Abstract:

Nitrogen injection in Tahe oilfield is one of the important measures to enhance oil recovery in carbonate reservoirs, and has achieved good effect of reducing the gradual decrease and increasing oil production. However, the purity of nitrogen produced by membrane separation and carbon molecular sieve separation is 97 % and 99 %, respectively. And the injected gas contains a certain amount of oxygen. With the increase of gas injection cycle rounds, borehole corrosion becomes more and more serious. In order to solve the problem of wellbore corrosion in nitrogen injection wells, dynamic corrosion simulator of high temperature and high pressure was used to study the corrosion mechanism of nitrogen injection wellbore, and the effects of oxygen content of gas injection, temperature, injection pressure and injection rate on corrosion rate were measured. Experiments show that the corrosion rate of pipe in simulated downhole environment increases by 1.3~2.0 times from 70 ℃ to 110 ℃, reaching 4.23 mm/a; the oxygen content increases from 0.7 % to 1.5 %, and the corrosion rate of pipe in simulated downhole environment increases by 0.7~1.0 times, reaching 2.91 mm/a. Thus, the main controlling factors of wellbore corrosion are oxygen content and temperature. Meanwhile, corrosion countermeasures of nitrogen injection wellbore are discussed, and the idea of improving purity of nitrogen production, reducing oxygen content, matching liner and corrosion inhibitor is put forward. It can effectively control corrosion of nitrogen injection wellbore.

Key words: nitrogen injection, oil recovery, oxygen corrosion, corrosion mechanism