From field experience, even if a borehole is stable while drilling through a shale, it may turn unstable with time when the well is left static for a sufficiently long period of time. This may in some cases be explained by the collapse limit being located somewhere in-between the equivalent circulating density (ECD) and the corresponding static density (ESD) of the mud. In such cases, the combination of formation conditions and properties, and well conditions will determine the timing for possible onset of instability in the vicinity of the borehole wall.
We present a set of highly specialized hollow cylinder (HC) laboratory tests on a field shale core. While subject to a confining stress and an applied pore pressure, the borehole pressure has been systematically varied according to scaled-down field conditions to simulate the effect of ECD vs. ESD at connection on borehole stability. The laboratory results suggest that the mud weight (MW) may be kept below the classic static collapse limit for shorter periods of time, like those occurring at connections, without inducing borehole collapse due to low permeability, plastic nature, and time dependent strength of the shale. The borehole diameter may however gradually decrease due to irreversible plastic deformation induced during each low pressure cycle. When at total depth (TD) and the when well is left static, the ESD may have to be increased to maintain stability, compensating for the loss of ECD.
We illustrate the dynamic stability effect with a North Sea field case where the near horizontal well was stable while drilling, but turned unstable when left static for a period of time after reaching TD. The observations are compared to corresponding modelling of the time dependent stability.
This shows the importance of accounting for the potential time dependence of borehole stability while drilling through shale. The combined effect of ESD, ECD, and timing may determine whether the borehole will be sufficiently stable from an operational point of view, allowing the well to be drilled and completed without excessive problems.