A Reserves And Deliverability Study Of A Middle East Gas Reservoir
Authors: Shehabi J. 
Published: December 2007

Various estimates of the gas in place have been made, depending on the availability of reservoir data. The reservoir was discovered in 1948, but due to the lack of a market for gas, was not developed until 1969. Soon afterward, deliverability studies were undertaken to plan well in advance a concrete schedule of well drilling, compressor installation, and other investments. Four computer programs have proved to be extremely useful in this study. proved to be extremely useful in this study

Introduction

Two very large and highly prolific gas reservoirs are found on the Bahrain structure within the 2100-ft thick Khuff Formation. The two main productive zones appear to be of regional extent, but productive zones appear to be of regional extent, but continuity in minor Khuff porosity developments is still speculative. The Khuff is composed mainly of dolomitized carbonate rocks of middle and late Permian age. Productive limits encompassing Permian age. Productive limits encompassing approximately 80,000 acres is based on a reasonable estimate of the gas-water contact being at 11,000 ft subsea. Nominal capacity of producing wells is 50,000 Mcf/D with a wellhead pressure of 2500 psig.

Khuff gas was discovered in 1948 when crestal Well 52 was deepened. Wells 88 and 128 also penetrated and evaluated the Khuff—their objective penetrated and evaluated the Khuff—their objective being to discover an oil column. The latter wells merely proved the large extent of Khuff gas accumulation. Two exploratory wells, Sitra No. 1 and Jarim No. 1, also partially penetrated the Khuff, but their structural positions placed them in the Khuff aquifer. All of the above mentioned wells abandoned the Khuff.

Lack of commercial market for Khuff gas at the time, in conjunction with paucity of reservoir data, precluded any emphasis on precise definition of precluded any emphasis on precise definition of Khuff gas reserves. As a result, reserves up to 1970 were quoted in the range of three to seven times the current estimate.

In the late 1960's, Bahrain planned to embark on an industrialization program that would rely on Khuff gas as the main source of energy. In the latter part of 1968, commitment was made to deliver 66 MMcf/D Khuff gas to a consortium of international companies planning to set up an aluminum smelter commencing in Jan. 1971. Two Khuff wells, Nos. 254 and 255, were drilled and completed during the last half of 1969 to fulfill this commitment. The first detailed study of Khuff gas reserves then was completed in May 1970, and was updated in 1972, using additional data and a slightly deeper gas-water contact. At this stage, it was evident that the Khuff reserves are much less than earlier estimates. Production at the time of the study was 200,000 Production at the time of the study was 200,000 Mcf/D. Firm future demand stood at 342,000 Mcf/D.

In addition, there were substantial potential demands over and above the firm demand. In view of these additional demands, it was essential to undertake a Khuff gas reserves and deliverability study so as to render an opinion on the feasibility of making further commitments.

RESERVOIR DESCRIPTION

The Khuff Formation, of middle and late Permian age, consists in the Bahrain area of about 2100 ft of largely dolomitized carbonate rocks with many anhydrite interbeds. The upper contact is gradational with the overlying Triassic Sudair Formation. The lower contact is a marked unconformity with the Khuff dolomites overlying deeply weathered continental sands of presumed Devonian age.

These reservoirs consist of dolomitized microsucrosic oolite and fine to coarse pellet grainstone.

The porosity is largely of restored intergranular type resulting from solution removal of earlier mineral cement during late diagenesis.

Fig. 1 is a structure map contoured on the top of the Khuff Formation. The general shape of the structure is based largely on projection from Arab Zones depth, although refinement has been necessary in the light of actual well control.