SurfChem is a Danish Hydrocarbon Research and Technology Centre (DHRTC) SPRINT project granted to Nico Bovet which ran from 1st October 2016 to 31st December 2016.
Title: Surface Chemistry of Chalk
The use of outcrop chalk analogues to study the behavior of reservoir rocks is common in the industry. Analogues are chosen to have the same characteristics in terms of depositional environment, sedimentary facies, geological history, mineral composition, grain size, permeability and porosity. However, an important aspect is not taken into account. That is the exact chemical composition of the pore-fluid interface, which can dramatically affect wettability.
As a result of a previous project on sandstone Nano-Sand, using techniques that can see at the nanometer scale, we have proven that there is organic material adsorbed on all minerals, even in pores that have not been exposed to oil or gas. Outcrop analogue rocks and reservoir rocks have different organic material, mainly because of the exposure of the analogue rocks to air, rain water, bacteria and other organisms, and erosion. This organic layer plays an important role on the surface chemistry and reactivity of the rocks. Thus properties such as wettability and the ability to adsorb specific ions used in advanced water flooding are influenced by the chemical composition of the surface. We propose to investigate chalk to test for differences in surface composition between reservoir rocks and outcrop analogue rocks, to be able to better understand the behaviour of analogues during core plug testing.
Results – executive summary:
The use of outcrop chalk as analogue for reservoir chalk during core plug test is common in the industry. We have shown, using a technique that can see at the molecular scale, that the surface chemistry of these two types of chalk is different. The amount of organic matter adsorbed at the surface of reservoir chalk is larger than the one from the outcrop chalk, meaning that more functional groups are available at the pore surface in the reservoir. This can affect directly surface properties such as the ability to adsorb ions during advanced water flooding and the wettability of pore surfaces. We proposed that the surface chemistry assessment should be part of the core plug test protocol. The new information would help interpreting the results of such tests.