Assessing the Shear and Plastic Properties of Danian Reservoir Chalk Under Different Conditions of Stress, Saturating Fluid and Temperature: a Quantitative Comparison with Maastrichtian Chalk (North Sea)

The North Sea provides a high economic contribution to four major countries (Denmark, Norway, UK and Netherlands) due to oil and gas discoveries from the 1960’s. Yet, seafloor subsidence, well deformation and damage of casing had occurred in some of the North Sea hydrocarbon fields caused by changes in situ conditions (stress, saturating fluids, temperature) that in turn result from deformation of chalk reservoirs during drilling, hydrocarbon production and waterflooding activities. Later, extensive studies on the mechanical behavior of chalk were done to avoid undesired consequences. This paper aims at assessing the effect of temperature, water saturation and porosity on the brittle and plastic deformation of Danian chalk. 178 mechanical tests from the Joint Chalk Research library (JCR) were reviewed (i.e., hydrostatic, uniaxial, shear and two-step). The dataset covered the main porosity range (35% to 42.5%) in which chalk deforms in a reservoir. Zero% and 100% water saturated chalk were considered to assess deformation in the oil-bearing reservoir, water-bearing interval as well as water-flooded reservoir. The effect of temperature at 131 oC was studied that represents the maximum temperature of chalk reservoir in the North Sea (Ekofisk field has a temperature of 131oC and buried at three km). The results indicated softening of chalk strength towards higher temperatures and in case the pore space is saturated with water instead of oil, i.e., the water weakening effect. The strength of the rock also decreased with an increase in porosity. Furthermore, a comparison between chalk of different ages was considered (Danian and Maastrichtian) due to differences in their petrophysical and mineralogical properties. Results showed a stiffer mechanical behavior and a larger transition zone between the onset of pore collapse and plastic deformation for the Danian oil saturated specimens. In addition, scattering of data caused by undetected defects (stylolites, micro-cracks) as well as diagenesis and mineralogy is typical in laboratory studies. Thus, high uncertainties may arise that should not be neglected while interpreting chalk deformation under different conditions of stress, saturating fluid, and temperature. Therefore, a 95% confidence interval was plotted as a statistical method to achieve a high-quality interpretation of results that previously published laboratory studies have not taken it into account. The dataset collected and assessed during this study as well as the extrapolated trends represent key input data for geomechanical modelling. In a next step, the outcome of this work aims at building more realistic compaction simulators used to forecast seafloor compaction and field deformation and thereby mitigating risks associated with hydrocarbon exploitation in chalk reservoirs.