The Evolution of Khuff Completion

Abstract

Abstract This paper reviews the changes and improvements in the design of Khuff Well completions in Bahrain; functions of the components will be emphasized and their performance evaluated. performance evaluated. In general, there have been three basic completion designs. Early wells have 5" liner cemented across the Khuff zones with 5" tubing tied back from the liner to the surface. Subsequent wells were completed with 7" casing with a mechanical set seal bore packer and 5" tubing anchored to it. The latest group of wells are equipped with hydraulic set packers and the tubing is tied back through a floating seal assembly to allow for movement. All designs use various configurations of landing nipples below and above the liner or packer seal assembly. Early designs also packer seal assembly. Early designs also utilized a sliding side door sleeve (circulating device) above the seal assembly. The later designs incorporate a landing nipple below a perforated pup in the tail section for perforated pup in the tail section for flowing bottom hole pressure measurements. Introduction The Khuff Reservoir has been the source of natural gas in Bahrain for the past 16 years. The reservoir, initially discovered in 1948, was not developed until late 1960's. The development of this reservoir can be broadly traced in three phases. Two wells were drilled in 1969 to meet phases. Two wells were drilled in 1969 to meet the gas requirements for an aluminium smelter plant. During the second phase in 1970's, nine plant. During the second phase in 1970's, nine more wells were added. The gas from these wells was utilized (a) to replace the rich Arab gas used for pressure maintenance in an oil reservoir; (b) for artificial lift; (c) for power generation and (d) as a feed stock and fuel in the refinery. With the advent of rapid industrialization in Bahrain and to meet the ever increasing demand for power, ten more wells were drilled from 1982-85. Thus the total number of Khuff gas producers in Bahrain now stands at 21. A daily average production of 530 MMSCF/day and a peak production of production of 530 MMSCF/day and a peak production of 753 MMSCF/day was achieved from these 21 wells in 1986. Changes and improvements have been implemented in all Khuff related activities, including drilling, completion, testing, production and processing since the early stages of development. processing since the early stages of development. RESERVOIR CHARACTERISTICS The Khuff formation is of middle and late Permian age. It consists of approximately 2100 Permian age. It consists of approximately 2100 feet of dolomitized carbonate rocks with thin beds and nodules of anhydrite. It is overlain by the Triassic Sudair formation. The lower contact is marked by an unconformity. The Khuff is divided into four zones, Khuff 0, I, II and III with Khuff I and II being the principal gas producing zones in the field. producing zones in the field. Khuff 0 is about 290 feet thick and includes oomoldic porosity zone (10 - 15 feet). It has porosity as high as 36 percent and permeability porosity as high as 36 percent and permeability ranging from 0.01 to 640 md, the average being 54 md. Khuff I is about 500 feet with an average porosity of 17 percent and permeability of 295 porosity of 17 percent and permeability of 295 md. There is extreme vertical variation in porosity and permeability. porosity and permeability. Khuff II is about 325 feet. The main porosity occurs at the top of the zone. porosity occurs at the top of the zone. P. 737