RIASSUNTO
ABSTRACT
Geomechanical tests (triaxial) have been carried out on outcrop chalk samples from Sigerslev Quarry, Stevns, Denmark. The chalk is of Maastrichtian age and has a matrix composed of skeletal debris from calcareous nannofossils, mainly coccolith. The samples had the same high porosity (~46%), the same low permeability (~4 mD), and the same brine saturation (~25%). Half of the samples were left at the initial water wet wettability. The other half of the samples were dynamically aged with crude oil at 80°C in order to change the wettability towards a more mixed and neutral wettability. The geomechanical tests showed that the water wet chalk samples had a very uniform behavior and strength, whereas the samples with the changed wettability showed a more diffuse and scattered behavior and a lower mechanical strength than the water wet group.
1. INTRODUCTION
Water weakening of chalk has been studied extensively since the discovery of reservoir compaction and seabed subsidence at the Ekofisk Field in the North Sea in 1984 [1]. Many theories and explanations have been suggested in order to describe and predict the geomechanical behavior of chalk, when the fluid saturation in the chalk reservoir changes during oil production; especially due to waterflooding. Wettability of chalk has been studied extensively in order to understand the mechanisms governing fluid flow in chalk. The interaction between spontaneous imbibition of water from the fractures and into the chalk and viscous waterfloods has in particular been emphasized. However, the studies have focused on flooding experiments without mechanical properties taken into account. Most geomechanics tests on chalk are carried out on either outcrop chalk or chalk, which has been cleaned. In both cases, the wettability of the chalk is or becomes water wet, i.e. the chalk surface has a preference for having water coating the grains, and it will spontaneously imbibe water, if available. Mechanistically, fully oil saturated water wet chalk is stronger than fully water saturated water wet chalk [2]. Water wet chalk, which is saturated with 25% water and 75% oil, behaves as if it was fully water saturated [3]. However, how much does the location of the oil and water in the chalk affect the strength? That is, does the wettability affect the geomechanical strength of the chalk? Risnes & Flaageng [4] pointed out that wettability is an important parameter in chalk-fluid interaction. They carried out a few tests on neutral to oil wet reservoir chalk and found that when waterflooding oil saturated chalk it showed a strengthening effect. This is exactly the opposite of what will happen when waterflooding a water wet oil saturated chalk. Strand et al. [5] determined tensile failure by carrying out Brazilian tests on Aalborg and Liége chalk (Sw~20%) and found that samples, which had been exposed to crude oil (with or without temperature) were weaker than samples exposed to Heptane. 2. SAMPLE PREPARATION We have studied chalk samples from the same block of high-porosity outcrop chalk from the Sigerslev Quarry near Stevns Klint in Denmark. The Stevns chalk is a very clean chalk with 99% CaCO3 and less than 1% impurities.