Abnormal variations of pit volume are a prime indicator of mud losses and formation fluid influxes. However the detection of gain and loss at the pit level can be difficult because of the transient behavior of several components in the hydraulic loop. For instance, during circulation the compressibility of the drilling fluid changes the total volume of mud needed to fill the well compared to static conditions. Similarly, changes of the drilling fluid temperature modify the volume of mud due to its thermal expansion. Furthermore, the transport and separation of cuttings also influences volume variations in the pit. Finally, the retention capacity of some of the mud transport and treatment equipment (e.g., return flow-line, shakers, sand-trap, degasser, transfer pit) has a direct impact on the active pit level. All of these effects are transient and can cause substantial variations of the active pit volume that may be interpreted as gain or loss while they actually have natural origins.

Most of these effects are well known and are dealt with in a pragmatic way by using finger printing between the current pit volume variation and a reference pattern obtained in similar drilling conditions. Nevertheless, the finger printing method has its limitations when the current drilling condition variation does not have an obvious reference pattern. In addition, it necessitates a selection of the reference pattern by an operator and therefore is rather difficult to automate.

This paper describes in detail a transient hydraulic model of the downhole and surface equipment effects on the pit volume variations. Several use cases are also described including early kick detection with small margins during pump shutdown and detection of tiny gains during draw-down tests used in an MPD (Managed Pressure Drilling) operation.

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