A Physical Model for Stress Cages

Abstract Well designs are constrained by the variation of both the pore pressure and fracture gradients throughout the depth of the well. Each hole section is designed such that the pressure profile within the hole at any time during drilling will not exceed the fracture pressure profile at any point throughout that section. The maximum pressures catered for in the design are invariably dictated by permeable formations with the highest pore pressure gradient. The casing depths are set to put behind pipe formations with too weak a fracture gradient to resist the planned pressure profile expected during drilling. This prevents a weak formation from failing and cross flow occurring between that failed zone and any high pressure permeable formations within the same hole section. The fracture gradient is typically determined by measuring the pressure at which losses begin to occur in the hole section and converting the downhole pressure into an equivalent mud weight. Most operators and mud companies have observed that the addition of some mud additives has influenced the pressure at which these induced losses begin. However, the use of those additives has been unreliable in many instances. Recent work at BP has resulted in the development of a physical model that describes the mechanism that allows the fracture resistance to increase above conventional minimum horizontal stress through the addition of mud additives. These additives result in the formation of a "stress cage" which is a near wellbore region of high stress induced by propping open and sealing shallow fractures at the wellbore/formation interface. With the development of the physical model it is now possible to analyze the effects of different drilling practices upon the reliability and stability of those induced stress cages. The development of stress cages is influenced by a number of properties including the diameter of the borehole, the width of fractures induced in a formation, the range of particle sizes which can be used as proppant in the fracture, the sealing properties of the mud, and the permeability of the formation. The successful implementation of the stress cage mechanism is dependent upon the use of appropriate constructive drilling practices and avoidance of detrimental practices which may destabilize the stress cages.