Abstract

In the Central Luconia area, offshore Sarawak, substantial gas reserves are present in Miocene carbonate buildups. The carbonates consist of present in Miocene carbonate buildups. The carbonates consist of limestones and dolomites with porosities ranging from 0 to 40 percent. From core analysis it became evident that there exists a potential problem with regard to the compaction of the carbonate reservoir matrix as a result of effective stress increase as the reservoirs are depleted. Triaxial compaction tests were carried out on core samples from several carbonate reservoirs. It was found that highly porous mouldic limestones show pore collapse at relatively low effective stresses. After pore collapse the uniaxial compressibility coefficient increases significantly.

Reservoir compressibility parameters can be derived from core data, for collapsing as well as non-collapsing rocks. These can be used to calculate reservoir compaction from well logs for a particular pressure decline. An estimate of the expected surface subsidence can be made based on the theory of poro-elasticity and the nucleus of strain concept as described by Geertsma. Predicted subsidence figures have been taken into account in platform design. platform design. The gas reservoirs have large aquifers within the carbonate buildups. As these aquifers have not all been fully appraised, there exists some uncertainty concerning their actual compaction behavior.

Introduction

In the Central Luconia area off shore Sarawak substantial gas reserves are present in Miocene carbonate buildups. Sarawak Shell Berhad, operating present in Miocene carbonate buildups. Sarawak Shell Berhad, operating under a production sharing contract with PETRONAS, recently developed 2 gas fields (the E11 and F23 fields) in Central Luconia and the development of further fields is planned, supplying gas to a 6 million tons/annum capacity LNG plant located at Bintulu, Sarawak. The gas bearing carbonates consist of limestones and dolomites with porosities ranging from 0 to 40 percent. A considerable amount of reservoir rock consists of highly porous carbonates exhibiting a mouldic porosity. Mouldic porosity occurs when part of the rock matrix has been dissolved by fresh water leaching. The part of the rock matrix has been dissolved by fresh water leaching. The carbonates have Brinell hardness values (reference 7) ranging from as law as 1.6 kgf/mm2 in the highly porous mouldic limestones, to about 12 kgf/mm2 in dolomites.

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