Abstract

Rotliegendes sandstones have been found to display a wide range of petrophysical and mechanical properties stress sensitivity behaviour. The extent of stress related changes has been found for permeability, electrical conductivity and Young's modulus to be related by the depositional facies. It has also been noted that the relationship between some rock properties which are thought to exist in situ cannot be replicated unless measurements conducted at near in situ stress levels are employed. These results lead to the conclusion that the structure reflected by properties measured at low confining pressures is not representative of that which exists in situ.

Introduction

When core is cut and retrieved to the surface the magnitude of the stresses acting on the rock change. The stress state also changes from the polyaxial conditions in the formation ahead of the core bit, where the principal earth stresses are not equal (), to the hydrostatic stress state () acting on the core in the barrel due to the hydrostatic pressure of the drilling mud. When the in situ stresses are relaxed, the strain energy which will have accumulated in the load-bearing framework over geological time due to burial, tectonic and thermal forces will be released. This will occur at a comparatively rapid rate of several hours in contrast to the millions of years over which the strains developed.

The change in magnitude and stress state will contrive to alter the intimate relationship between the load-bearing framework of detrital and authigenic minerals and the pore structure of the rock. As a result the key petrophysical properties of the rock such as porosity, permeability and electrical conductivity may be significantly altered. A great deal of experimental evidence has shown that the low, nominal confining pressures employed in the laboratory for routine core measurements are generally insufficient to reverse these structural changes. The resultant discrepancy between the value of a particular property evaluated at laboratory stress levels (generally between 250 psi and 1000 psi) and in situ stress levels gives rise to the concept of stress sensitivity. This is most frequently expressed as the ratio of the value of the property measured at effective reservoir stress levels to its value measured in the laboratory.

Stress sensitivity in sandstones and carbonates has been studied experimentally since the 1950's; however, most of the work to date has concentrated on the stress sensitive behaviour of porosity, permeability and electrical conductivity. The stress sensitivity of these properties directly reflect changes in the pore structure of the rock. Much less work has been conducted to examine the effect of stress relaxation on the mechanical properties, which directly reflect the behaviour of the load-bearing structure of the rock. The load-bearing framework is composed of the detrital mineralogy and any authigenic mineral phases, such as mineral cements, which alters the area over which the earth stresses are distributed. Understanding the behaviour of the load-bearing framework due to stress changes may be an important facet of stress sensitivity analysis because the characteristics of the load-bearing framework ultimately control the response of the other properties to stress changes.

This study set out to examine the stress sensitivity of both petrophysical and mechanical properties of a low permeability sandstone formation. The influence of the depositional facies on the stress sensitivity is identified for permeability, electrical conductivity and the elastic (Young's) modulus. This is considered to arise from the remnant influence of the depositional process on the load-bearing structure even though the sediments have all experienced extensive phases of burial and diagenetic mineralisation to varying extents.

THE GEOLOGY OF THE ROTLIEGENDES SANDSTONES OF THE UK SOUTHERN NORTH SEA The Rotliegendes Sandstone Group in Northwest Europe are of early Permian age (270 ma) and were derived from continental clastic sediments deposited in desert and semi-desert conditions which affected the mid-continental regions of Pangea during the Permian and Triassic periods.

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