Abstract
Understanding the distribution and variation of subsurface formation pressure is key to preventing geo-hazards associated with drilling activities such as kicks and blow out. To assess and prevent such risk in drilling offset wells in the Hamoru field, prediction of pore pressure was done to understand the pressure regime of the field using well logs in the absence of seismic data. Two commonly used methods for formation pressure prediction; Bower’s and Eaton’s methods were adopted to predict pore pressure and determine the better of the two methods that will be more suitable for the field. The cross-plot of Vp against density disclosed that compaction disequilibrium is the prevalent overpressure mechanism. The prediction of Pore pressure with Eaton’s method gave results comparable to the acquired pressure in the field, typical of what is expected when compaction disequilibrium is the dominant overpressure mechanism. Since the result of Bower’s method over estimated formation pressure, Eaton’s method appears to be the better choice for predicting the formation pore pressure in the field. Analysis of the predicted pore pressure reveals the onset of overpressure at depth of 2.44 km. The formation pressure gradient ranges from 10.4 kPa/m to 15.2 kPa/m interpreted as mild to moderately over pressure.
 Keywords: Geohazard, over-pressure, Eaton’s method, Bower’s method, normal compaction trend
Highlights
Formation pore pressure consideration is vital for drilling plan, geo-mechanical and geological evaluation to prevent some geological hazards (Jincai, 2011)
The well data available for this study was obtained from Shell Petroleum Development Company (SPDC)
Well log data were used to predict formation pore pressure using Eaton’s and Bower’s method to determine the better of the two methods to adopt for pore pressure prediction in the Hamoru field in order to understand the pressure regimes of the field to prevent geo-hazards during drilling of any proposed offset wells in the field
Summary
Formation pore pressure consideration is vital for drilling plan, geo-mechanical and geological evaluation to prevent some geological hazards (Jincai, 2011). The fluid pressure in rock pore spaces is described as pore pressure. It varies from normal or hydrostatic pressure; to critical overpressure i.e. when the formation pressure greatly exceed the regional hydrostatic pressure. Prediction of formation pore-pressure obtained from shale properties acquired from wire line log were probably first attempted by Hottmann and Johnson (1965). They indicated that as pore fluid is expelled, there is a reduction in porosity as depth increases with sediments compact normally. If the rate of sedimentation is very high, compaction disequilibrium occurs which may lead to overpressure build up (Osborne and Swarbrick, 1997; Mouchet and Mitchell, 1989)
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