When drilling into a naturally fractured tight reservoir,mud losses into the fractures can easily induce reservoir damage,block the pore throats and fractures in the reservoir formation,thereby reducing the permeability of the reservoir and hindering the stable production of oil and gas.In this study,the buried hill fractured tight metamorphic rock reservoir in a block in the Bohai Bay Basin was taken as the object of study,experiments such as fluid sensitivity,dynamic damage by drilling fluid and pressure bearing capacity of mud cakes were conducted.With these experiments,the degree of drilling fluid dynamic damage to the reservoir and the pressure bearing capacity of mud cakes were understood and the mechanisms of reservoir damage by fluid sensitivity were analyzed.The study results show that the matrices of the reservoir rocks have strong salt sensitivity,moderate to weak alkali sensitivity and weak water sensitivity.The incompatibility between the drilled solids particle sizes and the opening widths of the fractures in the reservoir causes the solids particles to invade into the deep fractures,plugging them and damaging the permeability of reservoir rocks.The drilling fluid used has good ability to plug fractures with opening widths≤150 μm,but is unable to effectively plug fractures with opening widths≥300 μm.The fractured tight metamorphic rock reservoir contains plenty of clay minerals and heavy minerals,mud losses cause the clay minerals to become hydrated and swollen,and migration of particles takes place,reacting with the heavy minerals to produce iron hydroxide precipitates which in turn exacerbate reservoir damage.When drilling metamorphic rock reservoirs,the protection of fractures with opening widths greater than 300 μm should be strengthened,and special attention paid to the reservoir damage induced by the interaction between the metamorphic rocks and the work fluids.Also presented in this paper are the measures to be taken in protecting reservoirs from being damaged.
FracturedTight reservoirReservoir damageLossFluid sensitiveHeavy mineral
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