Estimating Near-Wellbore Cooling Due to Drilling Circulation and its Effect on Drill Stem Test

When drilling oil, gas or geothermal wells, the temperature difference between the wellbore and the formation can have a significant impact on its surroundings. Fluid circulation after drilling to rid the wellbore of drilling mud causes significant cooling around the wellbore near the bottom-hole. Even after shutting the well in, it often takes a long time for this near-wellbore zone to get back to the original formation temperature. This zone of altered temperature around the wellbore can give rise to various phenomena. It may influence wellbore stresses that may cause wellbore instability in high temperature formations that may lead to formation breakdown, or loss of circulation, or even a false kick. It may also play a part during any formation testing procedures carried out right after drilling. In the initial stages of flow after perforation, during clean-up, fluid flows through this cooled near-wellbore zone and enters the wellbore at a lower than undisturbed formation temperature. Any inference about this early fluid's properties or their flow rates needs to account for the near-wellbore cooling. The estimation of compressible fluid flow rate is further complicated during Drill Stem Tests (DSTs) due to the transient nature of the temperature profile and having to account for multiple fluid entry points. At the start of the production period or when the production rates are changed in multi-rate DSTs, thermal transients take much longer to stabilize than the pressure. Because DSTs are often conducted before the near-wellbore temperature has become stabilized, DSTs are also affected. Thus, estimates of the near-wellbore temperature is often important. This paper presents a model for estimating near-wellbore zone (NWZ) temperature as a function of radial distance and time. Heat lost (or gained) by this zone due to circulation is used to establish temperature distribution in the affected zone using a line-source solution of the heat diffusivity equation.A recently developed model for circulating fluid temperatures in tubing and annulus for offshore wells allows us to estimate heat transfer from the formation to the circulating fluid in the well.Note that various approaches, including the Horner method, are available to estimate any heat recovery of the affected zone for a well that has been shut-in after circulation. The model allows the computation of fluid flow rates and their entering temperature in the wellbore at early times. These estimates can then be used as inputs in DST analysis for more accurate characterization of the reservoir. Thus, the model impacts the deliverability results and in turn affects project economics. The model is validated using a set of field data.