Two-dimensional study of chemical floods for viscous oils

There is a vast deposit of viscous and heavy oil especially in US North Slope, Canada and Venezuela. Typically, waterflood and thermal methods are used to recover viscous oil. However, waterflood is not very efficient because the water fingers through the oil due to adverse viscosity and thermal methods are inefficient when the depth of the reservoir is high and pay thickness is low. Non-thermal chemical enhanced-oil-recovery (EOR) methods are being developed as alternatives. The alkaline-surfactant-polymer (ASP) flood has been shown in the past to reduce residual oil saturation to almost zero for light oils. In addition to ASP, a new low-cost chemical EOR technology called alkali-cosolvent-polymer (ACP) flooding has been developed which does not use any synthetic surfactant. ASP/ACP floods for light oils are designed to be stable where the mobility ratio of the oil bank to the ASP slug is kept close to one. For viscous oils, such stable recovery processes may be unreasonably slow due higher oil viscosity. The goal of this work is to study unstable ASP and ACP flooding (i.e., oil to ASP/ACP slug mobility ratio < 1) in secondary and tertiary modes. ASP/ACP formulations that achieve ultra-low tension were developed. Since achieving high sweep efficiency is challenging in viscous oil reservoirs, ASP/ACP floods were studied in a quarter 5-spot lab model. This research is the first of a kind ASP/ACP study in a multidimensional medium. There are currently no published work available that focuses on studying ASP and ACP in a two-dimensional porous medium. Experimental results were numerically simulated and matched using an in-house simulator UTCHEM. The results of the experiments indicate that tertiary ASP/ACP flood with oil to ASP/ACP slug viscosity ratio between 2 to 4 could recover more than 80% of the oil with a reasonable pressure gradient. This unfavorable mobility of the chemical slug is beneficial considering both oil recovery and pressure drop. The timing of the start of the tertiary flood did not change the cumulative oil recovery very much. However, a shorter waterflood resulted in an earlier oil recovery. It has been observed in field tests that ASP flooding after polymer flooding is quite effective. It is impossible to compare the effectiveness of ASP floods after polymer floods to ASP floods after waterfloods in the same reservoir. This comparison has been conducted here in a heterogeneous quarter 5-spot, where one half of the sand pack is 10 times more permeable than the other half. One side of the quarter 5-spot model was built to be transparent. The interaction of sweep efficiency and displacement efficiency of ASP flooding was studied through visual observations. The experiments demonstrated that the cumulative oil recovery for the polymer flood-ASP flood combination is significantly higher than that for the waterflood-ASP flood combination. The incremental recovery efficiencies due to ASP floods are similar for both the cases. Most of the benefit comes from the higher sweep efficiency of the polymer flood over that of the waterflood.