Title : Drilling challenges worldwide due to the geomechanics misconception
Abstract:
Safe drilling operation is contingent on the mud weight tolerance to the permissible drilling window. Usually, the drilling window is dictated by the difference between the fracture pressure and the subsurface formation pressure. Pore Pressure Prediction (PPP) is the foundation of estimating the drilling window especially in Deepwater Wildcats. The limits of this window are subject to scrutiny by governmental and safety agencies especially with Deepwater regulators. PPP takes place before drilling the exploratory wildcat. Applying the effective stress modelling technique is the most successful and common method to predict subsurface pressure (derived from extracting seismic velocity). Geomechanics is the backbone of forecasting and calculating PPP. Most of the PPP estimations and software settings are designed based on the general assumption that the principal stress (S1) is equal to the Overburden. Applying the conventional geomechanical setting of S1> S2 > S3 in non-extensional geologic structural setting can lead to unintended PPP calculation pitfalls. This can take place in Salt basins, tectonics plates, shale diapirs, compressional tectonic areas where the stress vectors exchange their positions. Some of the wide-ranging drilling challenges related to misinterpreting the PPP profile are:
- Assuming the subsurface pressure column is divided into normal hydrostatic and abnormally pressured (geopressured). This can cause PPP modelling defect. The recent subsurface research considers the presence of four compartments instead of two. This is proven worldwide especially in clastic active sedimentary basins where shallow water and mud flow takes place.
- Geopressure compartmentalization causes the sudden pressure increases between seals and reservoirs. This should be incorporated in PPP modelling process. It is a common cause of kicks and blow outs especially in HC bearing formations.
- Calibrating PPP models in real time utilizing measured pressure in reservoir beds (RFT, MDT .etc.).
- The Narrow Safe Drilling Window in Deepwater can cause serious loss of circulations, stuck-drill pipes, side tracks and bypasses. These challenges are widespread in the Deepwater Salt Basins. Adding extra casing shoes and liners, as a result, is costly and leads to NPT (non-productive time).
To overcome some of these issues and avert the unsought surprises:
- Geomechanical – Pore pressure prediction models should be tailored to the geological building blocks as their basic numerical output.
- Sequence Stratigraphy in collaboration with seismic velocity shed light on high versus low pressures gradient (PG) areas. Kicks, flows and mud cuts take place in high PG sections, whereas Loss of circulations (LOC), sticking and stuck pipes take place in low PG section.
- Real time modification of mud weight against the bore-hole breathing is important for drilling a successful safe bore hole.
- Dual Gradient Drilling is designed to reduce the drilling mud pressure in the annulus zone to avoid LOC, formation break-down and reduces the number of casing seats needed to reach TD in Deepwater.
- Managed Pressure Drilling requires additional equipment to be assembled and synchronized with the drilling rig mud life line. It has its pros and cons as well.
