Water Performance in Toe-Point Injection Wellbores at Supercritical State

Most of the previous works were focused on the saturated/superheated steam flow in wellbores coupled with conventional single-tubing injection method. With the rapid development of technology. Supercritical water coupled with toe-point injection technique is proposed. Compared with conventional method, supercritical water could heat the reservoir to a higher temperature, obtain a larger heated radius, and obtain a higher thermal cracking efficiency etc. Besides, toe-point alternating heel-point injection could release the phenomenon of unequal absorption of steam when the horizontal wellbore is extremely long or the reservoir is of serious heterogeneity. This paper presents a model for estimating thermal properties of supercritical water along the inner tubing (IT) and annuli in the horizontal section of the wellbores with toe-point injection technique. Firstly, a flow model in wellbores is proposed based on the mass, energy and momentum conservation equations. Secondly, coupled with flow model in reservoir, a comprehensive mathematical model is proposed. Thirdly, type curves of supercritical water flow in horizontal wellbores with toe-point injection technique is obtained by finite difference method on space and iteration technique. Finally, sensitivity analysis is conducted. Results show that: (a) supercritical water temperature decreases rapidly from heel-point to toe-point in IT. The temperature decrease rate near toe-point of wellbores becomes smaller. (b) The larger the pressure difference, the larger the mass injection rate from annuli to oil layer. (c) When the mass injection rate is small, heat loss from fluid to reservoir plays an important role on temperature drop. (d) When the injection rate is high enough, the effect of heat loss on temperature drop becomes weak. (e) The pressure of supercritical water at a certain place in IT or annuli decreases with injection rate.