ABSTRACT The use of offset mud weight in combination with geologic information and drilling operations report can be very useful in estimating formation pressure in appraisal and exploratory well planning. It has been established that conventional shale pressure modeling is an effective tool for pore pressure prediction William Standifird et al 2005. The model can be updated as new data is acquired to reduce the uncertainty window and optimize drilling. This paper aims to generate shale based pore pressure profiles for the Sahara wells in northern depositional mega structure of the Niger delta using equivalent depth method and compare shale based gradient with predicted gradient from offset mud weight
The offset mud weight was expressed in pressure gradients allowing for computation of hydrostatic head of the mud and subsequently backing out the overbalance. This provided a fair estimate of the formation pressure. The estimated pressure was corrected for hydrocarbon buoyancy. The fracture gradient was derived from the leak off test of the offset well data. The lithostatic gradient is based on the regional gradient for the northern depo belt of Niger delta of Nigeria. In the shale based pore pressure estimate, the overburden gradient was modeled by superimposing the density log of the wells due to the paucity of density log. The Leak off Test of two offset wells was also utilised used to define the fracture gradient. Equivalent depth method was used in the computation of the pore pressure in the three wells
Comparison of the formation pressure profile generated using the offset mud weight from X, Y and Z wells and shale based pore pressure prediction using proprietary software indicated the two methods shows tolerable difference fit for purpose well planning.