Abstract
With the continued growth of drilling activities and the cost of performing them in depleted sands, loss of productive time is more important than ever. For years the use of LCM has been the preferred way for treating lost-circulation problems (Messenger 1981). However, the lost circulation problem was not totally resolved. Recent studies have shed much light on its cause and potential solutions. However, wellbore strengthening or stress cage implementation has been recognized as an effective means of dealing with lost circulation during drilling operations. One of the mechanisms developed for strengthening a wellbore has been to prop induced and existing fractures with particulate lost circulation materials (LCM) to effectively increase hoop stress in the near wellbore region. However, a good understanding of this mechanism is necessary in order to avoid a potentially flawed design and implementation process which could adversely affect job success during well operations. One of the main issues is depleted sand/shales stringers stability under the strengthening conditions. This paper will describe those factors which are important in designing wellbore strengthening jobs and address the conditions necessary to help ensure depleted sands stability, as determined through geomechanics analysis. The strengthening of a wellbore by propping fractures has been discussed in a previous investigation (Wang et al. 2007a). In this paper, various parameters that affect the strengthening of the wellbore are addressed in detail. In-depth discussion of how each of those parameters affects the process of wellbore strengthening will be presented. This study was accomplished using a boundary element numerical analysis coupled with the linear elastic theorem. Hence, the stress cage concept is an approach developed to enhance wellbore pressure containment (WPC). It has been found that a weak wellbore can contain much higher pressure if the wellbore fluid is treated with particulates and as such understanding the mechanism of stress caging is recommended to design the treatments for specific field application and to advance the development of the technology for application across a wider mud weight window and with a higher success rate while drilling in depleted sands. On-the-fly preventive methods appear to be favored as they cut down on non-productive time and reduce costs in the longer term while drilling in depleted sands as can be seen using the new 3D MUDSYST model.