Abstract

Tabnak gas field is located in the Fars province, Southern Iran. Severe lost circulation problem were encountered while drilling the long interval of large size hole in the highly fractured and porous carbonate formations. To resolve the severe lost circulation problem, Air and air/foam drilling technology was chosen as a main means to drill through the top intervals and an air /foam drilling program was worked out. The field application proved that air and air/foam drilling could resolve the severe lost circulation problem in the highly fractured and porous carbonate formations with very low formation pressure and speed up the penetration rate significantly. The drilling cost also reduced dramatically.

Introduction

At the end of 2001, GreatWall Drilling Company Ltd started to execute the turnkey drilling service contract in Tabnak gas field southern Iran using air and air-foam drilling technology. This is CNPC's first drilling service project in Iran. The basic contract work amount is to drill 19 wells and service 2 existing wells with a total estimated footage of 68400m. The service scope includes drilling engineering, top drive services, mud services, air/ foam drilling services, coring, directional drilling, casing running, fishing, cementing, logging, production test, acid stimulation and completion. The project was concluded at the beginning of 2004. Air and air/foam drilling technology was used to drill through the top sections of all the wells including 36 ", 26", 171/2 "and 12 1/4" borehole sections with a length of up to 2600m. During execution of the project we have tackled a number of technical problems and the penetration rate was increased dramatically resulting in lower costs.

Tabnak Structure Characteristics and Technical Problems
Structure Characteristics and Stratigraphy

Tabnak gas field is an exposed anticline located in mountain area of the sub coastal Fars province, southern Iran. At surface the anticline is about 80 km long and 15 km wide with an altitude of about 1200m. In relation to the top of the Dehram group, Tabnak is a deeper horizon when compared with the other anticlines of Assaluyeh range. The northeastern flank of the structure is steep dipping. From exposed outcrops to the target, formations are consisted of mainly limestone and dolomite and secondly shale with interbeds of gypsum. The upper formations are interbedded with chert and yellow iron ore. Laffanh and Kazhidumi are consisted of shale primarily. The formations below Gadvan are dominated by limestone and dolomite with beds of blackish gray, grayish-green and mauve shale. Some limestone is much argillaceous. The formation sequence and the lithology are same as the other three structures.

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