Waterflooding Huff-n-puff in Tight Oil Cores Using Online Nuclear Magnetic Resonance

Abstract

Given the difficulty in developing waterflooding in tight oil reservoirs, using waterflooding huff-n-puff is an effective method to improve oil recovery. Online nuclear magnetic resonance (NMR) can detect the change in internal oil and water during the core displacement process, and magnetic resonance imaging (MRI) in real time. To improve the tight oil reservoir development effectiveness, cores with different permeability were selected for a waterflooding huff-n-puff experiment. Combined with online NMR equipment, the fluid saturation, recovery rate, and residual oil distribution were studied. The experiments showed that, for tight oil cores, more than 80% of the pores were sub-micro-and micro-nanopores. More than 77.8% of crude oil existed in the sub-micro-and micropores, and movable fluids mainly existed in the micropores with a radius larger than 1 µm. The NMR data and the MRI images both demonstrated that the recovery ratio of waterflooding after waterflooding huff-n-puff was higher than that of conventional waterflooding, and, therefore, residual oil was lower. Choosing two cycles' of waterflooding, huff-n-puff was more suitable for tight oil reservoir development. The production of crude oil increased by 22.2% in the field pilot test, which preliminarily proved that waterflooding huff-n-puff was suitable for tight oil reservoirs.

Evaluation of Displacement Effects of Different Injection Media in Tight Oil Sandstone by Online Nuclear Magnetic Resonance

“…Secondly, the instrument carried on the phase coding and frequency coding to the target layer under the gradient magnetic field. Thirdly, receiver collected echo signal, and then the gray level image of the target layer was obtained by Fourier transform [44]. Finally, MRI processing software was applied to add pseudo color to get easily recognizable images.…”

“…The core NMR T 2 spectrum can be converted into the core pore distribution [13,54]. For tight oil sandstone, the pores were divided into nanopores, micro-nanopores, sub-micropores, and micropores according to pore radius [44,55], which were 0.05, 0.1, and 1 µm. The distribution of pore radius was analyzed by NMR data which was acquired from the 16 cores saturated with simulated formation water.…”

“…Online NMR spectrometer was applied to log NMR T 2 data of each core which was filled with crude oil. The formula for calculating initial oil saturation S oi is shown as [44]:…”

“…Oil recovery ratio refers to the proportion of the crude oil production to initial oil reserves. In the displacement process, NMR data can be measured in real time by online NMR spectrometer, and then the oil recovery ratio can be calculated according to Equation (3) [44]:…”

“…Online NMR technology has effectively solved the above problems. Online NMR technology combines NMR and displacement equipment [44]. The material of core gripper and inner rubber sleeve is nonmagnetic [45].…”