Abstract
Gamma field, a brown onshore field in Niger Delta, Nigeria, has been producing oil and gas from normally hydrostatic pressured reservoirs for over 40 years. In 2017, a drilling campaign, targeting two HPHT natural gas reservoirs, kicked off as part of a gas development project to supply gas to the export gas market. This was the first of its kind in SPDC with seven wells planned to be drilled in a campaign mode to optimize project cost and timing. Being the first experience of drilling and completion of HPHT wells, there were several challenges encountered. Drilling challenges includes stuck pipes and loss of BHAs leading to several hole sidetracks. Completions challenges encountered were sustained high casing head pressures due to material corrosion and inflow impairment due to drilling and completions fluids used.
A very significant challenge was the loss in well productivity of these high-rate HP gas wells due to the heavy barite-weighted drilling mud used to drill the reservoir section (for well control) was also left in hole for over 6 months before well clean up and production. Therefore, there was a mixed outcome from the drilling campaign, with over 50% of the wells experiencing a potential loss (the wells were tested to rates of 20 – 85MMscf/d against an expectation of over 100MMscf/d.
The loss of productivity was adjudged to be due to the caked barite across the reservoir section below the Formation Isolation Valves (FIV) which plugged the sand control screens, resulting in very high drawdowns. Furthermore, two of the wells produced with high casing head pressures (High A-Annulus pressures) equaling the Maximum Annulus Allowable Surface Pressures (MAASP) of the wells and were closed in for well integrity reasons.
As part of well productivity restoration, some premium bespoke cocktail of stimulation chemicals was selected (Imbazi, Oyeintonbra, et al, 2019) and with an effective well intervention program was designed and executed by joint team of local and global Experts. Currently, 5 of the 7 originally planned wells have been restored post well intervention and are producing above expectation.
This paper discusses how the value of these wells are ultimately realized, what was done, what was achieved and how the learnings from the first drilling campaign were incorporated to achieve better results in later wells.
