*4.2. Circulation Time*

Figure 8 displays the TDR and annulus temperature distribution with various circulation times. The circulation time refers to the period of drilling and subsequent clean-up operations just before the perforation. Although circulation time can be varied significantly depending on the geologic and operational issues, one, three, and seven days of circulation are in the reasonable range in typical onshore fields. In Figure 8a, the minimum TDR depths indicate around 1000 m regardless of the different circulation times. The temperature dis-

tribution justifies this observation; the three temperature curves intersect the formation temperature line around 1000 m (Figure 8b).

**Figure 8.** The sensitivity analysis of thermal disturbance with varying circulation times: (**a**) TDR and (**b**) fluid temperature in the annulus.

A noteworthy finding in Figure 8 is that the temperature variation with the various circulation times is somewhat trivial, while the TDR variation is significant. The TDR is around 17 for one day of circulation. The TDR increases almost double when the circulation lasts one week. This observation can be illustrated from the perspective of heat transfer rate in the two different domains: the formation and the wellbore wall. At the wellbore wall, the circulating mud is powered by pumps at the surface. The dominant forced convection by the pumping takes away heat from the formation. The heat removal triggers heat transfer from the outer formation boundary to the wellbore. As the fluid in the reservoir porous medium is static before the perforation, the flowability is insufficient to induce convection. Thus, conduction exists as the only heat transfer mechanism in the formation. The heat conduction in the ground is less effective than convection heat transfer in fluid. The cooling effect appears to be moving radially towards the outer boundary. The thermal disturbance continuously propagates further from the wellbore, keeping the increase in the TDR.

A lengthy circulation time leads to a large TDR, which may impair temperature measurement accuracy after drilling. The circulation time is not easily manageable because it heavily depends on the total depth and ROP. Moreover, when operational problems take place, managing the circulation time becomes even more challenging. When the circulation time becomes longer, any alternative is required to compensate for the accuracy of the measured temperature.
