Surfactant applications to improve mud removal performance in drilling & oil recovery during hydraulic fracture treatments in unconventional formations
Effective spacer design is a critical criteria for cement to develop an excellent bond between cement-casing and cement-formation. Applications of the spacer are mud displacement, mud cake removal, and preventing cement contamination with drilling mud. Furthermore applications can be altering the formation and casing wettability, especially when drilling operations involves drilling with oil-based mud, where the production zone will be most likely oil-wet. Oil wet formation can reduce cement bonding with formation, which could lead to pressure communication between hydraulic fracture stages and poor zonal isolation. Therefore, the objective of the study is to investigate the impact of adding surfactants to cement spacer design under reservoir temperature, where the surfactants will help to mitigate the oil base mud impact on cement bond performance, reduce compatibility issues with drilling mud, change formation – casing wettability from oil-wet to water-wet, and reduce interfacial tension (IFT) between the spacer and oil-based mud to have a better mud displacement. Core samples from Wolfcamp were used to conduct the study. The core samples were cut to small chips and aged in (8.8 ppg, 12.5 ppg) oil-based mud and crude oil under reservoir temperature for ten days to mimic the horizontal-drilled section of the oil well. Then, the chip's wettability was measured using the captive bubble method. After that, the chips were immersed in a spacer mixture of (water and surfactant). Three different surfactants were used to conduct the study (Anionic surfactant, Nonionic surfactant, and Zwitterionic surfactant). After the chips were exposed to the aforementioned spacers, the chip's wettability was measured again to examine the effect of surfactants on altering the wettability. Moreover, the interfacial tension (IFT) between the spacer and crude oil was determined. Oil-based mud and spacer compatibility tests were conducted to examine the mixability issue with the presence of surfactants in the spacer design. Additionally, mud removal efficiency using different surfactants was examined using beaker and rotor cleaning tests. The results demonstrate the benefits of adding surfactants in the cement spacer design. Where spacer without surfactant showed compatibility issues particularly at high OBM density, having surfactants in spacer design resulted in an adjustment of the formation wettability from oil-wet to water-wet. Anionic surfactant showed the best performance among all the surfactant types that have been examined. Also, having surfactant in the spacer design will reduce compatibility issues between oil base mud and spacer significantly. Moreover, more surfactant volume will be required when displacing heavy oil-based mud to prevent compatibility problems. Regardless of the outperforming of non-ionic surfactants in alternating the chip's wettability and reducing IFT, remarkable compatibility issues were detected when non-ionic surfactants were added to the spacer recipe. The necessity of reducing the need for high horsepower to frack formation led to the use of friction reducers intensively; however, the consequences of pumping friction reducers could lead to poor fracturing fluid flow-back. The unrecovered fluid creates a water blockage at the matrix-fracture interface in the reservoir, which could impact the oil recovery factor eventually. Therefore, adding surfactants is believed to mitigate the water blockage and increase the flow back by reducing the oil-water interfacial tension (IFT) and altering the matrix-fracture interface wettability. Although many studies have shown the benefit of adding surfactants, there is a quite lack of experimental studies about the impact of surfactants and friction reducers on water blockage. Thus, this study emphasizes studying the effects of surfactants and friction reducers on oil recovery factors. Core samples from Wolfcamp were utilized to conduct the study as core samples were cut to small chips and aged in dead crude oil for two months under a reservoir temperature of 155° F. Then, the chip's wettability was measured and immersed in a solution mixture of water and surfactants (nonionic surfactant, anionic surfactant, and zwitterionic surfactant). The wettability was measured again to identify the best surfactants candidate. Moreover, the IFT was measured between crude oil and the fracture solution mixtures. To study the effect of friction reducer on oil recovery factor, two types of friction reducers (FR), an anionic friction reducer and a cationic friction reducer, were added to the surfactants mixture to investigate the impact of FR on the RF. The results of the study showed adding surfactants will improve the oil recovery factor significantly and increased the oil recovery factor. The results also reveal that using a cationic friction reducer in fracture fluid can help in reducing water blockage damage as a higher oil recovery factor was observed compared to when an anionic friction reducer was utilized. Non-ionic surfactants showed a good capability in reducing the IFT between oil and water, unlike anionic and zwitterionic surfactants. Including non-ionic surfactants in fracture fluid systems can increase the ultimate oil recovery factor.