Predicted vs. Measured Pore Pressure: Pitfalls and Perceptions
Authors: Shaker Selim S. 
Published: October 2008

There can be great confusion between predicted and measured pore pressure. Pore pressure (PP) is measured in the reservoir quality rocks (sand) and is predicted in relatively impermeable beds (shale and clay). The mistake of assuming there is immediacy between the pressure in the sand and the sandwiching shale leads to serious drilling problems. Understanding the geological setting and the expected hydrocarbon presence helps establish this relationship. Foreseeing pressure shift across bedding interfaces is beneficial for explorationists and drillers.

Introduction

Subsurface compartmentalization dictates the development of the pore pressure profile. In relatively clastic young (Pleistocene - Tertiary) sediments, porosity indices such as sonic slowness is widely used to calculate predicted pore pressure (PPP) in the shale and clay. Geopressure analysis pioneers (Eaton, 1975; Matthews and Kelly, 1967) used rock mechanic effective stress in relation to the change of petrophysical properties to establish several empirical pore pressure calculation methods in the Gulf of Mexico coastalbelt. Sonic, resistivity and density are used during and post drilling to estimate PPP. Prior to drilling, seismic velocities are very helpful in estimating PPP.

In reservoir type rock (sand, sandstone, oolites,.etc.) pore pressure (actual pore pressure) is measured using wire-line tools and drilling stem gauges. The measured pore pressure (MPP) in wet sand usually follows the main hydrostatic gradient in the region as long as the formation water density stays the same. The MPP in pay zones follows the gradient of the hydrocarbon which, in turn, depends on the density of the oil or gas column.

The envelope shift in the pressure gradient (PG) between two compartments usually takes place in the seals (shale and clay). Most of the bore-hole problems such as sloughing shale, enlarged and backed off-hole take place where the PG slope changes from linear in the reservoir to exponential in the shale, especially near the seal base. Blow-outs, kicks, flow-kill-breakdown and loss of circulation problems usually occur along the interface zone between the seal and thecompartment below. The structural setting of a prospect and the fault plane lithology juxtaposition play a substantial role in pressure differential distribution in sand vs. shale. On the structural crest MPP usually exceeds the PPP and in trough position it is vice versa. The presence of hydrocarbon, especially gas, usually leads to a significant increase of the MPP in the reservoir relative to the PPP calculated in the seal.

It is important to recognize and diagnose the difference between PPP and MPP. Pore pressure rectification across beds interface due to geological environment, structure setting and expected hydrocarbon presence is vital for successful drilling and exploration programs.

Concepts and causes

The divergence between the pore pressure in the shale (PPP) and the pore pressure in the sand (MPP) is a consequence of compaction and entrapment of the formation fluids in the geopressured section. MPP usually exhibits the hydrostaticgradient of the formation fluid (0.46 psi/ft in GOM) and progresses in a cascade fashion with depth.