Mechanical effects ___________________________________________________ 64 Chemical effects_____________________________________________________
64 1.1.1 1.1.2
Common Misconceptions _________________________________________ 64 The Mechanisms of Wellbore Instability - Mechanical Aspects____________ 66
Unconsolidated Formations ____________________________________________ 66 Competent Formations ________________________________________________ 67 Increased well inclination ______________________________________________ 70 Open hole time ______________________________________________________ 70 Pre Drilling ________________________________________________________ 71 PlanningStage ______________________________________________________ 72 Symptoms and Remedial Action_________________________________________ 74
Mud weight window_____________________________________________ 70
Preventative Action - Mechanical __________________________________ 71
1.5.1 1.5.2 1.5.3
The Mechanisms of Wellbore Instability -Chemical Aspects______________ 75 Preventative Action - Chemical ____________________________________ 76
Pre Drilling ________________________________________________________ 76 Oil Based Mud - Engineering Comments __________________________________ 77 Water Based Mud - Engineering Comments ________________________________ 78 Symptoms and Remedial Action_________________________________________ 79Salt Formations _____________________________________________________ 80 Coal Formations__________________________________________________
1.7.1 1.7.2 1.7.3 1.7.4
Special Cases __________________________________________________ 80
1.1 Introduction The maintenance of wellbore stability is one of the most critical considerations in any drilling operation. Anunstable wellbore will reduce drilling performance, result in drilling and tripping difficulties and in the worst case could result in the loss of the hole throug h borehole collapse. Wellbore instability can occur as a result of:
• Mechanical effects, • Chemical effects,
• Combination of both.
1.1.1 Mechanical effects In simple terms, mechanical effects are usually related to :• Inadequate mud weight (too high or too low). • Inappropriate drilling practices (rate of penetration, vibration
effects, torque and drag, poor practices, and frequency of trips). 1.1.2 Chemical effects Chemical effects are mud type related for formation being drilled and may result due to
• Inappropriate mud type
• Inadequate inhibition.
The following sections give more detail on this,and provide a guide to minimise wellbore instability in the planning, implementation and drilling phases. 1.2 Common Misconceptions Before detailing aspects of hole stability problems some of the common misconceptions should first be discussed.
1. Well control purposes do not solely dictate the required mud weight. 2. Hole collapse is not simply a result of drilling with insufficient mudweight. 3. Drilling with a nominal safe overbalance will not necessarily ensure hole stability. The logic behind the above statements are fundamentally flawed as certain weak formations may need overbalance in excess of 1000 psi to prevent hole collapse, whereas some mudstones can be drilled problem free in “underbalanced” conditions. 4. Increasing the mud weight will rectify hole instabilityproblems. Increasing mud weight can in instance amplify problems. E.g. More rapid failure could occur in fractured rocks, whereas in some porous formations the resulting higher fluid losses and thicker mud cakes could promote differential sticking. 5. Oil-based mud will prevent any problems occurring while drilling in shales. Hole instability can still occur however particularly if mud weight or water...