Lost circulation is a complicated situation in the drilling operation, wasting a lot of time and mud during processing. A serious lost circulation can cause many hazards, such as internal pressure drop, affecting normal drilling, causing well instability, inducing formation fluid into wellbore and blowout. The current plugging technology can solve some of lost circulation leakage, but there are still some problems in conventional plugging technology, such as particle size of plugging material does not match crack width, slippage of plugging material in blocking zone, and weak adhesion of plugging material additive to reservoir, which restricts the success rate of lost circulation plugging operation.

Variable structure has variable network affected by stress compound with elastic plugging material forming a plugging mixture for different leakage speed and crack width.

The rigid plugging materials with different particle sizes and different materials are mixed in a certain proportion to form a compound plugging agent for different leakage speeds and different leakage crack widths. Combined with the variable structure gel, a new type of lost circulation plugging system is used to seal the fracture-cavity reservoir leakage.

Through indoor experiment simulation, the on-site plugging formula which is suitable for fracture- vug reservoir under different temperature is formed and determined.

Scanning electron microscope shows the compound plugging mixture (CPM) has a variable network structure. Solution experiments show CPM is not diluted with water. Single plugging gel can plug 0.04-0.8 mm cracks and pressure capacity is greater than 3 MPa. With different types and sizes rigid plugging materials and elastic plugging material can block crack width of 0.15-5mm and pressure capacity is greater than 7.5MPa.

The novel CPM material has been tested and used for more than 50 wells. The porosity or micro- fracture leakage and the leakage rate of less than 30m3/h, the success rate of plugging is up to 100%, and the loss rate between 30 and 60m3/h, the success rate of plugging is beyond 85%. And the leakage rate of more than 60m3/h, the success rate of plugging is more than 80%.Leakage loss time is less than 80% shorter than conventional plugging technology.

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