Review of basic theory for lost circulation control

The theories for lost circulation control are divided into five major categories, including Rigid Plugging Theory, Flexible Plugging Theory, Lost Prevention by Lowering Pressure Theory, Lost Prevention by Bearing Pressure Theory, and Fuzzy Plugging Theory, based on plugging mechanisms. Before the emergence of plugging theories, the main causes of lost circulation were considered as the lost circulation medium, including fractures and caves in the formation. The medium was attempted to be blocked by plugging materials, without considering the size matching relationship between plugging materials and formation lost circulation medium. By Rigid Plugging Theory, including Packing Plugging Theory and Bridging Plugging Theory, the lost circulation is controlled by accumulating particles or bridging in the medium. By Flexible Plugging Theory, the lost circulation is controlled by the special properties of the polymer which invades the formation porous medium with working fluids and generates a special area to kill the lost circulation by physical and chemical reaction of some special ingredients. The Lowering Pressure Theory prevents lost circulation by lowering fluid column pressure to lower the pressure difference between bottomhole pressure and reservoir pressure. The Bearing Pressure Preventing Lost Theory plugs the micro-fractures to improve the bearing pressure capacity of wellbore, according to the principle of fracture stability. The Fuzzy Plugging Theory controls lost circulation by the particles, which are described by the fuzzy mathematics for the parameters of the pores and fractures such as size, shape, and number. Nowadays, the main plugging theory is Rigid Plugging Theory. The Lowering Pressure Prevention Lost Theory will not be used extensively since it is largely limited by formation and facilities, though it develops rapidly. The Fuzzy Plugging Theory may have a bright future for its compatibility between the natural formation and the property of the lost circulation materials.