A new quantitative model and application for overpressure prediction in carbonate formation.

The prediction of pore pressure in subsurface formations is important for the estimation of hydrocarbon accumulation and for safe drilling activities. Typically, pore pressure is estimated from either well logs (e.g., P-wave velocity, resistivity) or seismic data (seismic velocities, acoustic impedance) using empirical relations based on compaction laws. Due to the high density and extremely heterogeneous physical properties, it's not feasible to calculated pore pressure in carbonate rocks with these empirical methods. The result of ultrasonic velocity measurements in carbonate rocks shows that velocity is affected by multiple factors, e.g., stress condition, porosity, rock composition, fluid composition, and other rock properties. It's practicable to predict pore pressure with velocity when other factors are kept constant or evaluated quantitatively. In this paper, a quantitative model to compute the elastic attributes as a function of pressure and petrophysical properties is proposed. The model as a rock physical model is based on the poroelasticity theory and describes the effect on the elastic property of stress conditions. The model is validated using laboratory measurements. A case study for this model application from the northeastern Sichuan Basin, including a set of well logs and borehole measurements is also presented. The pore pressure gradient in carbonate reservoirs is ranging from 0.99 to 2.13 MPa/100 m. The predicted result shows the relative error between model predictions and observations measured while drilling is from 0.60% to 10.22%. The method proposed in this study is effective for pore pressure prediction in carbonate formation. • Pore pressure in carbonate rocks is estimated by a new rock physical model. • The model is validated by laboratory measurements and tested using well-logging data. • The model can be considered as a new tool to predict overpressure in carbonate rocks. [ABSTRACT FROM AUTHOR]