Permeability Impairment Due To The Presence Of Kaolinite In The Matrix Of Reservoir Rock

Abstract

Abstract It is well known that clay minerals reduce the permeability of a reservoir rock. Significant problems were encountered in the Cormorant field when it was found that most of the clay was composed of kaolinite, a non-radioactive mineral. This paper discusses a method for identification of the presence of this mineral. Section 1 INTRODUCTION 1.1 Description of the field The Cormorant field is located in the Northern sector of the North Sea, see fig. 1, and covers blocks 211/21 and 211/26. Oil was discovered in 1972 and since then appraisal drilling has continued in order to define the extent and size of the discovery. The field has been divided into two areas: North Cormorant in block 211/21 and South Cormorant in 211/26, see fig. 2. Development plans for South Cormorant are proceeding but plans for North Cormorant have been complicated by the discovery of unexpectedly low permeabilities in some wells. 1.2 Geological aspects of the field This paper is concerned with the Middle Jurassic Sands which are the main oil-bearing rocks in the Cormorant field; they are subdivided into five lithological units - Broom, Rannoch, Etive, Ness and Tarbert. Mineralogically the sands are predominantly quartz grains with subsidiary feldspar, mica, lithics and clay matrix, with calcite and siliceous cements in varying minor amounts. The fractions of mica and clay matrix present together with the grain size are the principle variables differentiating the lithological units. The Broom Sands are the product of sediments which have been resorted by subsea wave action and are poorly sorted, burrowed, pebbly sands. The Rannoch Sands represent a shallowing of the prehistoric sea with the result that the sands become progressively less shaly, more porous and permeable towards the top which is characterised by the presence of mica rich laminae. Calcite cemented bands are present especially in the upper part of the section; they are considered to represent local concentrations of shell debris which have undergone solution and recrystallisation. The Etive Sands represent the terminal phase of the shallowing and may be locally cut by channels. These sands are the best reservoir rock in Cormorant as they are cleaner, coarser and contain only traces of mica compared to the Rannoch Sands. The Ness Sands consist of sand-shale sequences deposited in a deltaic environment and they generally exhibit good porosities and permeabilities. The Tarbert Sands have been definitely identified in two wells but due to a lack of data no comment on the mineralogy can be made. 1.1 Description of the field The Cormorant field is located in the Northern sector of the North Sea, see fig. 1, and covers blocks 211/21 and 211/26. Oil was discovered in 1972 and since then appraisal drilling has continued in order to define the extent and size of the discovery. The field has been divided into two areas: North Cormorant in block 211/21 and South Cormorant in 211/26, see fig. 2. Development plans for South Cormorant are proceeding but plans for North Cormorant have been complicated by the discovery of unexpectedly low permeabilities in some wells. 1.2 Geological aspects of the field This paper is concerned with the Middle Jurassic Sands which are the main oil-bearing rocks in the Cormorant field; they are subdivided into five lithological units - Broom, Rannoch, Etive, Ness and Tarbert. Mineralogically the sands are predominantly quartz grains with subsidiary feldspar, mica, lithics and clay matrix, with calcite and siliceous cements in varying minor amounts. The fractions of mica and clay matrix present together with the grain size are the principle variables differentiating the lithological units. The Broom Sands are the product of sediments which have been resorted by subsea wave action and are poorly sorted, burrowed, pebbly sands. The Rannoch Sands represent a shallowing of the prehistoric sea with the result that the sands become progressively less shaly, more porous and permeable towards the top which is characterised by the presence of mica rich laminae. Calcite cemented bands are present especially in the upper part of the section; they are considered to represent local concentrations of shell debris which have undergone solution and recrystallisation. The Etive Sands represent the terminal phase of the shallowing and may be locally cut by channels. These sands are the best reservoir rock in Cormorant as they are cleaner, coarser and contain only traces of mica compared to the Rannoch Sands. The Ness Sands consist of sand-shale sequences deposited in a deltaic environment and they generally exhibit good porosities and permeabilities. The Tarbert Sands have been definitely identified in two wells but due to a lack of data no comment on the mineralogy can be made.