The North Field-South Pars accumulation is the largest single gas accumulation in the world, extending across the Iran-Qatar maritime border. The Permo-Triassic Khuff, Dalan, and Kangan reservoirs have been producing since 2002 in Iran and 1991 in Qatar.

To boost production, both polymer and surfactant acid stimulation techniques have been employed. However, the heterogeneous nature of the Khuff reservoir has caused challenges when implementing effective acid stimulation. Porosity and permeability in the reservoir vary widely, ranging from 4-30% and 3-1800 mD, respectively (Figs. 1 and 2), which is attributed to facies variations and diagenetic processes such as differential leaching, dolomitization, and anhydrite and calcite cementation, which has added to the complexity of the reservoir (Fig. 3).

Despite these challenges, acid stimulation has been carried out successfully, leading to sustained production rates of approximately 78 MMCF/day for two years. This success was achieved through a combination of techniques:

• Acid stimulation was performed in stages, utilizing high pump rates and chemical diversion techniques to direct the acid to the targeted locations.
• Acid was bullheaded at high rates into lower permeability zones to create back pressure, facilitating the movement of acid into those areas.
• Testing indicated that using 28% HCl alternating with fibre-laden acid diversion would be successful.

These techniques have meant that areas of lower permeability that contain important reserves can be appropriately accessed and produced.