Design, Execution and Evaluation of Acid Stimulation Job in a Subsea Deepwater Well

Olatunji, Idris (Shell Nigeria Exploration and Production Company) | Ogunsina, Oluseye (Shell Nigeria Exploration and Production Company) | Okosun, Joshua (Shell Nigeria Exploration and Production Company) | Horsfall, Ipalibo (Shell Nigeria Exploration and Production Company) | Agbahara, Chidi (Shell Nigeria Exploration and Production Company) | Smith, Seun (Shell Nigeria Exploration and Production Company) | Yahaya, Ismael (Seplat) | Ogoja, Oluwafemi (Schlumberger)



This well was drilled and completed as a Frac & Pack oil producer with subsea wellhead in 3300 ft water depth. Despite all the care taken to maintain cleanliness during completion, initial performance on starting up the well indicated that it was severely impaired during the drilling phase with drawdown as much as 2300 psi observed. The well initially failed to reach the Hydrate Dissociation Temperature (HDT) of 35C within the specified period of 1 hour.

The well was eventually successfully started and some performance improvement was observed as the well was cleaned up gradually over the next few months but the potential of the well had to be revised down from the planned 30kbpd to 12kbpd to honour drawdown limit of 350psi.

This paper documents the effort made to restore the well’s planned production of 30kbpd by acid stimulation treatment by bullheading from the rig. From the well history, the source of impairment was attributed to drilling fluids. Thus, a half-strength mud acid was chosen as the treatment fluid and the placement was by high-rate bullheading from the rig connected to the subsea Xmas tree via a riser and EDP/LRP. The risk of flowing unspent acid through subsea flowlines and topsides equipment on the FPSO was assessed. Corrosion simulation studies showed the subsea flowlines can withstand the possible unspent acid with proper dilution by flowing high water cut wells at high rate through the same bulk flowlines being used to unload the just treated well. The dilution also helped protect the topsides equipment while further protection was provided by injecting diluted sodium hydroxide into the flow stream as it enters the topsides.

The result of the treatment was that productivity index increased by a factor of 3 from 27 bpd/psi to ca. 90 bpd/psi and the well is now producing at 30 kbopd.