Types of Losses
Lost returns or mud loss is a situation of partial or complete loss of circulation in the well. It’s a common downhole problem and is more prevalent in certain regions like the Middle East, North Africa, and Latin America. As per 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 cause well control issues and environmental damage as well. The 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. In general, it is 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 they are sufficiently blocked by the solid particles in the mud. 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, and both are collectively referred to as 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 very big, severe levels of losses are encountered where even complete circulation can be lost. If the pore throats are larger than 1/16” in diameter, the rock is termed a vugular rock. These vugular pore throats cannot be easily plugged and losses are harder 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 any uncompacted formation.
4. Fracture losses: This type of loss occurs in fractured formations. The fractures may be natural fractures, induced fractures, 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 quickly block pore throats in most sandstones. Hence for losses to occur, either the fractures should be large enough or should be induced wide enough for barite to enter the opening.