Pore Pressure Estimation From Velocity Data: Accounting for Overpressure Mechanisms Besides Undercompaction
- Glenn L. Bowers (Exxon Production Research Co.)
- Document ID
- Society of Petroleum Engineers
- SPE Drilling & Completion
- Publication Date
- June 1995
- Document Type
- Journal Paper
- 89 - 95
- 1995. Society of Petroleum Engineers
- 2.4.3 Sand/Solids Control, 1.6 Drilling Operations, 3 Production and Well Operations, 5.5.11 Formation Testing (e.g., Wireline, LWD), 5.6.1 Open hole/cased hole log analysis, 1.14 Casing and Cementing, 4.1.2 Separation and Treating, 5.3.4 Integration of geomechanics in models, 1.10.1 Drill string components and drilling tools (tubulars, jars, subs, stabilisers, reamers, etc), 4.1.5 Processing Equipment
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A new method for estimating pore pressure from formation sonic velocity data is presented. Unlike previous techniques, this method accounts for excess pressure generated by both undercompaction, and fluid expansion mechanisms such as aquathermal pressuring, hydrocarbon maturation, clay diagenesis, and charging from other zones.
The method is an effective stress approach; the effective stress is computed from the velocity, and the result is subtracted from the overburden stress to obtain pore pressure. To include multiple sources of overpressure, a pair of velocity-vs.-effective-stress relations are introduced. One relation accounts for normal pressure and overpressure caused by undercompaction. The second is applied inside velocity reversal zones caused by fluid expansion mechanisms.
Example applications of the method are presented from the U.S. gulf coast, the Gulf of Mexico, and the Central North Sea. Some other pore pressure estimation approaches are also examined to demonstrate how these techniques have unknowingly accounted for overpressure mechanisms other than undercompaction.
It is also explained how velocity-vs.-effective-stress data can be used to identify the general cause of overpressure in an area. For instance, the empirical correlation of Hottman and Johnson indicates that overpressure along the U.S. gulf coast cannot be due only to undercompaction.
Numerous methods have been developed for estimating pore fluid pressure from geophysical data, and the list continues to grow. Empirical approaches equate departures from the normal trend line of some porosity-dependent measurement to an equivalent pore pressure gradient. Recent methods have followed the more fundamental effective stress approach pioneered by Foster and Whalen,1 Ham,2 and Eaton.3
All current pore pressure estimation methods fail to formally take into account the cause of overpressure. It will be demonstrated that this can lead to significant errors. For a given velocity at a given depth, the pore pressure can vary by 4 lbm/gal or more, depending upon how the excess pressure was generated.
This paper presents a method for estimating pore pressure from sonic velocity data that systematically accounts for the cause of pressure. When applied to wireline sonic logs, it is preferable to only use shale data to minimize the effects of lithology changes. However, the method is also applicable for pre-drill predictions from seismic interval velocities.
The paper begins with a review of fundamental aspects of shale compaction behavior that form the foundation of this method. This is followed by a discussion of how different causes of overpressure affect the sonic velocity. Some current pore pressure estimation methods are then examined in light of these concepts. The new method is then described, and example applications are presented and discussed.
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