Types of Losses
Lost returns or mud loss is a partial or complete loss of circulation in the well. It’s a common downhole problem in regions like the Middle East, North Africa, and Latin America. As an estimate, lost returns account for nearly 10% of non-productive time worldwide. Lost returns not only cause operational delays and increase drilling costs but can also cause well control issues and environmental damage. Fluid loss to rocks takes place in the following four ways.
1. Seepage Loss: It is a slow and steady loss of volume of drilling fluid. It is generally termed seepage loss if the loss rate is less than 30 barrels per hour BPH. Seepage losses are caused in highly permeable rocks. Seepage losses can be stopped by blocking the pore throats of the rock with solids or adding ‘Lost Circulation Material (LCM)’ to the mud system. The flow of mud into the pore throat of the rock is stopped when the solid particles in the mud sufficiently block them. Causes, Effects, and Control of Seepage Loss
2. Filtrate loss: Drilling fluid consists of solids in a liquid phase. Filtrate loss is the loss of the liquid phase into the rock. Operationally, the industry does not differentiate between seepage and filtrate losses; both collectively refer to seepage loss. Filtration control materials are added to the mud system, but filtrate loss cannot be stopped unless effective blockage of the pore throat is achieved.
3. Vugular loss: If the pore throats of rock are large, severe losses are encountered where even complete circulation can be lost. If the pore throats are bigger than 1/16” in diameter, the rock is called a vugular rock. These vugular pore throats cannot be easily plugged in, and losses are more challenging to control. Since vugular losses are in significant volume, they are measured in ‘Barrels Per Minute (BPM)’ rather than ‘Barrels Per Hour (BPH)’. Vugular-sized pore throats are commonly found in carbonate, gravel, or uncompacted formations.
4. Fracture losses: This type of loss occurs in fractured formations. The fractures may be natural, induced, or a fault connecting to fractures. The fractures are induced if the wellbore pressure exceeds the resisting rock strength. Usually, the barite particles are large enough to block pore throats in most sandstones quickly. Hence, for losses to occur, the fractures should either be large enough or induced broad enough for the barite to enter the opening.