Prediction of Formation Pore Pressures for Tophole Well Integrity
Authors: Peuchen Joek , Klein Martijn 
Published: May 2011

Knowledge about formation pore pressures is important for planning and control of offshore well drilling operations. Particularly, a combination of unconsolidated clay/shale sediments and in-situ pore pressures exceeding hydrostatic values can create a distinctly adverse setting for tophole well integrity. If unanticipated, well completion may fail.

Pore pressure prediction for the tophole section generally relies on use of multiple methods, as no single technology is available. Actual measurement of in-situ pore pressure is necessary. Pore Pressure Dissipation Testing PPDT with a push-in piezoprobe or piezocone penetrometer is currently the most efficient and accurate method for actual measurement of formation pore pressure in unconsolidated sediments. The method can be used in a pre-well site survey, during pre-setting a tophole well section or combined with well installation itself.

Capabilities and limitations are highlighted for predicting and accurately measuring formation pore pressures in shallow unconsolidated clay/shale sediments. Use is made of a unique database containing more than 160 PPDT piezoprobe measurements. The database probably covers a majority of piezoprobe measurements ever completed for offshore tophole sections. It includes (1) hydrostatic pressure profiles, (2) excess pore pressures due to natural phenomena and (3) excess pore pressures due to formation damage caused by earlier well drilling. Depth coverage is up to 400 m below seafloor.

Achievable accuracy for measurement of pore pressure is typically estimated at about 50 kPa (= 5 m water column) or 5% of penetration pore pressure, whichever is the greater. Difficulties are noted for establishing formation pore pressures where (1) clay/shale sediments have been affected by formation damage caused by earlier drilling and (2) clay/shale sediments include shallow gas zones. Opportunities for improvement are discussed.

Introduction

Knowledge about formation pore pressures is important for planning and control of offshore well drilling operations. Particularly, a combination of unconsolidated sediments and in-situ pore pressures exceeding hydrostatic values can create a distinctly adverse setting for tophole well integrity. If unanticipated, well completion may fail.

The tophole section is defined here as the zone down to 200 m to 400 m below seafloor. This section typically shows unconsolidated clay/shale sediments, particularly for offshore wells installed in water depths exceeding 200 m. This section can also include unconsolidated and uncemented sand layers.

A typical offshore well will cover the tophole section by a conductor and a surface casing (Figure 1). The conductor may be installed by (1) open hole drilling or jetting followed by cementing, (2) impact driving or (3) a combination of driving, drilling, jetting and cementing. Installation of the surface casing is by drilling and cementing. Tophole well installation by impact driving would mitigate integrity issues. However, this installation method is feasible only to typically 80 m depth below seafloor. Conductor driving to about 200 m depth has been achieved for exceptionally weak formations. The driving depth limitation is because of high steel stresses, fatigue damage and directional stability issues during conductor installation.