Large-Scale Two-Dimensional Imaging of Wettability Effects on Fluid Movement and Oil Recovery in Fractured Chalk
- Arne Graue (U. of Bergen) | B.G. Viksund (U. of Bergen) | B.A. Baldwin (Phillips Petroleum Co.) | E.A. Spinler (Phillips Petroleum Co.)
- Document ID
- Society of Petroleum Engineers
- SPE Journal
- Publication Date
- March 1999
- Document Type
- Journal Paper
- 25 - 36
- 1999. Society of Petroleum Engineers
- 5.4.1 Waterflooding, 5.3.1 Flow in Porous Media, 4.1.9 Tanks and storage systems, 5.2 Reservoir Fluid Dynamics, 4.1.5 Processing Equipment, 5.8.7 Carbonate Reservoir, 6.5.2 Water use, produced water discharge and disposal, 4.3.3 Aspaltenes, 4.1.2 Separation and Treating, 1.6.9 Coring, Fishing, 5.4.9 Miscible Methods, 5.5.2 Core Analysis, 5.3.2 Multiphase Flow, 4.3.4 Scale, 5.6.5 Tracers
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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. Two-dimensional (2D) 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 wet. Waterflooding 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 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.
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 work1-4 which are pertinent to the assessment of fluid flow in fractured chalk include: (A) The monitoring of saturation distribution during spontaneous axial imbibition in stacked cores using both horizontal and vertical configurations; (B) 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; (C) saturation distributions produced by gravitational drainage; and (D) free gas. 2D saturation distributions have been monitored in larger strongly water-wet chalk blocks with several fracture orientations.5,6 Techniques have been developed which reproducibly alter the wettability of outcrop chalk to mimic the less than strongly water-wet chalk in a reservoir. 7,8
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. 5,6
Three blocks, approximately 20 cm×12 cm×5 cm thick, were cut from large pieces of Roerdal outcrop 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).
Before the blocks were epoxy coated, local air permeability was measured at each intersection of a 1 cm by 1 cm grid on both sides of the blocks using a minipermeameter. An example map of the air permeability for block CHP-2 is shown in Fig. 1. The average air permeability for the block was 3×10-3 µm2 and the water permeability across the block was 2×10-3 µm2 However, an area of 2 cm×2 cm at 11 cm from the inlet end and 7.5 cm from the bottom had an average air permeability of 1×10-3 µm2 both at the front and the back side of the block. This indicates a heterogeneous zone in CHP-2, shown in Fig. 1. A corresponding air permeability map was obtained for each of the blocks, but for the blocks, CHP-5 and CHP-6, a homogeneous air permeability map was obtained.
To study the effects from wettability on oil recovery in fractured chalk blocks we have established a reproducible technique to systematically alter the wettability in chalk.8 Core plugs cut in the same direction from large blocks of Roerdal chalk have been aged in a North Sea stock tank crude oil at 90°C for different time periods in duplicate sets. Because of possible wax problems, the crude oil was flushed out with decahydronaphthalene (decalin) which again was exchanged by flushing with n-decane at 90°C. After the exchange of oil, oil recovery by spontaneous room temperature imbibition, followed by a waterflood, were performed on the aged cores, producing the Amott water index w, 9 shown in Fig. 2. A consistent change in wettability towards a less water-wet state with increasing aging time was observed. Imbibition rate and endpoint saturation after spontaneous imbibition decreased with increased aging time corresponding to consistent change towards a less water wet state. Repeated imbibition tests after aging confirmed stable wettability conditions.
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