Search

The causes are discussed of well bore instability, including mechanical effects such as failure of the rock around the hole because of high stresses, low rock strength or inappropriate drilling practice, and chemical effects which arise from the interaction between the rock, generally shale, and the drilling fluid. Uncontrollable factors include naturally fractured or faulted rock formations, tectonically stressed formations, high in situ stresses, mobile formations, unconsolidated formations and natural and induced over-pressured shale collapse. Controllable factors include bottom-hole pressure, well inclination and azimuth, transient well-bore pressures, physical and chemical fluid/rock interactions, drillstring vibration and drilling fluid temperature. Direct and indirect indicators of well-bore instability are described and the development of models for well-bore stresses is discussed in relation to shear and tensile failure mechanisms., The causes are discussed of well bore instability, including mechanical effects such as failure of the rock around the hole because of high stresses, low rock strength or inappropriate drilling practice, and chemical effects which arise from the interaction between the rock, generally shale, and the drilling fluid. Uncontrollable factors include naturally fractured or faulted rock formations, tectonically stressed formations, high in situ stresses, mobile formations, unconsolidated formations and natural and induced over-pressured shale collapse. Controllable factors include bottom-hole pressure, well inclination and azimuth, transient well-bore pressures, physical and chemical fluid/rock interactions, drillstring vibration and drilling fluid temperature. Direct and indirect indicators of well-bore instability are described and the development of models for well-bore stresses is discussed in relation to shear and tensile failure mechanisms.

Underbalanced drilling (UBD) conditions occur when the effective downhole circulation pressure of the fluid system in contact with the rock formation is lower than the existing formation pressure. The reduction in damage to the formation has potential benefits including increased production rate and ultimate recovery. The technology is becoming accepted as a standard method for handling the drilling and completion of wells. The benefits and limiting factors are discussed and the selection of fluid system is considered with reference to gas (air, nitrogen), mist, foam, gasified mud and liquid systems, whose effects on circulation, reservoir damage, sticking, drilling rate. hole cleaning, wellbore stability, water inflows, downhole fires and corrosion are considered., Underbalanced drilling (UBD) conditions occur when the effective downhole circulation pressure of the fluid system in contact with the rock formation is lower than the existing formation pressure. The reduction in damage to the formation has potential benefits including increased production rate and ultimate recovery. The technology is becoming accepted as a standard method for handling the drilling and completion of wells. The benefits and limiting factors are discussed and the selection of fluid system is considered with reference to gas (air, nitrogen), mist, foam, gasified mud and liquid systems, whose effects on circulation, reservoir damage, sticking, drilling rate. hole cleaning, wellbore stability, water inflows, downhole fires and corrosion are considered.