally pressured formations before a well is drilled. These techniques are applicable to North Sea drilling operations, particularly in Northern waters. Based on data from twenty two wells, a correlation of pore pressure and sonic log measurements in pressure and sonic log measurements in shales has been made. This correlation permits a reasonably accurate prediction permits a reasonably accurate prediction of pore pressures both before and during drilling operations.

Introduction

The existence of abnormally pressured formations in the North Sea is well known, particularly in the waters above 55 degrees particularly in the waters above 55 degrees North Latitude. Conoco and Partners have drilled seven wells in this area since 1971 in both United Kingdom and Norwegian waters. The data from twenty two wells was analysed in an attempt to find a correlation between well log measurements and formation pressure. Figure 1 shows the area of interest in the North Sea.

This paper gives several illustrations of abnormal pressure in this area of the North Sea and describes the technique used to develop a correlation between shale acoustic travel time and formation pressure. The results of this work are applicable to both sonic log and seismic measurements of travel time. Although the data sample available for this analysis was small, the correlation gives us some confidence in our ability to predict pressures both before and during drilling operations.

THEORY

The technique used to analyse the well data is that presented by Hottman and Johnson in 1965. The paper by Hottman and Johnson contains an excellent discussion of the compaction theory of abnormal pressure origins and the application of the theory to the Tertiary sediments of the Gulf of Mexico. Briefly, the theory is that abnormally pressured formations are undercompacted pressured formations are undercompacted because the fluid removal is restricted by impermeable beds.