Considering The Stratigraphic Discontinuities Characteristics Of Wellbore Stability

It always contains fractures, poorly cemented or non-cemented fracted zone in real formations, moreover, some fractures are induced during drilling, so the formation around the well is not continuous. For simplification, it is often neglected when the discontinuity is not severe. But when there are many fractures in the formation, the feature of formation discontinuity will strongly affect the feature of wellbore stability. The terminology "discontinuous formation" is used in this paper to indicate the formations which have too many fractures or so. Discontinuous formations mainly contain friable hard mud shale which has many beddings or fissures, fracted zone and fracted formations (such as coal seam). Wellbore enlargement and wellbore instability (flaking and cave-in) are familiar stubborn difficulties in these formations at drilling spot.For wellbore instability in continuous formation, the practice at drilling spot is to increase the density of drilling mud or change the mud system. This is usually effective for chemosensitive continuous formation. But often it can not bear fruit for non-chemosensitive discontinuous formation, in some cases, wellbore instability becomes more severe when increasing mud density. This explains that the mechanism of wellbore instability in discontinuous formation is not the same with that in continuous formation.The occurrence of discontinuous fractures changes the mechanical properties of rock formation strongly. It not only reduces the rock strength but also varies the stress distribution in rock when it is loaded. Therefore, the rock becomes easier to be destroyed. Meanwhile, the occurrence of fractures makes it easier for drilling mud filtrate to invade into the interior of rock mass. The water power splitting and hydration promotion effect of drilling mud aggravate wellbore instability much more.In view of the complexity of the problem, this paper simplifies the discontinuous property of the real subterranean formation. It analyzes the stress distribution in wellbore vicinity and the fracture outspread stability considering the effects of drilling mud, the sealing effects of mud cake, fracture, the seepage field, stress field of the wellbore...