PETROPHYSICAL ANALYSIS OF CARBONATED WATER INJECTION EFFECTS ON LACUSTRINE CARBONATES OF MUPE MEMBER, LOWER PURBECK GROUP (UPPER JURASSIC), UK

Abstract

This research studies the dissolution and mineralogical alteration caused by carbonated water injection (CWI) and its effects on the petrophysical properties (porosity and permeability) of limestone samples from the Mupe Member, composed of lacustrine microbialites from the Upper Jurassic, part of the Purbeck Group lower portion, located in southern England and northern France. These limestones are a partial analogue of the Brazilian pre-salt Aptian carbonates, the most important oil reservoir in the country and which presents large amounts of CO2 that is reinjected into the reservoir (which, given the high reactivity of carbonate rocks in the presence of carbonic acid generated by the reaction between CO2 and water, can cause damage to the formation). Due to the few studies carried out so far directly related to the theme using these rocks, this research is presented as interesting and relevant. To achieve the proposed objectives, the samples (four with low permeability values and two with very high permeability) underwent laboratory tests carried out before and after the carbonated water (desulphated sea water saturated by CO2) coreflood. The tests aimed to characterize (1) the porous space of the rock through quantification of the volume of grains, total porosity and permeability to gas (via routine petrophysical tests), characterization of the pore size distribution (with the use of nuclear magnetic resonance - NMR) and 3D imaging of the porous space in detail (through X-ray computed microtomography or micro-CT) and (2) the chemical and mineralogical composition through powder X-ray diffraction – XRD (identify all the mineral phases present in the samples on a macro level) and description of petrographic thin sections (allows the interpreter to analyze in detail the mineralogy of the rock and characterize the type of porosity, in order to assist in the identification of the porous space by micro-CT imaging). After characterization, a single-phase flow of saline carbonated water was carried out to identify the physical-chemical and petrophysical changes generated by the CO2 interaction with the rock (different pore volumes were injected in each sample). These changes are vital for optimizing CO2 injection rates in carbon sequestration projects. During percolation, the effluent brine was sampled periodically, and ion chromatography identified elements added to the fluid due to the reaction with the rock. Finally, post-injection tests of the plugs were carried out, performing some of the analyzes mentioned above to compare the results and identify the chemical and petrophysical alterations – changes in the values obtained by the routine petrophysical tests, variations in the pore size distribution obtained by NMR and visible changes by micro-CT in the porous space – generated by the carbonated water coreflooding. The experimental results show that samples with high permeability showed a small decrease in permeability, indicating formation damage, while low permeability samples presented a significant increase in permeability with little change in porosity, indicating feasibility for Carbon Capture and Storage (CCS) in similar samples in likewise conditions. For samples with more pore volumes injected, the pressure stabilization seems to have favored dissolution in the later injection stages, indicated by the highest output of calcium ions in the effluent brine. Salt precipitation presented itself as a possible issue, especially in more heterogeneous rocks. carbonated water injection; coreflood; CO2 injection; petrophysics; pre-salt analogue.