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Causes of disparity between predicted and measured pore pressure

Geopressure Analysis Services, Houston, Texas

Corresponding author: ssshaker@compuserve.com

The first 20% of the full text of this article appears below.

The term pore pressure can be confusing. Pore pressure is predicted in relatively impermeable beds (shale and clay) and measured in reservoir quality rocks (sand). However, in many occasions, there is not a direct link between the pressure in the sand and in the sandwiching shale. Assuming such an immediacy can lead to serious drilling problems. The relationship between predicted (PPP) and measured (MPP) pore pressure is often complicated and establishing the relationship involves analysis of the geologic setting and expected hydrocarbons. This article will discuss the various causes for differing PPP and MPP and construction of an accurate pore pressure profile.

In relatively clastic young sediments (Pleistocene-Tertiary), porosity indices (e.g., sonic slowness) are widely used to calculate PPP. Prior to drilling, seismic velocities are very helpful in estimating PPP.

In reservoir type rock (sand, sandstone, oolites, etc.), pore pressure is measured using wireline tools and drilling stem gauges. MPP in wet sand usually follows the main hydrostatic gradient in the region as long as the formation water density stays the same. 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 borehole problems (e.g., sloughing shale, enlarged and backed off-hole) take place when 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 the compartment below.

The age of the deposits, rate of sedimentation versus rate of accommodation, structural setting of a prospect, and the fault plane lithology juxtaposition play a substantial role in pressure differential distribution in . . . [Full Text of this Article]