Importance of drilling-related processes on the origin of borehole breakouts — Insights from LWD observations

Logging while drilling (LWD) images are widely used for the analysis of borehole stability. In this context, borehole breakouts are a crucial indication of rock failure developing when the circumferential stress around the borehole exceeds the yield value of the rock. This study investigates the impact of drilling-related processes (DRPs) on the origin of borehole breakouts. DRPs, for instance, include connections or tripping operations. For this purpose, we analyze data from 12 boreholes in different geological settings throughout the Norwegian and Danish North Sea, containing a total of 208 borehole breakouts. The extensive data acquisition of LWD offers the unique possibility to link the imaging to real-time drilling operations and to monitor anomalies of e.g., bottom hole pressure. These records allow us to connect any thermal, hydraulic, or mechanical interaction next to the borehole wall to perturbations of the stress field. This analysis resulted in an apparent strong coincidence of borehole breakouts, representing major stress perturbations, with DRPs. The causal relationship is highlighted by one order of magnitude higher occurrence of DRPs in depth sections containing breakouts. Major pressure reductions in the annulus of the borehole seem to be the most significant cause of drilling-related wellbore failures. This applies in particular to shutting off the pumps during connections, where pressure reductions of up to 16 % of the annulus pressure led to higher circumferential stresses. This process will increase the likelihood of compressive and shear failure, therefore causing borehole breakouts. These observations further open the perspective of counteracting wellbore instabilities by pressure modification. In addition to the initiation of breakouts, their temporal evolution – as seen in relogs – can also be ascribed to DRPs. This study indicates that not only plasticity but also mechanical interaction from DRPs is a key driver of the temporal growth of borehole breakouts

Transport mechanisms of hydrothermal convection in faulted tight sandstones

Motivated by the unknown reasons for a kilometre-scale high-temperature overprint of 270–300 ∘C in a reservoir outcrop analogue (Piesberg quarry, northwestern Germany), numerical simulations are conducted to identify the transport mechanisms of the fault-related hydrothermal convection system. The system mainly consists of a main fault and a sandstone reservoir in which transfer faults are embedded. The results show that the buoyancy-driven convection in the main fault is the basic requirement for elevated temperatures in the reservoir. We studied the effects of permeability variations and lateral regional flow (LRF) mimicking the topographical conditions on the preferential fluid-flow pathways, dominant heat-transfer types, and mutual interactions among different convective and advective flow modes. The sensitivity analysis of permeability variations indicates that lateral convection in the sandstone and advection in the transfer faults can efficiently transport fluid and heat, thus causing elevated temperatures (≥269 °C) in the reservoir at a depth of 4.4 km compared to purely conduction-dominated heat transfer (≤250 °C). Higher-level lateral regional flow interacts with convection and advection and changes the dominant heat transfer from conduction to advection in the transfer faults for the low permeability cases of sandstone and main fault. Simulations with anisotropic permeabilities detailed the dependence of the onset of convection and advection in the reservoir on the spatial permeability distribution. The depth-dependent permeabilities of the main fault reduce the amount of energy transferred by buoyancy-driven convection. The increased heat and fluid flows resulting from the anisotropic main fault permeability provide the most realistic explanation for the thermal anomalies in the reservoir. Our numerical models can facilitate exploration and exploitation workflows to develop positive thermal anomaly zones as geothermal reservoirs. These preliminary results will stimulate further petroleum and geothermal studies of fully coupled thermo–hydro–mechanical–chemical processes in faulted tight sandstones