Drilling operations involve significant heat transfer between the drilling mud, downhole tubulars, and surrounding formation. Such heat transfer causes changes in drilling fluid temperature that alters drilling fluid density and viscosity, as well as changes near-wellbore formation temperature. Temperature changes in the near-wellbore formation need to be understood so that useful interpretation of often available temperature data from multiple discrete temperature sensors (MDTS) may be made.
In deepwater assets, fluid circulation through cold water makes the problem more complex. Deepwater drilling operation could be viewed as consisting of four processes: (1) mud circulation in the riser affected by surrounding cold sea water; (2) mud circulation in the cased and cemented zone; (3) mud circulation through the target zone (open hole); (4) shut-in after drilling through the target zone. Forced convective heat transfer dominates in the first three processes while conductive heat transfer is dominant during the shut-in period. Estimating temperature in the wellbore during and after circulation is critical for mud rheology, tubular thermal stress, and cement design. Application of “rule-of-thumb” and/or complicated numerical simulation is often unreliable and/or impractical.
This paper presents analytic models to estimate temperature profile during and after drilling fluid circulation in deepwater environment. Steady heat transfer is assumed in during fluid circulation, and transient modeling is performed for shut-in periods. Energy balance is set up over the differential control volume to develop the models. The end of circulation would provide the initial condition for the shut-in period. The models are used to estimate bottomhole temperature distribution during and after circulation. The analytical model is verified using data from a real deepwater well that had permanent downhole gauges (PDGs) installed at the bottomhole.