Pore pressure analysis of Oko, Olo and San Fields, Niger Delta

Abstract

An analysis of pore pressure in Oko, Olo and San fields in the Niger Delta have been carried out using relevant logs, check shot and biostratigraphic data. We obtained mud weight specific gravity data with depth from well completion report, calculated mud weight pressure gradient (psi /ft) and hydrostatic pressure. A plot of hydrostatic pressure and mud weight pressure were made to identify onset of overpressure. We later plotted gamma, sonic and density logs to ascertain compaction trend at points of onset overpressure. With the help of shale analysis from gamma ray log, applied to a porosity indicator log, the calculated pore pressure was then obtained by relating the deviation of the shale points from the trend line with an empirical equation. Finally, pore pressure and fracture gradient were estimated using Eaton’s relation. An examination of sonic log at Oko 001 (which normally decreases with depth) shows an observed departure to the right of the normal compaction trend line at 8585.41 ftss corresponding to 115.354s. The normal trend line corresponding to this point is 101.546 s. This was corroborated by the significant increase in the density of shale at the same depth (103.784 API). This marked the onset of overpressures. The pore pressure at this interval was obtained by reading the corresponding values on the respective curves (Tn and To) and applying in Eaton’s relation. The bulk density log has been integrated depth wise to obtain the overburden stress gradient. The Poisson’s and Stress ratio have been derived from Gamma ray log. The plots show the estimated pore pressure and fracture gradient curves for Oko 001. In areas where pore pressures were known, it was necessary to know both the pressure of the fluid and the pressure at which the formation will fracture. The Fracture pressure gradient was then calculated. The entire results have far reaching implications on well drilling.