Determining an Appropriate Pore-Pressure Estimation Strategy
Authors: Bowers Glenn L. 
Published: October 2008

A procedure is presented for using wireline or MWD data to identify when high pressure techniques are necessary to predict pore pressures inside velocity reversals. Shale sonic velocity and density data are smoothed and cross-plotted. If the reversal data lie on the same trend as points from lower pressure intervals, shale pore pressures can be computed with an Equivalent Depth approach. A high pressure technique is necessary when the reversal velocities track a slower trend.

Introduction

An indicator of possibly high overpressure is a velocity reversal, i.e., a zone in which the velocities all drop below the value at some shallower depth. When velocity-effective stress data from a reversal diverge from the compaction trend defined by shallower formations, the Equivalent Depth method can significantly underestimate pore pressures (Fig. 1). The same will hold true for any pore pressure estimation method that relies upon a single velocity-effective stress relation.

Bowers (1995) discussed ways to account for high pressure situations. For instance, a second velocity-effective stress relation can be introduced, the exponent in Eaton's pore pressure equation (1975) can be increased, or the standard Eaton equation can be combined with an exponential normal trend (Fig. 2). Wilhelm (1998) employs velocity effective stress relations that steepen with decreasing cation exchange capacity (CEC) and, to a lesser degree, increasing temperature (Fig. 3). Temperature defines the shape of the shallow normal trend curve. High pore pressures are matched by reducing the CEC values for an interval.

Not all velocity reversals warrant high pressure techniques (Bowers, 1995). In cases where the Equivalent Depth approach is appropriate, high pressure methods can cause pore pressures to be significantly overestimated. Therefore, it is important to have some systematic way to determine which type of pore pressure approach is required within a velocity reversal.

Until recently, the best criterion for determining an appropriate pore pressure method was thought to be a direct comparison between computed and measured pore pressures. However, with dipping sands having good hydraulic continuity and significant vertical structure (Fig. 4), so-called "centroid" effects can cause mismatches between the pore pressure measured in a sand and those calculated in nearby shales (Traugott, 1997; Stump, et. al., 1998). Calibrating a pore pressure estimation method to match crestal or basal sand pressure measurements can lead to erroneous results at other depths. Therefore, it is also important to know when a pore pressure estimation method should not match observed pressures.

This paper presents a procedure for using a crossplot of shale sonic velocity vs. density data to identify when high pressure techniques are required inside velocity reversals, and when the Equivalent Depth method is appropriate even when it fails to match measured pressures. The paper begins with a review of the theory the procedure is based upon. The procedure is then presented, followed by a discussion on how velocity-density crossplots can be used to refine the Bowers (1995) pore pressure estimation method. An example from Eugene Island 330 is then presented and discussed.