Progress in Immiscible WAG Modelling

Abstract

The paper reviews recent development in modelling of immiscible WAG processes (IWAG). Several different approaches have been made to model three-phase relative permeability in particular. In most cases capillary pressure has been neglected in application of these models in numerical simulations of IWAG. The argument behind eliminating capillary pressure is to simplify the model, and the assumption that capillary pressure is of less importance for the problem analysed or because there are no experimental data available. This study also shows the consequence of neglecting capillary pressure. A novel approach to include three-phase capillary pressure is discussed, and also a procedure for estimation of three-phase capillary pressure from more available two-phase capillary pressure data. A practical case study of history matching IWAG core floods is also presented. Introduction Numerical simulation of any EOR process is a key to the prediction of incremental oil recovery. Water -alternating -gas (WAG) injection has found an increasing application in both clastic and carbonate reservoirs and at both miscible and immiscible gas conditions 1,2. The addition oil recovery is typically in the range of 5–10 percent of the original oil in place. WAG injection is an oil recovery method initially aimed to improve sweep during gas injection. Possible improved microscopic efficiency in three-phase zones of the reservoir may come as an added benefit from the WAG injection. Today the WAG process (both miscible and immiscible) is considered for a number of new fields in the North Sea. WAG is defined as any injection of both water and gas into the same reservoir. This including definition covers MWAG (miscible), IWAG (immiscible), HWAG (hybrid), SWAG (simultaneous), and also tertiary gas injection or tertiary water flooding. When miscibility is developed along the gas slug, as gas displaces oil, it is referred to as a miscible WAG. In this case the main purpose of the water slug is to increase the volumetric sweep since the residual oil saturation will be low after the miscible front has passed. Immiscible WAG (IWAG) is pulses of gas and water injected, where the gas cannot develop miscibility with oil. Still some compositional exchanges between gas and oil may be important for the fluid characterization and oil recovery. Hybrid WAG is when a large slug of gas is injected followed by a number of small slugs of water and gas. Simultaneous injection has also been performed, but is reported to give reduced injectivity in some cases. In this paper immiscible WAG processes will be discussed. This process involves both drainage and imbibition processes, three phase flow modelling approach, and hysteresis in relative permeabilities and capillary pressure. The complexity of the WAG process is further complicated by mass exchanges (swelling and stripping of the oil by the injected gas). The focus is here on fluid flow functions and the mass exchanges are here ignored. Simulation studies of IWAG3–7 have shown that analytical models like Stone 8 and Jenkins 9 may strongly underestimate the extent of the three-phase zone. The extent of the three-phase zone is influenced by phase trapping and three-phase relative permeabilities, see Figure 1. The residual oil saturation in the three-phase zone may also be reduced compared to two-phase flow, due to effect of trapped gas on residual oil and also three-phase relative permeabilities 10–12. Experimental studies showed accelerated oil production and higher core flood oil recovery as a result of three-phase flow 10–13. The oil recovery has been related to the trapped gas saturation 11–13, and the influence of the effect of trapped gas is found to be varying with core wettability 13. Experimental results have also shown that both gas and water relative permeability may be reduced during three-phase flow. Simulation results have shown that the two-phase relative permeability hysteresis models were unable to describe the three phase experiments. The match of WAG experiments was improved by the three-phase relative permeability approach 14,15.

Investigation of Significance of Critical Components in Optimization of WAG Injection Processes under Uncertainty

“…High residual oil saturation from water injection can be avoided by using miscible gas because of phase transitions and inter-phase mass transfer between oil and gas (Bedrikovetsky et al 1996;Christensen et al 2001). A comprehensive classification of the WAG process was suggested by many authors, such as miscible (MWAG), immiscible (IWAG), hybrid (HWAG), simultaneous (SWAG), and selective (SSWAG) (Christensen et al 1998;Al-Mamari et al 2007;Skauge et al 2007).…”

Interpretation of Immiscible WAG Repeat Pressure-Falloff Tests

“…Egermann et al (2000) proposed a new three-phase-flow model based on a theoretical analysis validated on a set of experimental approaches. Skauge and Dale (2007) proposed the inclusion of three-phase capillary pressure estimated from a larger set of available two-phase capillary pressure data. While the impact of three-phase relative permeability hysteresis on the predicted recovery from the WAG process could be large at field scale, such an investigation was outside the scope of the work presented in this paper.…”

Histerese nas curvas de permeabilidade relativa trifásica em carbonatos usando tomografia computadorizada de raios-x

“…Nos últimos anos vem aumentado o número de projetos de injeção alternada de água e gás, com processos de deslocamentos de óleo e gás tanto miscíveis quanto imiscíveis, não se podendo descartar a influência da transferência de massa entre fases na recuperação de óleo, mesmo nos processos imiscíveis [Skauge and Dale, 2007]. O primeiro projeto de WAG citado na literatura data de 1957, no Canadá.…”

“…Apesar de WAG ser um método largamente utilizado, ainda permanecem muitas incertezas em relação à qual modelagem dinâmica seria a mais apropriada na simulação de reservatórios [Hosseini et al, 2012]. Muitos campos carbonáticos sob injeção de WAG apresentaram/apresentam alterações nas curvas de permeabilidade relativa ao longo dos ciclos do processo de injeção de WAG, mudanças estas decorrentes do fenômeno de histerese [Skauge and Dale, 2007]. Em adição, diversos autores reportam a presença do efeito de histerese em medidas experimentais de curvas de permeabilidade relativa, conforme será mostrado nas seções a seguir.…”

“…Diversos autores investigaram o impacto do uso de diferentes modelos de permeabilidade relativa nas predições de recuperação petróleo, encontrando resultados bem destoantes entre os diversos modelos [Spiteri, 2005], sendo que nenhum modelo é capaz de ajustar dados de mais de um experimento. Isto ocorre em função das discrepâncias entre as propriedades dos fluidos e das rochas, variações nos resultados entre medições em regimes transientes e permanentes (apresentadas na próxima seção) e, principalmente, pelas diferenças de molhabilidade das amostras [Skauge and Dale, 2007]. Spiteri and Juanes [2006] compararam resultados obtidos por diferentes modelos com dados experimentais, concluindo que: as curvas de permeabilidade relativa à água não apresentam comportamento significativo de histerese em rochas molháveis à água; os modelos de interpolação geram resultados muito distintos, principalmente em regiões de baixas saturações de óleo; o coeficiente de Land pode apresentar grandes variações dependendo do ângulo de contato em condições não-estáticas; os modelos superestimam a permeabilidade ao gás durante injeções cíclicas (WAG).…”