Prediction of Formation Pore Pressures for Tophole Well Integrity

摘要

Knowledge about formation pore pressures is important for planning and control of offshore well drilling operations.rnParticularly, a combination of unconsolidated clay/shale sediments and in-situ pore pressures exceeding hydrostatic values canrncreate a distinctly adverse setting for tophole well integrity. If unanticipated, well completion may fail.rnPore pressure prediction for the tophole section generally relies on use of multiple methods, as no single technology isrnavailable. Actual measurement of in-situ pore pressure is necessary. Pore Pressure Dissipation Testing PPDT with a push-inrnpiezoprobe or piezocone penetrometer is currently the most efficient and accurate method for actual measurement of formationrnpore pressure in unconsolidated sediments. The method can be used in a pre-well site survey, during pre-setting a tophole wellrnsection or combined with well installation itself.rnCapabilities and limitations are highlighted for predicting and accurately measuring formation pore pressures in shallowrnunconsolidated clay/shale sediments. Use is made of a unique database containing more than 160 PPDT piezoprobernmeasurements. The database probably covers a majority of piezoprobe measurements ever completed for offshore topholernsections. It includes (1) hydrostatic pressure profiles, (2) excess pore pressures due to natural phenomena and (3) excess porernpressures due to formation damage caused by earlier well drilling. Depth coverage is up to 400 m below seafloor.rnAchievable accuracy for measurement of pore pressure is typically estimated at about 50 kPa (= 5 m water column) or 5%rnof penetration pore pressure, whichever is the greater. Difficulties are noted for establishing formation pore pressures wherern(1) clay/shale sediments have been affected by formation damage caused by earlier drilling and (2) clay/shale sedimentsrninclude shallow gas zones. Opportunities for improvement are discussed.