Effect of Chemical Partition Behavior on Oil Recovery by Wettability Alteration in Fractured Tight Reservoirs

Various compounds have been studied as aqueous wettability modifiers that alter the matrix wettability for enhanced oil recovery from shales. This paper compares the oil recovery performance/characteristics using different wettability modifiers that have different partition behaviors into oil and water. We used two types of wettability modifiers: (1) a naturally occurring sulfate-rich injection brine (IB) of 13 500 ppm salinity containing 3900 ppm of sulfate and (2) 1.0 wt % 3-pentanone in IB (3pIB). Sulfate is highly polar, while 3-pentanone is miscible with oil and only slightly soluble in IB. Contact angle experiments using oil-aged calcite surfaces confirmed that IB and 3pIB changed the wettability from 122 degrees to 69 degrees and 72 degrees, respectively, within 3 days. Furthermore, spontaneous imbibition and forced imbibition experiments determined the Amott index to water of 0.68 for IB and 0.67 for 3pIB, indicating similar levels of wettability alteration. This result posed an important question as to how both modifiers would perform in oil recovery from the matrix in huff-n-puff, where the rapid transfer from the fracture to the matrix is an important step. Therefore, we tested IB and 3pIB in huff-n-puff experiments using Wolfcamp shale outcrop cores that were initially saturated with dead oil. Results showed that 3pIB started producing oil from the matrix from the first cycle and continued to produce until the fifth cycle, with a final oil recovery of 57%. IB did not show oil recovery until the second cycle, reaching a final oil recovery of 15% after 5 cycles. IB, however, showed no clear difference in brine composition between the injected and produced water. This is the first comparative study of wettability modifiers with different levels of polarity, which affect the mass transfer between the fracture and the matrix volumes in huff-n-puff.

Automated summary

This paper compares the performance of different wettability modifiers in enhancing oil recovery from shales. The results show that 3pIB performed better than IB, with 3pIB starting to produce oil from the matrix earlier and achieving a higher overall oil recovery rate. Additionally, there was no significant difference in brine composition between the injected and produced water for IB.