Analyses of Wellbore Instability in Drilling Through Chemically Active Fractured Rock Formations: Nahr Umr Shale

Abstract

Numerous time-dependent wellbore instability problems have been reported when drilling through the chemically active Nahr Umr shale formation in the Arabian Gulf. The shale is characterized by the abundance of not only bedding planes at the macro scale but also networks of smaller natural fractures at the micro scale. The presence of fractures weakens the shale mechanically and produces higher permeability fluid flow paths within the low permeability rock formation. Because of different fluid diffusion rates, there are two distinct pore pressure fields in saturated fractured shales. Additionally, in chemically active shale formations, osmotic pressure arises due to the imbalance in mud/shale chemical activity. These facts necessitate a dual-porosity/dual-permeability approach to analytical modeling that incorporates time-dependency, fracture networks, and chemical effects on wellbore stability. However, it is difficult or almost impossible to properly characterize the necessary shale modeling parameters due to the lack of standard-size samples required by conventional testing techniques. In this paper, a comprehensive approach to simulating instability problems for drilling in fractured shale formations from an innovative laboratory characterization technique for "tiny" shale samples to field analytical modeling is presented. Introduction Nahr Umr shale is "in-situ-fractured" when it comes to drilling operation and wellbore stability planning! Drilling induced fractures in addition to the fractured nature of Nahr Umr shale in the Gulf area necessitate a nonconventional approach to wellbore stability and stuck pipe problems. The nature of that claim is the result of an innovative testing technique to characterize shale rock at a "tiny" scale. In other words, classical wellbore stability analysis and mudweight windows are not good enough to estimate mud density and chemistry to prevent wellbore collapse and delayed borehole enlargement. Numerous wellbore instability problems have been reported when drilling through the chemically active Nahr Umr shale formation in the Gulf area. What has escaped many drilling operators in this area is the in-situ fractured nature of the shale formation and the pronounced bedding planes in many wellbore FMI observations;1 especially that the ultra-low permeability shale rock matrix is highly fractured at not only the macro scale (bedding planes) but also the micro scale (natural fractures). It is well known that the wellbore collapse in shale formation is caused by excessive effective stress concentration at or near the borehole wall where the drilling mud lacks the density and chemistry balance necessary to prevent the rock failure. Since both the pore pressure differential and mud/shale chemical activity modify the effective stresses, the cures for such intricate instabilities are therefore in a wellbore mechanics solution that accounts for the time lapse and fractured/compact shale matrix pore pressures and porosities. It is evident that time delayed failures are typical in these formations and necessitate a time-dependent poroelastic analysis. In addition, the dual-porosity fractured shale formation will exhibit a dual pore pressure system when subjected to stress and fluid pressure perturbations. This approach in classical mechanics, where a dual pressure system is exhibited, demands dual Darcy's law fluid flows that take into account the dual-permeability nature of the fractured formation. Further complication lies in the fact that it is extremely difficult or next to impossible to retrieve intact shale core samples required for conventional testing techniques for proper strength parameter characterization. A comprehensive approach to simulate instability problems for drilling in fractured shale formations, from shale mechanics laboratory characterization testing techniques to field in-situ analytical modeling, is documented herein. Nahr Umr Shale Formation Nahr Umr shale formation is found throughout the southern part of the Arabian Gulf and forms the cap rock to many major reservoirs in the region. Major wellbore instability problems when drilling through this shale formation have often arisen not only in new wells but also in re-entry wells, especially with the rise of water-based muds and stricter environmental control, making wellbore stability in this shale an extremely challenging operation for drilling/mud engineers.

Changing Shale Strengths With Invert Emulsion Drilling Fluids: Theory, Measurement, and Modeling

“…In particular, some researchers have studied the transport of water in shales 4-6 as a key factor in producing mechanical failure. Similarly, many wellbore stability models have been constructed in which the key failure mechanisms of shale have been coupled to give predictions for compressive shear as well as tensile failure [7][8][9][10] . Most recently, the complete models taking into account the three-dimensional wellbore geometry and the effects of time have also been coupled with earlier models 11 to give a poromechanical analysis of the generation/diffusion of pore pressure in shales in contact with drilling fluids 12,13 .…”