Measurement of Formation Pressure from Drilling Data
Authors: Rehm Bill, , McClendon Ray 
Published: December 2007

This paper was prepared for the 46th Annual Fall Meeting of the Society of Petroleum Engineers of AIME, to be held in New Orleans, La., Oct. 3–6, 1971. 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 paper is presented. Publication elsewhere after publication in the JOURNAL OF PETROLEUM TECHNOLOGY or the SOCIETY OF PETROLEUM ENGINEERS JOURNAL is usually granted upon request to the Editor of the appropriate journal provided agreement to give proper credit is made. provided agreement to give proper credit is made. Discussion of this paper is invited. Three copies of any discussion should be sent to the Society of Petroleum Engineers office. Such discussion may be presented at the above meeting and, with the paper, may be considered for publication in one of the two SPE magazines.

Abstract
A series of drilling equations have been proposed for the measurement of formation pressure. proposed for the measurement of formation pressure. They appear to work well and are as accurate as log-derived pressure techniques. While it probably is not possible to develop one equation that will work under all conditions, one of the series should work quite well where good drilling practices are used. Calculations have been made using these methods in all major drilling areas of the free world and accuracies approaching 0.2 lb/gal have been attained.

Introduction
There are two reasons why a drilling/pressure equation should work.The basic theory of abnormal pressure due to porosity or compaction indicates that as the bit enters the high pressure zone, the rock becomes more porous and less dense than it was previously. This has been proved by the methods previously. This has been proved by the methods of determining pressures from well logs, shale densities and seismic velocities. The principle of variation from normal was proposed by Hottman and Johnson in 1951. They firmly established not only that variation from normal was an indicator of high pressure, but that high pressure could be measured by the departure of the velocity from what might be expected to be normal velocity for the depth under consideration.Since velocity is intimately related to rock density, the techniques involved in determining pressure from velocity or density should work equally well with drilling rate insofar as drilling rate is related to density or porosity.The other drilling rate/pressure factor involved is the effect of differential pressure. As the differential pressure between the wellbore and formation decreased, drilling rate increases. The increase approaches a hyperbolic function and often has an inflection point at about 500 psi overpressure against the formation (Fig. 1).
It may be noted that both phenomena work in the same direction at the same time. Reduction in differential pressure tends to come with the entrance into the overpressured zone of greater porosity and both increase the drilling rate. porosity and both increase the drilling rate. However, it appears that the differential pressure phenomenon is the more sensitive function. phenomenon is the more sensitive function. These observation led to the belief that not only was the determination of overpressured zones possible, but that it was possible to determine formation pressures from drilling rate. The main difficulty appeared to be resolving the sensitivities involved in the drilling rate function.