Drilling of horizontal wellbores has been increasingly popular since early 2000’s in reservoirs where the recovery of hydrocarbons cannot be economically feasible using conventional techniques. The completion of these type of wellbores may require cementing the horizontal section to hydraulically seal the annulus preventing undesired leakage. The design and success of the cementing operation in such configuration is constrained by operational and geological factors.The geology of the zone where the borehole is drilled greatly influences its stability. To prevent the borehole from collapsing, several variables must be considered: the mud pressure, the pore pressure, the anisotropic effects of the rock, and the in-situ stresses. Even stable wellbores however exhibit a variety of geometrical irregularities or deviations from the ideal circular borehole, as is often evidenced in caliper readings. We refer to large-scale irregularities as where significant variations in the diameter of the borehole, persist through lengths of tens of meters along the well trajectory. Examples of these type of irregularities cover keyseats, washouts and breakouts. The keyseats are asymmetric oval holes resulting from wear against the drill string at depths where the inclination of the well changes. Washouts are overall enlargements in the diameter of the hole; this anomaly is especially found in unconsolidated zones and dipping beds. The breakouts are symmetric oval enlargements resulting from compressive shear fracturing in a preferential direction.In this work, we study the effect of oval irregularities during primary cementing of a horizontal section of well, using characteristic caliper data. We use a simplified 2D gap-averaged model to compute the displacement of a drilling mud by a spacer within an elliptical annulus that represents the oval irregularity. The rheology of the mud is typically a Herschel-Buckley fluid with a high yield stress, commonly used to prevent the sediment of solids while drilling and cleaning the borehole. The rheology of the spacer is often a shear-thinning viscous fluid and can be adjusted according to particularities of the well. The 2D gap-averaged model has been extensively used in both computational and experimental studies previously. The model allows to introduce smooth variations of the borehole wall after minor modifications; however, this approach has not been presented before. We also present a series of 3D numerical simulations, using a Volume of Fluid method to capture the interface between the fluids. The 3D model allows us to study the effects of more local irregularities, e.g. including roughness and washouts.The dynamics of the displacement of two fluids in a horizontal circular annulus is governed by buoyancy, eccentricity and the rheology of the fluids. A positive density difference combined with a slow mean pumping speed promotes slumping of the second fluid towards the bottom of the annulus. Nevertheless, high eccentricity values (e=1-standoff) are common due to the weight of the casing pulling downwards, opposing the buoyancy force. Finally, the rheology of the fluids is relevant to determine the presence of un-displaced layers of mud, e.g. at the walls. The same competition described above holds true in the elliptical annulus. The main difference is that the elliptical shape incorporates an additional way of altering the velocity field around the annulus.Although industrial practice deals with the effects of large-scale irregularities by introducing an annular volume excess percentage, here we show that the actual shape of the irregularity can substantially affect the mud removal in the annular space of the well.

Cementing Irregular Horizontal Wellbores

Paper Proceedings: https://asmedigitalcollection.asme.org/OMAE/OMAE2021/volume/85116

OMAE 2021

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