Critical Review of the Effect of Water Saturation Variation on Pore Pressure Estimation Technique

Pore pressure estimation is a critical process in terms of safety and economy of time and money during exploration, drilling and production operations. The knowledge of pore pressure gradients is invaluable in modelling correct mud weights and designing casing configuration for well construction. The properly estimated pore pressure gradients allow avoiding, or at least mitigating, the risks associated with drilling such as pressure kicks, well blowouts, formation damage and loss of circulation. Conventional well-log based methods of pore pressure prediction convert porosity indicators, such as sonic travel time/velocity and resistivity, into pore pressure estimates. The methods are developed in application to normal water bearing shale with 100% water saturation. This research study reviewed the effect of gas presence and associated water saturation variation in unconventional shale gas formations on conventional pore-pressure estimation technique, as well as on workflow of pore pressure analysis. The review of previous publications discovered that: (a) in normal shale, water saturation always equals unity and it does not affect a pore-pressure estimation technique; (b) in unconventional shale gas formations, water saturation alteration, through the gas presence, indirectly affects the pore-pressure estimation technique reducing its accuracy of prediction. Particularly, it causes the sonic compressional velocity (Vp) to be slower / transit time (Δtc) to increase. However, strictly speaking, the original cause of this effect is the gas presence, and water saturation alteration is associated with the gas saturation (Sw = 1 – Sg). To correct the gas effect on compressional velocity (Vp), a downhole measured shear velocity (Vs) should be used due to its low response to gas in porous medium. The method using corrected compressional velocity is applicable to predict pore pressure both in unconventional shale gas formations and in gas-bearing formations in conventional reservoirs. The inspection of correlation between water saturation and pore pressure, calculated after the correction of gas effect, in such kind of reservoir as shale gas formation, following the proposed methodology, was given as a recommendation for future project work.