Large Scale 2D Imaging of Impacts of Wettability on Oil Recovery in Fractured Chalk

Abstract

The effect of embedded fractures on the movement and recovery of hydrocarbon from larger outcrop chalk blocks at different wettabilities has been measured in the laboratory. 2-D nuclear tracer imaging was used to produce in-situ fluid saturation distributions during oil production. Emphasis was on determining the oil recovery mechanisms by tracking the flow path of the advancing water. Two sequential waterfloods were performed on each of the three different blocks: first before fracturing, and then after fracturing. The same fracture network configuration was used for all three blocks, which were strongly water wet, moderately water wet and near neutral water wet. Waterflooding of the unfractured blocks, at high initial water saturation, occurred with minimal water banking while waterflooding at low initial water saturation produced distinct water bank formation. Waterflooding of the fractured blocks showed that the "closed" fractures produced a significant effect on fluid movement in the strongly water wet block, but only minor effect for the moderately and near neutral water wet blocks. The open fracture affected flow in all the blocks. Total oil recovery was higher in the strongly water wet block than in the moderately water wet block with the lowest oil recovery observed in the near neutral wet block. Introduction In fractured chalk reservoirs it is generally believed that oil production results from spontaneous imbibition of water from the fracture network and subsequent movement of the expelled oil through the fractures to the producing wells. However. if there were a significant amount of capillary continuity between adjacent blocks, viscous displacement of oil could also play a role and the matrix pore network could provide an alternate path for oil movement toward the production wells. Viscous displacement in a fractured chalk should be most important near water injection wells, during waterfloods and in reservoirs which are less than strongly water-wet. Previous experimental work which are pertinent to the assessment of fluid flow in fractured chalk include:The monitoring of saturation distribution during spontaneous axial imbibition in stacked cores using both horizontal and vertical configurations.Measuring saturation distributions as a function of time for the spontaneous imbibition and waterflooding of cores of different length and the area and configuration of exposed faces.The saturation distributions produced by gravitational drainage andfree gas. 2-dimensional saturation distributions have been monitored in larger strongly water-wet chalk blocks with several fracture orientations. Techniques have been developed which reproducibly alter the wettability of outcrop chalk to mimic the less than strongly water-wet chalk in a reservoir. This study was conducted to follow fluid movement and investigate hydrocarbon recovery mechanisms in fractured chalk at less than strongly water-wet conditions. The fracture network for these larger chalk blocks is similar to previously reported experiments on strongly water-wet fractured chalk. Experimental Three blocks, approximately 20 cm × 12 cm × 5 cm thick, were cut from large pieces of Roerdaloutcrop chalk obtained from the Portland quarry near Alborg, Denmark. This chalk material had never been contacted by oil and was strongly water-wet. The blocks were oven dried for three days at 90 C, end pieces were mounted and the whole assembly was epoxy coated. Each end piece contained three fittings so that entering and exiting fluids were evenly distributed with respect to height. The blocks were vacuum evacuated and saturated with brine containing 5 wt% NaCl + 5 wt% CaCl2. Porosity was determined from weight measurements and the permeability was measured across the epoxy coated blocks, see Table 2. P. 559^

Wetting Phase Bridges Establish Capillary Continuity Across Open Fractures and Increase Oil Recovery in Mixed-Wet Fractured Chalk

“…During the water floods, the core plug was positioned horizontally such that the open sagital fracture was aligned with gravity. The cutting process does not measurably alter the saturation or its distribution (Graue et al 1999a(Graue et al , 2001a. During assembly, A and B were separated by 1.3 mm thick elongated POM-spacers.…”

“…The oil recovery mechanisms involved while waterflooding fractured chalk blocks were found to be dependent on the wettability of the chalk (Viksund et al 1996(Viksund et al , 1997Graue et al 1999aGraue et al , 2000aGraue et al , b, 2001aGraue et al , b, 2002aGraue et al , b, 2003Aspenes et al 2002a). At strongly water-wet conditions, water accumulated in the preceding matrix block until the spontaneous imbibition endpoint was reached.…”

Wettability and Imbibition: Microscopic Distribution of Wetting and Its Consequences at the Core and Field Scales

“…Graue and co-workers [70,71,72] applied nuclear tracer techniques to monitoring in situ saturation changes with special attention to recovery from chalk. In one of several studies of the combined effect of fracture geometry and wettability, oil recovery from blocks of strongly waterwet chalk were compared with that for moderately water-wet, and near neutral-wet chalk prepared by treatment with crude oil.…”

Analytical Solutions for Linear Counter-Current Spontaneous Imbibition in the Frontal Flow Period

“…The saturation profiles can be detected by some physical methods such as NTI (nuclear tracer imaging) (Graue et al 1999), MRI (magnetic resonance imaging) (Baldwin and Spinler 2002), and CT (computerized tomography) (Akin et al 2000).…”