Petrophysical Changes due to CO₂ - Reservoir Rock Interaction – A Study Case

This article analyzes the alteration of petrophysical properties due to the rock-CO2 interaction in saturated Cretaceous sandstone with high geological CO2 content. It is based on the response of different physical principles tools and the data integrated into a workflow previously applied successfully in several mature fields. The knowledge generated sheds light on the causes of oil production of a sand reservoir falling rapidly, up to approximately 25% per month, from more than 1000 bbls/day initial rate.

Specifically, we analyze the impact of CO2 on the petrophysical properties of the rock by integrating X-Ray Diffraction, formation water composition, production history, decline index, stimulation results, Oddo-Thomson’s calcite saturation, and information from conventional and special logs in a typical well. This methodology can be easily adapted to other deposits with similar characteristics.

CO2 distorted the density logs and NMR logs readings. That causes porosity overestimation with the density logs and under-estimation with NMR logs. Therefore, this is an excellent indicator of CO2 presence in the pore space. To obtain the correct effective porosity, we used the density neutron logs.