In the development and production process of offshore heavy oil thermal recovery wells, the rise of annulus fluid temperature will lead to extremely high annular pressure, which will threaten the casing safety and wellbore integrity. Nitrogen injection is often applied to release the annular pressure. However, insufficient injection can't solve the problem and too much nitrogen will lead to economic loss and poor effect. Therefore, there exists an optimal injection volume in nitrogen injection operation.

In this study, based on the thermodynamic principle and the PVT equation, the reducing mechanism of the nitrogen physical properties and injection volume for annular pressure was analyzed and a calculation method is presented. Then a well in Bohai sea of China is taken as an example to calculate the optimal injection volume and analyze the influence of nitrogen physical properties, annulus temperature and casing strength parameters on the optimal injection volume. At last, the prediction model of the optimal injection volume of annulus nitrogen and a safety evaluation method for production string under nitrogen injection are proposed and some optimization methods are given for nitrogen injection.

The results show that nitrogen injection can effectively reduce annular pressure over 40%. With the increase of nitrogen injection volume, the reduction rate of annular pressure and the injection efficiency decreases gradually. The optimal injection volume of annulus nitrogen is mainly determined by annulus temperature, intial temperature and pressure of nitrogen. Injecting nitrogen at the optimal volume can maximize the economic and safety benefit. It is necessary to control injection volume when taking nitrogen injection operation.

This work provides a prediction model for determining the optimal nitrogen injection volume to reduce annular pressure buildup, and a production string safety evaluation method was also proposed. These can provide guidance for field nitrogen injection operation.

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