Fanny Descamps

Assessing Mechanical and Petrographic Properties of Fine-Grained Formations From Samples Collected in Deep Oil Wells

Abstract Gas shales are fine grained fissile detrital sedimentary rocks. But shale is often used as a catch-all term including different rocks with a non-negligible clay minerals content which leads to typical physical-chemical characteristics. Several properties are generally assessed for shale gas plays (TOC, porosity…). However, regarding the access to the reservoir, drilling into the formations implies a good knowledge of their mechanical and petrographic properties. As classical cores are quite difficult to collect from deep wells, unconventional techniques using small samples are useful ( Tshibangu et al. 1999 ). When dealing with hydraulic fracturing, the formation brittleness is to be assessed. Several estimators exist among which we propose the so named plasticity index determined from punching tests. In the framework of a collaboration of more than fifteen years between the university and the drill bits manufacturer, samples coming from all over the World and associated with difficult drilling conditions have been collected and processed in the laboratory. This work involved thin section characterization and mechanical tests like FPMs abrasiveness, Shore hardness and punching tests. Recently, a database was then built in order to analyze the cumulated information ( Descamps et al. 2013 ). In the context of gas shales, this database includes various fine grained rocks like shales, claystones, fine grained sandstones (i.e. grain or crystal size below 125 microns). In this paper, two main axes are developed. First, we describe typical properties of fine grained rocks, especially when they are associated to difficult drilling environments. Then, we look to how those properties can be linked together. Particular interest will be taken on abrasiveness regarding the selection of drilling bits and on brittleness assessment in relation to hydraulic fracturing.

Tight Chalks - How Does Microtexture Affect Petrophysical and Geomechanical Properties?

High porosity chalks have been extensively studied over the past decades. Only recently, there has been an increasing interest in the understanding of low permeability chalks, forming potential unconventional reservoirs or intra-reservoir seals. In order to better understand the properties of those tight chalks, an integrated petrographical, petrophysical and geomechanical study was carried out on a set of 35 carefully selected outcrop samples, covering a wide range of lithotypes. The dataset gathered covers a broad spectrum of values with regards to determined petrophysical (e.g.porosities from 15 to 45%, pore throat diameters from 25nm to 1080nm) and geomechanical properties (e.g.: strengths from 3 to 50 MPa). The samples have also been characterized in terms of microtextures by integrating both geological and sedimentpetrological data. Tight chalks encompass different lithotypes, but the main factors controlling the microtexture are: (1) the non-carbonate content, either related to sedimentological settings (e.g. argillaceous chalks) or burial diagenesis (e.g. marl-seam chalks); (2) the degree of cementation, either eogenetic (e.g. hardgrounds) or mesogenetic. Those parameters strongly modify chalk microtexture and thus its porous network, reducing pore-throat and pore body sizes hence altering poroperm properties. Beyond petrophysical properties, cementation appears to be the main parameter controlling the strength of chalks.