Carbonate Petrophysical Properties: Computed Tomography (CT) for Experimental WAG Design Under Reservoir Conditions

Abstract

The understanding of single and multiphase flow behavior in porous media has been improved with the development of in situ saturation measurement techniques such as X-ray Computed Tomography (CT), mainly in specialized core analysis. On the other hand, effective experimental designs are necessary to advance knowledge on operations of Water Alternating Gas (WAG) and Carbon Capture and Storage (CCS) projects. The present study addresses the determination of petrophysical properties concerning fluid storage and displacement in carbonates by using CT images taken during core flooding runs. Eight displacement experiments were carried out in long core to analyze N2 and CO2 flooding under reservoir conditions (from 700 to 7000 psi) at temperatures of 22°C and 65°C. A carbonate core sample of 5 cm diameter by 76 cm long with porosity of 15% from a carbonate outcrop analogous to Brazilian pre-salt reservoir rocks was used in the displacement tests. The mixing of CO2 and brine was a key experimental procedure to evaluate the CO2 trapping. The parameters of porosity, permeability, distribution of initial non-wetting phase, irreducible brine saturation, trapped non-wetting phase saturation, displacement effectiveness and the effect of saturation history were investigated during drainage-imbibition cycles similar to those in the WAG process. Values for Land trapping coefficients were evaluated from on-line X-ray CT scan images. The trapped non-wetting phase saturation ranged from 8 to 16 percent for both N2 and CO2 floods. The results reveal that trapped saturations are higher for higher pressures and higher temperatures. Cross-section images show the enlargement of pore spaces induced by brine-CO2 flooding with a consequent increase of the trapping capacity. Porosity and permeability changed after a CO2 injection, along with the observed formation of short wormholes. In addition, some degree of dissolution of the rock was verified and solid particles of carbonate salts were collected at the outlet of CO2-brine runs. The results obtained emphasize the importance of using high-resolution saturation imaging to provide the main parameters for the experimental evaluation of CO2-WAG processes in carbonates. Introduction Carbon dioxide (CO2) flooding has been considered as one of the most important processes for enhancing oil recovery (EOR) from carbonate reservoirs since the 1980's [1]. Its use, though, is most of the time limited by the availability of an economic source. In the Brazilian pre-salt reservoirs, e.g., Tupi field, the solution gas contains a high proportion of CO2 [2]. The re-injection of the produced CO2 in this case represents the solution of two problems at once. It solves the discard problem, which is of growing environmental concern, at the same time it provides the resource for improving the difficult oil recovery. Both EOR and environmental processes demand new studies covering the application of WAG injection and the safety of geological storage of CO2. The alternating injection of water and gas was conceived in order to compensate the counter tendencies of gas rising upward and water falling downward within the reservoir by ‘breaking-up’ the continuous slug of gas into smaller slugs by alternating water banks [3]. On the other hand, injecting water with miscible gas reduces the instability of the gas/oil displacement, improving the overall sweep efficiency.

Impact of physical phenomena and cyclical reinjection in miscible CO2-WAG recovery in carbonate reservoirs

“…Parameter a was assumed to be equal to the work by Larsen and Skauge (1998), and the other two parameters were obtained from the work by Laboissière et al (2013). The three-phase water relative permeability was assumed to be 25% of the biphasic, an experimental observation due to Rogers et al (2000), because of the absence of isoperm data.…”

Histerese da permeabilidade relativa ao gás em rochas carbonáticas

“…A compreensão dos mecanismos in-situ requer experimentos complexos e análises detalhadas ao investigar as interações entre CO 2 -rocha-salmoura. As interações químicas entre os componentes são influenciados pela salinidade da salmoura, a taxa de injeção de CO 2 , o tipo de rocha, e as condições de pressão e temperatura (Grigg et al, 2005;Izgec et al, 2005;Zecri et al, 2009;Luquot et al, 2009;Bacci et al, 2010;Yatsuda et al, 2013;Laboissière et al, 2013). Kamath et al, (1998) concluíram que essas interações podem ser responsáveis por variações na injetividade dos poços e que este fenômeno tem sido investigado por diversos grupos de pesquisa, mas com resultados variados.…”

“…Figura 34. Configuração do aparato (A) para todos os ensaios de embebição utilizando N 2 e salmoura (Laboissière et al, 2013 (Laboissière et al, 2013).…”

Caracterização de fluido e simulação composicional de injeção alternada de água e CO2 para reservatórios carbonáticos molháveis à água

“…Por esse motivo, se decidiu fazer no Caso 1 uma análise adicional considerando apenas a histerese da permeabilidade relativa trifásica. Nessa análise, ao invés de fixar os parâmetros em parâmetros conhecidos de um sistema molhável a água, foram variados os parâmetros a, α e S gr_máx para nove combinações de parâmetros observadas experimentalmente por Laboissière et al (2013)…”

“…Observou-se na análise anterior, quando ocorreu a histerese da permeabilidade relativa, que a operação ótima é mais influenciada pelo dimensionamento da pluma injetada, principalmente da vaporização aquosa e efeitos de salinidade, e uma vez que, devido aos demais resultados, a histerese era fundamentalmente mais importante que os outros dois fenômenos de difusão e solubilidade aquosa, uma matriz discreta de planejamento experimental fatorial completo(MONTGOMERY, 2005) foi redesenhada, mostrada na Tabela 3.5, para averiguar a otimização do WAG no Caso 2 sujeita ao fenômeno de histerese da permeabilidade relativa, solubilização aquosa e difusão, sem efeitos de salinidade e vaporização aquosa: Tabela 3.5 -Faixa de parâmetros de otimização investigados -Caso 2-análise de otimização na presença de histerese A histerese da permeabilidade relativa na análise inicial com três fenômenos simultâneos (análise 1) do Caso 1 Fluido 1 desta tese e no Caso 2 da tese foram consideradas em um cenário fixo para os parâmetros α, a e S grmax , o parâmetro a foi assumido como os mesmos de um dos valores do trabalho de Larsen e Skauge (1998), e os outros dois parâmetros foram obtidos deLaboissière et al (2013). Em todos os casos, a permeabilidade relativa trifásica da água foi assumida como 25% da bifásica, uma observação experimental coerente empregada por Rogers e Grigg (2000) devido à ausência de dados de isopermas.…”