Abstract

Steam assisted gravity drainage (SAGD) is an oil recovery technique utilized for the production of bitumen. Two horizontal wells are used; low pressure superheated steam is injected into the upper well thereby reducing the insitu oil viscosity and mobilizing the fluid which is produced from a lower well. On occasion wells need to be drilled into or close to existing steam chambers associated with the SAGD wells. When drilling such wells the drilling fluid temperature may become excessively high, endangering personnel at surface, as well as exceeding the borehole stability, bottom hole assembly and/or drilling fluid temperature limits. A thermodynamic model of the drilling circulating process was created to predict the wellbore, mud return and formation temperatures. The model may be used to outline drilling procedures and enable wells penetrating or proximal to the steam chamber to be drilled safely.

The transient thermodynamic model was created with Landmark’s WellCat™ and calibrated by matching predicted drilling fluid temperatures with actual drilling data obtained during the drilling of several cold wells. The calibrated model was used to predict the formation temperature of several hot wells for which drilling data was available. The model was also used to investigate the drilling fluid and wellbore temperatures during the drilling a generic well. This model may be applied for planning the drilling of future hot SAGD wells to ensure the drilling fluid temperatures are not prohibitively high, to calculate necessary cooling capacity at surface and delineate trip speeds/circulation rates necessary to avoid high temperature fluid being circulated to surface.

For instances in which the formation temperature was known the mud return temperature could be matched to within ±1°C. For the cases in which the formation temperature was being estimated by matching the predicted mud return temperature to the actual drilling data the formation temperature was predicted to approximately ±20°C. Formation temperatures of the wells reviewed range between 7° and 200°C. The generic drilling scenarios which were modeled demonstrate that, depending on the formation temperature and the well depth, it is possible for the mud to return to surface at temperatures approaching and potentially exceeding 100°C resulting in the water component of the mud flashing to steam. When entering a hot wellbore, tripping in slowly as well as circulating during the trip may be required to cool the wellbore and avoid returning excessively high temperature mud to surface.

A small number of SAGD wells have been drilled into steam chambers. The model allows this limited experience to be expanded in a controlled manner. The model may be used for planning of future drilling operations by predicting mud temperatures in a variety of drilling scenarios especially with regard to safety concerns. In addition, the need for employment of mud coolers or specific trip procedures can be evaluated. The approximate accuracy of the simulation software employed is also demonstrated.

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