Capillary Pressure, Fluid Distribution, and Oil Recovery in Preserved Middle Bakken Cores

Abstract

Understanding reservoir rock characteristics and the forces that mobilize oil in unconventional reservoirs are critical in designing oil recovery schemes. Thus, we conducted laboratory experiments in three preserved Middle Bakken cores using centrifuge and nuclear magnetic resonance (NMR) instrument to understand oil recovery mechanisms in the Bakken. Specifically, we measured capillary pressure, pore size distribution, and oil and brine distributions. A series of oil and brine replacement experiments (drainage and imbibition) were conducted in the preserved cores using a high-speed centrifuge. T2 time distribution and one-dimensional saturation profile measurements were obtained using a 2-MHz nuclear magnetic resonance instrument before and after centrifuge experiments. Moreover, pore size distribution was determined from Mercury Intrusion Capillary Pressure (MICP) and Nitrogen Gas Adsorption experiments. We conducted scanning electron microscope (SEM) imaging on polished core slabs to determine pore shapes and mineralogy of pore walls using a field emission-scanning electron microscope (FE-SEM). Our measurements show that, contrary to the common notion, the preserved Middle Bakken cores are not oil-wet—but show mixed-wet characteristics. Water resides in smaller pores and oil resides in larger pores in all experiments. Using a low-salinity synthetic brine of 50,000-ppm to surround Bakken cores of much higher salinity, we produced up to 6.33 % (of pore volume) oil from two higher porosity (~8%) cores, and 10.72% (of pore volume) oil from one lower porosity (~2%) core in spontaneous imbibition experiment. Moreover, up to 6.62 % (pore volume) oil from the two higher porosity cores and 11.23% (of pore volume) oil from the lower porosity core was produced in forced imbibition experiment. These experiments indicate that chemical osmosis overrides the wettability effects in tight Middle Bakken cores. The new observations regarding osmosis have altered our classical notion of capillary imbibition in shale reservoirs.

Mechanism of the Wettability Impact on Surfactant Imbibition in Dodecane-Saturated Tight Sandstone

“…When the wettability turns to be oil-wet in pores and throats imbibed with the water (surfactant) phase, dispersed oil droplets will spread, coalesce, and form a continuous phase gradually. With the movement of the surfactant solution, a continuous oil phase could also migrate further in the core under capillary pressure in the front of the oil/water interface, as shown in Figure e, which contributes to reduce residual oil saturation in the channels with water (surfactant) imbibition and improve displacement efficiency. − , On the other hand, contrary to the water-wet or intermediate-wet surface, there is a slippage effect for the water phase on the oil-wet surface, as depicted in Figure b. When the surface wettability turns to be oil-wet, a higher slip velocity and less wall shear would happen on the hydrophobic wall, which means that the water phase flow resistance decreases compared to the previous conditions.…”

“…As a result of the difference in the wettability of the rocks, the displacement work of the wetting phase is smaller than that of the non-wetting phase. Therefore, the area bound by the capillary force axis and water saturation axis can be compared to calculate the wettability index. − The USBM wettability index of a core sample was acquired by the following method:…”

Hydraulic Fracturing Tight Reservoirs: Rock Mechanics and Transport Phenomena

“…Natural fractures properties can be assessed from cores, outcrops, logs, and simulation results based on matching production history. In this part, some theoretical studies are applied for interpretation of some fracture properties in the core plugs [11]. If there is one natural fracture in the middle of the core plug, the fracture porosity and permeability are calculated using the following equations:…”

“…During desorption, gas is released from organic material. Therefore, the Langmuir adsorption coefficients are plotted against TOC as presented in [11].Error! Reference source not found.…”

“…The purpose of this study is to estimate the effective permeability of water and gas by using the multiphase flow analysis of production data. As it is illustrated in For the multiphase linear flow analysis in natural fractures, the following equation is used [11]:…”