Unraveling chalk microtextural properties from indentation tests

Abstract

The petrographical, petrophysical and geomechanical properties of the less commonly studied low permeability or tight chalks are presented in this study. The latter are relevant as potential unconventional reservoirs or intra-reservoir seals. Tight chalks encompass different lithotypes, in which the main factors controlling the petrophysical properties as shown in this study are the non-carbonate content and the degree of cementation. Those parameters strongly modify chalk microtexture and thus its porous network, reducing pore-sizes hence altering poroperm properties. In order to better understand the characteristics of tight chalks, an integrated petrographical, petrophysical and geomechanical study was carried out on a set of 65 chalk samples from Northwestern European outcrops, covering a wide range of lithotypes. The dataset gathered covers a broad spectrum of values for the determined petrophysical (e.g. porosities from 9 to 45%) and geomechanical properties (e.g. strengths from 3 to 60MPa). In the framework of this study, indentation tests were performed on the chalk samples. This technique proved to be a successful method to quickly estimate rock strength. Indeed, a good linear correlation (R2=0.90) has been established between indentation strength and UCS. Furthermore, indentation tests yielded valuable information about the chalk properties, both in terms of petrographical (cementation/clay-content), petrophysical (exponential relationship with porosity) and geomechanical (in relation to the elastic parameter and plasticity index) properties. This cheap and easy-to-operate method is a promising tool to indirectly estimate the mechanical parameters of chalks, when core samples are unavailable for laboratory testing in oil-field wells.