A well engineer from Slb recently demonstrated the tackling of injectivity issues on an offshore well during the Society of Petroleum Engineers’ (SPE) Nigeria Annual International Conference and Exhibition.
Characterised by high-permeability and unconsolidated sands in a prolific turbidite reservoir, pressure maintenance at the AX field were initially conducted by deviated frac-pack injectors which fell short in keeping issues such as fines migration and other formation damage mechanisms in check.
To address this issue, horizontal injectors were chosen, offering larger flow areas and lower velocities to mitigate sand production risks. This, however, did not turn out enough for the AX9 well, as water injectivity level still remained markedly low. It required proactive intervention to curb formation damage while ensuring long-term well integrity. Advanced surveillance techniques, including Hall plot analysis, were deployed to confirm that injectivity decline was caused by internal formation damage rather than wellbore restrictions. Mineralogical analysis revealed the presence of fines-prone clays such as Smectite, Illite, and Kaolinite, which contributed to plugging and injectivity impairment.
This gave birth to a stimulation strategy tailored to AX9’s specific reservoir characteristics. Early intervention was initiated while the well was still operating in matrix mode, ensuring maximum benefit from the stimulation process. In line with the AX9 formation, a matrix acid stimulation plan was developed utilising a nitrified mud acid system with continuous foam diversion. The metculous preparation of critical components were involved from high-pressure nitrogen units and fluid pumps to IWOCS systems, among others.
The nitrogen foaming-supported treatment sequence involved multiple stages, including ammonium chloride pre-flushes, acid pre-flushes, a main stage of 13.5:1.5 mud acid, and post-flushes. This made sure even acid distribution and effective fines removal.
Post-treatment the injection rate improved to 43 kbwpd, with a substantial pressure drop of 21% and the injectivity index rose from 20 to more than 80-100 bpd/psi. Besides the improved injection rates, the treatment helped boost production by an additional 5,000 barrels per day approximately.
