Pore-Pressure Prediction: Pitfalls in Using Porosity
Authors: Swarbrick R.E. 
Published: October 2008

Porosity based pore pressure prediction is based on mechanical compaction of fine-grained sediments with known compressibility behavior. The pitfalls of the prediction methods relate to several challenges in describing overpressured rocks. Firstly, techniques which compare porosity to a "normal compaction curve" assume the same lithology. Fine-grained sediments, in which most overpressure is sourced, are highly variable. Secondly porosity loss may not be solely due to mechanical compaction. Thirdly mechanical compaction should strictly be related to mean not vertical stress. Finally redistribution of fluids along inclined aquifers and up/down faults will modify the pressures. There are a number of emerging solutions to these challenges.

Introduction

Porosity is used as a rock property implicitly reflecting the degree of compaction (both mechanical and chemical) of sediment. Porosity is related to pore pressure through its relationship with effective stress. The porosity may be measured, derived from wireline response, or a porosity attribute may be used, for example velocity data derived from seismic. The methods employed in porosity based pore pressure prediction, first developed in areas such as the Gulf of Mexico, have been in use now for several decades. More recently the introduction of LWD/MWD tools has advanced the application of porosity-based pore pressure prediction to real-time. Furthermore newly emerging plays with high reserves replacement potential include the deep-water sediments of ocean margins worldwide, sub-salt reserves and high-pressure high-temperature environments, all of which are subject to significant overpressure. This paper examines the methodology and assumptions involved in extracting pore pressure prediction from wireline and seismic data sources. The pitfalls of the methods are reviewed and the challenges these pose for safe drilling.

It is widely known that different lithologies compact at different rates, and from contrasting starting porosities. Compaction is a function of the mean effective stress (Goulty, 1998; Harrold et al., 1999 and Figure 1a), although vertical effective stress is most frequently used in pore pressure estimation techniques as a proxy for mean effective stress. Compaction behavior is frequently described in pore pressure estimation using an Athy-type algorithm in which a starting porosity and compaction coefficient are related to depth and/or effective stress.

(A-1) ? = ? o   e − c ? '

Lithological variability is accounted for by "best fit" of the shallow data, assumed to be normally compacted if the porosity is decreasing with increasing depth. This "normal compaction curve" is used to compare actual porosity at any depth with its equivalent depth on the curve to estimate the effective stress and hence the pore pressure (Figure 1b). Soil mechanics relationships for porosity and effective stress are also used but similar limitations exist.

(A-2) ? ' = ? ' 100 exp ⁡ ( e 100 − e ) / ?

Both methods assume that the compaction is mechanical, and both can provide satisfactory pore pressure estimation when the origin of overpressure is disequilibrium compaction and the sediments are young and at low temperatures. Hence, porosity based pore pressure prediction works best in low temperature, young sediments, in which the lithology remains similar and where an upper shallow section exhibits a recognizable "normal compaction curve" or trend.