There is a clear advantage to drilling if we could strengthen the wellbore and drill at higher mud weights without losing fluid. A major prize is accessing difficult reserves in depleted reservoirs. Another application is in deep water drilling where the drilling window between pore pressure and fracture gradient is often narrow. This paper describes the approach taken by BP to produce a ‘designer mud’ which effectively increases fracture resistance whilst drilling, and which can be applied in both shale and sandstone. It works by forming a stress cage using particle bridging and an ultra-low fluid loss mud system. The theory is described and laboratory data show how the fluid system was developed. Field data are shown which quantify the increase in fracture resistance and demonstrate the value of the system. Logistics issues are discussed. Introduction Mud losses are a frequent problem encountered during drilling. Induced losses occur when the mud weight, required for well control and to maintain a stable wellbore, exceeds the fracture resistance of the formations. A particular challenge is the case of depleted reservoirs. There is a drop in pore pressure as the reserves decline, which weakens hydrocarbon-bearing rocks, but neighbouring or inter-bedded low permeability rocks (shales) may maintain their pore pressure. This can make the drilling of certain depleted zones virtually impossible - the mud weight required to support the shale exceeds the fracture resistance of the sands and silts. The potential prize is clear if we could devise a way to strengthen the weak zones and thereby access these difficult reserves. In fact, the value of wellbore strengthening is much more wide-ranging and includes the following applications/benefits:Access to additional reserves (depleted zones)Reduced mud losses in deepwater drillingLoss avoidance when running casing or cementingImproved well controlElimination of casing stringsAn alternative option to expandable casing This paper describes development work at BP to produce a drilling fluid that effectively strengthens the wellbore whilst drilling. The fluid can also be used in pill form. The effect is achieved by changing the stress state rather than by altering the strength of the rock itself. Such a system will have great utility if it can be engineered in a practical way. Previous studies have investigated wellbore strengthening with a view to preventing mud losses. One method1 suggests using temperature changes to alter the stress state around the wellbore. Mud heaters can be used to heat the circulating fluid and increase near-wellbore stresses, thereby giving a strengthening effect. However, this method might be difficult to control and would not be suitable in wells with an already-high bottom hole temperature. The 1992 paper by Fuh2 discusses the concept of adding granular particles to the mud to seal fractures and prevent losses; it is stated this could only work in permeable rocks where leak-off into the rock allows a cake to build in the fracture. Other work has discussed the concept of using fractures to cause stress changes in the rock - introducing the idea that fractures could increase the hoop stress around the wellbore. This is discussed by Morita and Messenger3,4. Alberty and McLean5 discuss how mud cake deposition in the fractures can affect near-wellbore stresses, and they give field examples suggesting large increases in fracture resistance. Recent papers by Sweatman et al6,7 have taken this concept and developed chemical treatments which could be squeezed into fractures to prop them open and seal them. Theoretical Approach In developing the above ideas, the approach we have taken is to actually allow small fractures to form in the wellbore wall, and to hold them open using bridging particles near the fracture opening. The bridge must have a low permeability that can provide pressure isolation. Provided the induced fracture is bridged at or close to the wellbore wall this method creates an increased hoop stress around the wellbore, which we refer to as a ‘stress cage’ effect. The aim is to be able to achieve this continuously during drilling by adding appropriate materials to the mud system, to produce what we have termed a ‘designer mud’. The concept is illustrated in Figure 1.

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