Understanding Loss Mechanisms: The Key to Successful Drilling in Depleted Reservoirs?

Abstract

Summary Mud losses are frequently observed when drilling in depleted formations. This is because of the decrease in the minimum in-situ stress during depletion. As a result of this decrease, the lost-circulation pressure—or fracture gradient (FG)—decreases, and the operational mud-weight window shrinks. Losses in such formations are often observed when drilling through sand/shale sequences. Preventing and curing losses requires a sound understanding of loss mechanisms. In this study, we investigate several mechanisms that might be responsible for the elevated risk of mud losses in differentially depleted sand/shale sequences. Numerical models of synthetic cases representative of lost-circulation scenarios in a high-pressure/high-temperature (HP/HT) field in the North Sea, under normal faulting conditions, are set up using the finite-element method. The simulations reveal that shear displacement at the horizontal sand/shale interfaces is unlikely to cause losses. On the other hand, shear displacement and losses might be induced at high-angle sand/shale interfaces, such as those found near faults. In addition, faults introduce an extra complexity to the stress distribution and stress-path coefficients. Stress anisotropy near faults might increase during depletion, making both lost-circulation issues and borehole-stability problems worse. The zone most prone to lost circulation in a faulted formation is located in depleted sand adjacent to the fault. The loss mechanism here is because of drilling-induced fractures (DIFs). In addition, depletion itself might induce fractures in shale adjacent to the depleted sand and located across the fault from it. Such fractures might then serve as escape paths for the drilling fluid during infill drilling. The loss mechanism here is caused by pre-existing, depletion-induced fractures. These findings are in agreement with field observations. A noncircular (elliptic or irregular) borehole cross section is found to reduce the fracture-initiation pressure (FIP). An irregular borehole cross section is, thus, another possible mechanism behind irregular loss patterns observed in depleted fields. The results from the study are important for establishing best practices when drilling in depleted formations.

Stability of highly inclined non-circular wellbores in isotropic formations

“…It is worth mentioning at this point that there are other relevant studies on hoops stress distribution around an elliptical borehole. Jaeger et al (2007) and Savin (1961) presented hoop stress equations for an elliptical hole in an isotropic body, subjected to a uniaxial far-field loading, with an arbitrary inclination angle to the horizontal axis. Setiawan and Zimmerman (2020)) provided a complete in-plane stress solution around an arbitrary-shaped hole.…”

Novel Torque and Drag Model for Drilling Two-Dimensional High-Angle Wells

“…), the hole will be exposed for longer time than required in vertical or conventional deviated wells. A number of time-dependent effects may occur, such as more cycles of pressure variations and D/S contact with borehole walls that reduces the strength of formations as a consequence of fatigue-type mechanisms. , …”

“…A number of time-dependent effects may occur, such as more cycles of pressure variations and D/S contact with borehole walls that reduces the strength of formations as a consequence of fatigue-type mechanisms. 22,23 1.1.3. Drilling Fluid Properties.…”