Abstract

The objective of the present project was to study how high temperatures affect the different mechanical parameters of chalks. Two types of high porosity outcrop chalks were used. Several series of Brazilian, uniaxial and triaxial compressive tests have been performed at both ambient and reservoir temperature, 130°C the Ekofisk temperature. Both water and glycol were used as saturating fluids, and in addition some test series on dry chalk were included. The results so far show that the yield envelope has a tendency to shrink with increasing temperature. Both cohesion and the hydrostatic yield stress are typically reduced about 20 % in going from 20 to 130°C, but the magnitude of this reduction may be chalk type dependent. For the friction angle there are some indications of a slight increase with increasing temperature, but some results point in the opposite direction. Experiments with dry chalk show some degree of strengthening, probably due to drying effects.

Introduction

During the last decades, mechanical behavior of porous chalks has been extensively studied in order to understand the mechanisms behind well instability, compaction and subsidence experienced in the North Sea chalk reservoirs. However, most of the laboratory work performed, has been carried out at ambient temperature. This greatly facilitates laboratory studies, and temperature effects on mechanical properties of chalks have received limited attention. But the temperature in the chalk reservoirs is rather high. For instance, the temperature in the Ekofisk reservoirs is around 130°C, and if mechanical properties are temperature dependent, the effects could be quite significant.

So far only few studies on temperature effects in chalks have been reported and the results point in different directions. Risnes1 (1990) observed a rather strong temperature dependence in a series of chalk extrusion experiments performed with reservoir chalk from a Danish offshore gas field. Cylindrical water saturated samples were compressed axially in a rigid oedometer type cell with an outlet opening in the bottom. The diameter of the opening was 10 mm. Tests were performed both at 20 and 90°C, and two typical stress-strain curves are shown in Fig. 1. At low stresses the increase in temperature seems to soften the chalk, but as the stress level increases, the high temperature chalk becomes stiffer, and a much higher extrusion pressure is needed. These experiments were carried out with deformation control, but the same temperature effect has later been seen with load control equipment (Bukkholm2, 1990).

A study by Charlez3 (1992), also on a Danish reservoir chalk, but from a different field, concluded in a strong temperature effect in pore collapse failure, but no effect at shear failure conditions. Another study by Addis4 (1989) showed no significant temperature effects in uniaxial strain compaction experiments.

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