STEVE O’CONNOR**, GUY MARKHAM**, CHRIS WARD**, BITRUS PINDAR**, NANCY UMOREN**, ADEDAYO ADEBAYO*, KINGSLEY NWANKWOAGU*, JEROME ASEDEGBEGA**, OLUSOLA SCOTT-OGUNKOYA** AND GBENGA LUFADEJU***
The onshore Niger Delta is divided into a series of depo-belts (Upper Miocene, Middle Miocene, Oligocene-Lower Miocene and Eocene). Density data shows low porosity shales are present at similar depths across individual depo-belts. This appears related to age of the depo-centre i.e. how long the shales have been in-situ. High observed densities, beyond those achieved purely by mechanical compaction are not consistent with simple disequilibrium compaction where porosity is retained with depth, and are suggestive of additional mechanisms such as unloading (elastic or inelastic).
Shale pressure prediction models were developed for each depo-belt, utilising relationships between velocity and vertical effective stress, using 37 wells. 25 blind test wells were tested, and the resulting shale pressure predictions gave results close to observed drilling history and matched kicks encountered. Each depo-belt has shallow and deep loading trends defined, linked to sedimentation rate and density. Unloading trends were also defined for each depo-belt, the onset being linked to temperature. Hence for any un-drilled location Onshore, an estimate of shale pressures (and if un-drained, also reservoir pressures) can be produced.
Historically many wells in the Onshore Niger Delta have terminated drilling at the occurrence of the first kick. These models will allow for a more robust understanding of shale pressures, allowing wells to reach their planned termination depth on a regular basis. This becomes even more critical as current wells are drilling to depths of over 20,000 feet below the surface in true HP/HT conditions and are hence very expensive.
* Sonar/Tusk, Lagos **Ikon Geopressure, UK and Lagos ***DPR