The currently popular concept and practice on "Wellbore Strengthening" for drilling applications was originated in part from our earlier SPE papers on "A New Approach to Preventing Lost Circulation While Drilling" 1 and "Theory of Lost Circulation Pressure".2 Based on our theory and earlier test results, a significant increase in formation fracture resistance could be achieved due to fracture sealing or plugging mechanism induced by a particle "screen-out" effect resulting from the drilling fluid loaded with an adequate amount of narrowly-sized granular materials. We have shown in both our laboratory and field tests that such an increase in formation fracture resistance is particularly valuable in helping the following well operations: 1) drill through depleted zones without losing fluid even when a much higher mud weight (than the normal fracture gradient) is used; 2) strengthen the weaker formations that usually require additional casing strings for protection; 3) avoid lost circulation during cementing operations as the strengthening operation also improves the strength of the cement sheath; and 4) drill high-angle well sections with high mud weights that would normally not be possible due to low formation fracture gradients, etc. We have recently conducted several Wellbore Strengthening treatments using a special type of lost circulation material, in terms of particle size range and loading density, for increased formation fracture resistance (or increased apparent fracture gradient) during normal drilling operations. A wide variety of wellbore problems encountered have been addressed by this technique. Using several field case examples, this paper will provide a review and discussion of the theory, potential, and limits of the wellbore strengthening technique. Lessons learned from our field treatments will also be discussed along with precautions for optimum drilling applications.
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