Abstract
Pre-salt strata are more and more common in recent years, challenging the petroleum industry. The reservoir in Kenkyak Oilfield (Kazakhstan) is found in deep formation beneath the ultra-thick salt dome, which brings lots of challenges such as mud loss, well kick, pipe-sticking, borehole caving, low ROP and so on. As for Halfaya Oilfield in Iraq, HF-10 is expected to be drilled as the first vertical exploration well in the region, targeting Baluti/Kurra Chine, but the complicated multiple pressure regime, cross-bedded high-pressure and low-pressure formations has been challenging the operator for decades, especially the two high-pressure formations above Yamama, one is the Lower Fars layer featured by salt-gypsum and a high pore pressure coefficient. How to safety and efficiently penetrate pre-salt strata and ultra-thick salt domes are rather important issues.
Logging data of all wells that had been drilled previously in the region were studied to analyze causes of low ROPs and downhole complexities of all kinds. A pilot well was drilled in Kenkyak Oilfield and optimized drilling technologies were applied on it, so as to make a comparison between it and adjacent wells. Rock cores in the salt-gypsum formation were collected to analyze its creep deformation law on a basis of rock mechanics and establish the creep deformation formula. Important findings have been analyzed, in order to deal with the similar scenarios in Iraq.
The upper Mesozoic formation contains sandstones and intercalated mudstones with ooze, which increases permeability and hydration grade. However, problems such as mud loss, borehole collapsing and bit ball-up always happen. Shallow secondary gas with a limited volume and high pressure was drilled in the pilot well in Kenkyak Oilfield. The polymeric mud system and integrated solid control devices were used for the large diameter borehole to deal with mudstone hydration. In the Permian ultra-thick salt dome salt and gypsum dissolve to cause borehole caving and creep deformation. Therefore, a mud system that resists to salt and gypsum contamination and a more advanced maintenance system was adopted. Compatible techniques and tools have been deployed to enhance ROP and mitigate potential challenges. As for HF-10 in Iraq, Low Fars that is featured by abnormally high pressure contains salt, thus two feasible well schematics have been proposed, in order to address challenges of creep deformation of salt-gypsum. Feasible mud systems have also been proposed, in order to mitigate loss, borehole sloughing and pipe sticking in the salt bed and pre-salt strata. Bit selection has been finalized, while a feasible vertical drilling tool has been proposed to mitigate deviation caused by salt.
42 wells were drilled in Kenkyak, among which 29 wells were abandoned due to engineering complexities. As for the pilot well, no downhole complexity happened, the average ROP in the salt dome was improved by twice. The mud density has been optimized. HF-10 that is expected to be drilled in 2026 is going to be the deepest vertical well in Halfaya Oilfield, integration of all of the solutions to challenge pre-salt strata contributes to reduction of risks and costs.
