Topic: Mining Engineering
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Integrating Optimization of Drilling Parameters with Wellbore Stability
The drilling of wells in deepwater and extended reach environments is characterized by staying within the window between the formation pore pressure and fracture gradient. The success of such wells having complex geometry or a high degree of difficulty hinges on maintaining wellbore stability (WBS) and optimization of drilling parameters. However, during the pre-well planning phase, geomechanical wellbore stability modeling generally focuses only on predicting the required surface fluid density. On the other hand, "optimized" drilling parameters are identified by modeling pressure drops, hole cleaning and rate of penetration without any consideration of wellbore geomechanics.
In this paper, wellbore hydraulics and geomechanical wellbore stability are integrated to assess the volume of rock that is subjected to shear failure due to annular pressure fluctuations during the wellbore construction process. Conversely, the effect of the failed rock on equivalent circulating densities (ECDs) and pump requirements for hole cleaning are also estimated.
The volume of rock subjected to failure due to annular pressure fluctuations during various phases in the well construction process is estimated. Next, the effect of the failed rock on ECDs and hole cleaning are estimated. It must be noted herein that a chemoporoelastic model accounting for chemical interaction between the drilling fluid and the formation, as well as full coupling between the change in pore pressure and effective stresses has been used.
Using the conditions before running casing and a drilling fluid surface density of 12.5 lbm/gal, the window for safe drilling between formation collapse and formation fracture initiation was determined.6 The pertinent input parameters used in the modeling were taken from the earlier study; the activity of the aqueous phase of the LTOBM was also input so the chemoporoelastic model could be used. At a 63º angle, the modeled drilling fluid density was predicted to be 12.7 lbm/gal, a level slightly higher than the final density used while running casing. With the hole giving some problems on the cleaning trip out, a density at interval TD of 12.0 lbm/gal was clearly inadequate, and densities of 12.35-12.4 lbm/gal could be considered "borderline" at best. In Fig. 1 the various mud densities and ECD values are detailed for the modeled case.
Source: www.aade.org
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