Large Scale 2D Imaging of Impacts of Wettability on Oil Recovery in Fractured Chalk
Authors: Graue A. , Viksund B.G. , Baldwin B.A. , Spinler E.A. 
Published: December 2007

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^