A Novel Practical Approach to Borehole Breathing Investigation in Naturally Fractured Formations

Abstract

Abstract Occurrence of reversible mud losses and gains while drilling in naturally fractured formations is of primary concern. Borehole breathing can greatly complicate the already difficult practice of fingerprinting the changes in the return flow profile, hence undermining the reliability of kick detection. Issues can also derive from misdiagnosing a kick and attempting to kill a breathing well. The objective of this work is to correctly address the phenomenon and increase insights of its physical characterization. The fluid progressively flows in and out of fractures as a consequence of three mechanisms: (1) bulk volume deformation, (2) fluid compressibility, and (3) fracture aperture variation. To represent this complex scenario, a model involving a continuously distributed fracture network is developed. A time-dependent, one-dimensional dual-poroelastic approach is coupled with a variable fracture aperture and a passive porous phase. Finite fracture length is considered and no limitation on the number of fractures is posed. The latter permits us to analyze long open-hole sections intersecting several fissures, which is a more realistic approach than the available single fracture models. The proposed model is able to quantify pressure distribution in fractures and pores, together with the flow rate entering or exiting the fractures. When the fissured space is reduced to zero and incompressible bulk volume is considered, the solution reduces to that of classical reservoir engineering. A sensitivity analysis is performed on the physical properties of the formation and the drilling fluid. The latter provides a deeper insight on the factors that significantly influence breathing phenomena (i.e. drilling fluid weight, rheology and formation mechanical properties). Furthermore, a very useful application of the model is proposed by suggesting its application as a breathing discriminator during kick diagnosis. The shut-in drill pipe pressure, recorded from a real kick, has been compared to one caused by a simulated breathing case. Although the two SIDPPs show great similarities, the correct modelling of breathing can significantly help the identification of the major differences between a kick and breathing. Altogether, a comprehensive in-depth characterization of borehole breathing can help with kick diagnosis and can be used to effectively design unconventional drilling techniques such as Managed Pressure Drilling.