Abstract
In order to give full play to the role of imbibition of capillary force and enhance oil recovery of ultralow permeability sandstone reservoir after hydraulic fracturing, the mixed water fracture technology based on functional slick water is described and successfully applied to several wells in oilfield. The core of the technology is determination of influence factors of imbibition oil recovery, the development of new functional slick water system and optimization of volume fracturing parameters. The imbibition results show that it is significant effect of interfacial tension, wetting on imbibition oil recovery. The interfacial tension decreases by an order of magnitude, the imbibition oil recovery reduces by more than 10%. The imbibition oil recovery increases with the contact angle decreasing. The emulsifying ability has no obvious effect on imbibition oil recovery. The functional slick water system considering imbibition is developed based on the solution rheology and polymer chemistry. The system has introduced the active group and temperature resistant group into the polymer molecules. The molecular weight is controlled in 1.5 million. The viscosity is greater than 2mPa·s after shearing 2h under 170s-1 and 100℃. The interfacial tension could decrease to 10-2mN/m. The contact angle decreased from 58° to 22° and the core damage rate is less than 12%. The imbibition oil recovery could reach to 43%. The fracturing process includes slick water stage and linear gel stage. 10% 100 mesh ceramists and 8% temporary plugging agents are carried into the formation by functional slick water. 40-70 mesh ceramists are carried by linear gel. The liquid volume ratio is about 4:1 and the displacement is controlled at 10-12m3/min. The sand content and fracturing fluid volumes of single stage are 80m3 and 2500 m3 respectively. Compared with conventional fracturing, due to imbibition oil recovery, there is only 25% of the fracturing fluid flowback rate when the crude oil flew out. When the oil well is in normal production, about 50% of the fracturing fluid is not returned. It is useful to maintain the formation energy and slow down the production decline. The average cumulative production of vertical wells is greater than 2800t, and the effective period is more than 2 years. This technology overcoming the problem of high horizontal stress difference and lack of natural fracture has been successfully applied in Jidong Oilfield ultralow permeability reservoir. The successful application of this technology not only helps to promote the effective use of ultralow permeability reservoirs, but also helps to further clarify the role of imbibition recovery, energy storage and oil-water replacement mechanism.
Highlights
Effect of surfactants on Imbibition oil recovery
Field application results show that slickwater fracturing with a certain proppant can improve conductivity obviously and enhance production and recovery efficiency of single well[3,4].At present, in addition to water and proppant, there is a small amount of additives in the slickwater fracturing fluid and the total content is below 1%[5]
The oil displacement effect of anionic surfactants and nonionic surfactants in a certain unconventional oil and gas reservoir shows that anionic surfactants in the initial system of weak water wetting can increase the contact angle, while a nonionic surfactant doesn't change in the contact angle[8,9]
Summary
Effect of surfactants on Imbibition oil recovery. Slick water fracturing fluid is defined as adding a certain amount of proppants and a very small amount of additives, such as surfactant, friction reducing agent and antiswelling agent in the water. The surfactant could be used to change wettability of the reservoir, reduce the capillary force and strengthen imbibition effect, which could be enhanced the oil recovery. With the fracturing development of ultra-low permeability and tight oil reservoirs, besides making fracture and carrying sand, the fracturing fluid has the effect of increasing the rate of imbibition oil recovery and supplementation of energy. The influence of wettability on imbibition is determined by analyzing the relationship between the change of contact angle and the imbibition oil recovery, fig. 2. The change of contact angle 0° is the wettability of distilled water, whose imbibition oil recovery is 20.7%. It can be seen that the fracturing fluid system can change the original state of the reservoir to water-wetting, which is beneficial to the enhancement of the imbibition oil recovery. This may be caused by the small influence of the core size, which is not fully reflected in the emulsification
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