Pore-pressure and stress prediction

Pre-drill predictions of pore pressure, fracture gradient and effective stress are critical to drilling wells in a safe and cost-effective manner. These estimates are used to predict borehole stability so that appropriate mud weights, casing and cementing designs can be determined. In addition, predictions of pressure and stress can identify potential drilling hazards, aiding in the avoidance of kicks, fluid losses, and blowouts.

Conventional prediction methods are based on estimates of elastic properties derived from seismic data. Variations of these properties with depth are related to the porosity changes that occur in shale during compaction under vertical loading. Over-pressured rocks have lower velocities (higher porosity) compared to normally pressured rocks at the same depth. These methods are typically most accurate when applied to shale mudrocks in rapidly deposited basins where overpressure is associated with undercompaction. However, compaction relationships are lithology dependent. There can also be other factors that affect porosity and elastic properties, especially those at higher temperatures associated with diagenetic changes in the rock such as cementation, dissolution, transformation of smectite to illite, etc. In addition, applying pressure predictions to drill well design requires accurate seismic time-to-depth conversion, which can be challenging in areas with velocity anisotropy. As a result of these and other issues, pressure and stress prediction is often inaccurate.

To support the application of pore pressure and stress prediction, CRGC is working in the following areas:

• compaction behaviour of clay sediments
• digital rock modelling based on X-ray microtomographic images acquired under high pressure and temperature conditions
• broadband acoustic measurements from seismic to ultrasonic frequencies
• laboratory measurements of intrinsic and stress-induced anisotropy used to calibrate seismic data
• seismic anisotropy from VSP and surface seismic data
• modelling stress-dependent properties of rocks
• modelling of attenuation and dispersion due to squirt flow in rocks saturated with viscous and viscoelastic fluid