Pubudu Hasanka Gamage

Evaluation Of The Effect Of Low Salinity Waterflooding For 26 Fields In Wyoming

This report evaluates the effectiveness of low-salinity waterflooding in the Minnelusa Formation in the Powder River Basin of Wyoming. The Minnelusa sandstone play constitutes a resource of over one-hundred fields with cumulative production of more than 600,000,000 barrels of oil. We conducted initial laboratory screening using Minnelusa oil and rock with synthetic brine, supplemented with geochemical models of low-salinity injection, to evaluate the potential for low-salinity waterflooding in this formation. The laboratory experiments showed little or no incremental recovery from low-salinity injection. Calculation and comparison of recovery factors for 51 Minnelusa reservoirs were used to further evaluate the effectiveness of low-salinity waterfloods at the field scale. There was no increase in recovery for fields that used low salinity injection (26) compared to fields with mixed or formation water injection (25). Since some Minnelusa fields have relatively fresh formation water, the amount of dilution was quantified using the salinity ratio (SR), defined as the ratio of salinity of injected water to salinity of formation water. This analysis showed that while some fields actually had little or any salinity reduction (13), the remaining fields with significant dilution (38) still showed no correlation between dilution and recovery factor. Since some postulated mechanisms involve change in wettability, injection of low-salinity water may produce later water breakthrough. Analysis of water breakthrough timing and watercut evolution for 23 fields found no significant difference between fields with low-salinity injection and mixed-water or saline injection. Introduction Low-salinity waterflooding has been widely studied during the last decade by various research groups as one of the most inexpensive methods of enhanced oil recovery (EOR). The level of investigation into low-salinity waterflooding has sharply increased in the past three years as more research groups have become involved (Webb et al. 2008, Alotaibi and Nasr_el_Din 2009, Austad et al. 2010, Boussour et al. 2009, Cissokho et al. 2009, Kumar et al. 2010, Lager et al. 2008, Patil et al. 2008, Seccombe et al. 2008, Pu et al. 2010, Rivet et al. 2010, RezaeiDoust et al. 2010, Gamage and Thyne 2011). Laboratory studies with synthetic formation water, reservoir and outcrop rocks and reservoir oil have been conducted with injected water diluted by a factor ranging from 2.5 to 100-fold compared to formation water. Many studies have reported increases in recovery of 2-30% original-oil-in-place (OOIP) varying with brine and crude oil compositions and rock types used. However, while both laboratory and field studies have had successful results, there are also examples in which low-salinity flooding does not create additional production (Sharma and Filoco 2000, Rivet et al 2010, Skrettingland et al. 2010). The fundamental observations of increased recovery from low-salinity flooding in the laboratory were made by Martin (1959) and Bernard (1967). This work was extended and brought to wider attention by various workers over the last 15 years (Jadhunanadan and Morrow 1995, Zhou et al. 1995a, Zhou et al. 1995b, Tang and Morrow 1997, Yildiz et al. 1999, Morrow et al. 1998, Tang and Morrow 1999a, Tang and Morrow 1999b, Maas et al. 2001, Robertson et al. 2003, Lohardo et al. 2008, Morrow et al. 2008, Pu et al. 2008, Kumar et al. 2010, Pu et al. 2010). The mechanism(s) is still a matter of debate (Austad et al. 2010, Kumar et al. 2010, Lee et al. 2010, RezaeiDoust et al. 2010, Sorbie 2010), but continued work shows diluting the salinity of injected water can often produce increased oil recovery. However, there are few field studies (Webb et al. 2004, McGuire et al. 2005, Robertson 2007, Seccombe et al. 2008, Lager et al. 2008, Seccombe et al. 2010, Skrettingland et al. 2010) and scaling laboratory results to the field is always challenging. Currently, laboratory tests are used to screen candidate reservoirs followed by single well tracer tests before implementation