The Sha-3 member V reservoir in the Gaoliu block,Nanpu sag is a typical interlayered thin sandstone and thin claystone tight reservoir with complex formation pressure systems and densely distributed artificial fractures resulted from many years of fracturing and injection operations.Horizontal drilling in this area is faced with problems such as mud losses,oil and water kicks as well as collapse of borehole walls.This paper describes,based on the formation characteristics of the Gaoliu block,the technical difficulties in horizontal drilling,and points out the causes of mud losses and borehole wall collapse.To solve these problems,an oil based drilling fluid was formulated with a nanometer/micrometer sized emulsified borehole wall collapse preventing agent,graphite microspheres,a micrometer flexible plugging agent and calcium carbonate of ultrafine particles,which render the drilling fluid capacities such as plugging,high pressure bearing and borehole wall stabilizing.The oil based drilling fluid has low filtration rate and has the ability of maintaining the strengths of the claystones that is 5 times higher than other drilling fluids.With this drilling fluid,the pressure bearing capacity of the sandstones of medium and high permeability can be increased to 18 MPa.Bridging and solidification mud loss control techniques were used to control mud losses.Using this drilling fluid,horizontal drilling through the tight reservoirs in the Gaoliu block was safely performed and no borehole wall collapse was encountered during drilling in more than ten wells.In formations where mud losses were encountered,the mud losses were controlled and the pressure bearing capacity of the formation was increased by 5.4 MPa,satisfying the requirement of subsequent drilling.
Tight reservoirComplex pressure systemHorizontal wellCollapseMud lossOil and water kick
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