Horizontal Matrix Injector Stimulation in Unconsolidated Sand

Abstract

The AX field is located in Deepwater Offshore Nigeria and has been in production for ca.12 years through FPSO with a waterflood scheme providing pressure maintenance and sweep for the reservoirs. Over the years, AX field has moved from the use of deviated frac-pack injectors to horizontal SAS injectors due to significant declining injectivity observed with the deviated injectors. The switch to horizontal injectors was premised on various studies proposing that horizontal water injectors in deep water are expected to have longer well life due to the larger flow area and ability to inject at lower flow velocities (matrix condition) compared to vertical water injectors. The benefits of going horizontal is further enhanced by keeping the well in Matrix mode for as long as possible before switching into frac mode. However, damage is inevitable afterwards, even for relatively clean injection water. This may be primarily due to various factors, one of which is fines influx into the well during abrupt shut-ins/trips from the FPSO. AX field started operating horizontal injectors since 2011. On the average, most horiziontal injectors in AX field begin to experience injectvty decline about 1 – 2 yrs after start of injection as was the case with AX9 injector. AX9 is a horizontal injector providing support to 2 producers in the field. Over the past four years injection had declined from 40 kbwpd to 21 kbwpd at 395 barg IBHP with II of 20 bpd/psi. This drop in the injectivity index led to a production curtailment of ca. 5 kbopd from the supported producers. From a cross learning opportunity, it was identified that there is higher possibility of success (POS) with stimulating horizontal water injectors while still in matrix mode. Leveraging on this knowledge we decided to attempt stimulating the AX9 well for better performance. Stimulation was carried out successfully under matrix condition using a deepwater rig in May 2018. Post stimulation, well was ramped up to 43 kbpwd at 330 barg IBHP. Increase of ca. 15 kbwpd with a corresponding 21% drop in BHP. Initial performance shows a four-fold improvement in injectivity index; current injectivity index is 80 – 100 bpd/psi. The success of the AX9 stimulation has validated early stimulation of injectors while still in matrix mode rather than later when the well would have switched to frac mode. Additionally, the acid recipe has opened opportunities to mitigating injectivity decline in deepwater water injectors.