New methods for enhanced oil recovery are typically developed using core flooding techniques. Establishing reservoir conditions is essential before the experimental campaign commences. The realistic oil-rock wettability can be obtained through optimum aging of the core. Aging time is affected by temperature, crude oil, formation brine, and coreplug lithology. Minimum time can significantly reduce the experimental cost while insufficient aging time can result in false conclusions.
Real-time online resistivity measurements of coreplugs are presented and a novel method is introduced for determining the optimum aging time regardless of variations in crude oil, rock, and brine properties. State of the art core flooding equipment has been developed that can be used for consistently determining the resistivity of the coreplug during aging and waterflooding using advanced data acquisition software. In the proposed equipment, independent axial and sleeve pressure can be applied to mimic stresses at reservoir conditions. 10 coreplugs (four sandstones and six chalk samples) from the North Sea have been aged for more than 408 days in total and more than 29000 resistivity data points have been measured to consistently investigate the change of wettability during aging.
At 60°C and 100 bars a homogeneous sandstone coreplug attained optimized wettability after 5 days, a heterogeneous coreplug required 30 days of aging. Chalk coreplugs needed 45 days of aging. This shows that coreplugs originating from the same field, when aged at equivalent conditions can have significantly different aging times because of minor variations in the coreplug properties. No fixed aging time can be recommended on the other hand a method is recommended which can determine the extent of aging. Coreplug aging patterns were studied for variation in pressure (20 to 130 bar) and temperature (60 to 130°C). Based on these experiments an algorithm has been developed which distinguishes the effect of wettability alteration, pressure, and temperature on coreplug resistivity.
This study highlights the use of hydraulic oil to avoid release of fluids in the effluent pipes during the aging process. Furthermore, the described multiple monitoring devices are useful in detecting any experimental error that may have occurred during mounting of the coreplug in the core holder. Thus imperfect waterflooding which can otherwise produce misleading data can be avoided.
The presented equipment can instantly and continuously calculate the mineral wettability throughout the aging process at any pressure, temperature condition and for any combination of rock and crude oil. Thus, using the stated core flooding equipment can not only decrease the cost and time of doing aging and waterflooding studies but can also significantly increase the accuracy in conducting core flooding experiments.