Reproducible Wettability Alteration of Low-Permeable Outcrop Chalk
- A. Graue (U. of Bergen) | B.G. Viksund (U. of Bergen) | B.A. Baldwin (Phillips Petroleum Co.)
- Document ID
- Society of Petroleum Engineers
- SPE Reservoir Evaluation & Engineering
- Publication Date
- April 1999
- Document Type
- Journal Paper
- 134 - 140
- 1999. Society of Petroleum Engineers
- 5.8.7 Carbonate Reservoir, 5.6.5 Tracers, 1.14 Casing and Cementing, 1.6.9 Coring, Fishing, 5.3.1 Flow in Porous Media, 5.4.1 Waterflooding, 4.3.4 Scale, 1.6 Drilling Operations, 4.1.9 Tanks and storage systems, 4.3.3 Aspaltenes, 5.3.2 Multiphase Flow
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A total of 41 chalk core plugs, cut with the same orientation from large blocks of outcrop chalk, have been aged in crude oil at 90 °C for different time periods, in duplicate sets. Different filtration techniques, filtration temperatures and injection temperatures were used for the crude oil. Oil recovery by spontaneous, room temperature imbibition, followed by a waterflood, was used to produce the Amott water index for cores containing aged crude oil and for cores where the aged crude oil was exchanged by fresh crude oil or decane.
The main objective was to establish a reproducible method for altering the wettability of outcrop chalk. A secondary objective was to determine mechanisms involved and the stability of the wettability change. The aging technique was found to be reproducible and could alter wettability in Rørdal chalk selectively, from strongly water-wet to nearly neutral-wet. A consistent change in wettability towards a less water-wet state with increased aging time was observed.
Wettability is defined as the tendency of one fluid to spread on, or adhere to, a solid surface in the presence of other immiscible fluids (Ref. 1). Wettability is a major factor controlling the location, flow, and distribution of fluids in a reservoir.
Hydrocarbon recovery results from a combination of capillary and viscous forces and gravity. In most chalk reservoirs spontaneous imbibition is the major recovery mechanism. This dominance of capillary forces is due to narrow pore throats, more or less water-wet conditions, and the low permeability of this rock. Thus the wettability is a very important parameter controlling the capillary pressure (Refs. 2, 3). However, wettability is not an explicit parameter in the equations that describe flow in porous media and the effects are therefore reflected by the change in capillary pressure and relative permeability (Ref. 4).
The literature (Refs. 1, 5-7) reports that polar components in the crude oil (CO), like resin and asphaltene groups, may alter the wettability of porous rock. These oil components can adsorb on the rock surface by different mechanisms including polar, acid/base, and ion-binding interactions (Ref. 7). Oil composition, surface rock mineralogy, history of the fluids exposed to the rock surface, pore roughness, water saturation, and water composition (Refs. 1-10) are all critical parameters affecting wettability alteration. However, the influence from each parameter on wettability is not well known and may well involve synergistic interactions.
This study was initiated by the need to quantitatively assess the relative importance of the forces acting during waterflooding fractured chalk blocks at different wettabilities (Ref. 11). A series of experiments were conducted at water-wet conditions which investigated spontaneous axial imbibition in stacked cores and the impact of fractures on hydrocarbon displacement mechanisms for large chalk blocks. During this study a dynamic in situ, nuclear tracer imaging technique was used to track the water advancement (Refs. 12,13). In addition the scaling of oil recovery by spontaneous imbibition in cores of various lengths and areas of the exposed faces to brine, i.e., one- (1D), two- (2D), and three-dimensional (3D) exposure, and the gravitational effects on vertically stacked cores have been investigated (Refs. 14-18). To extend this study to relevant reservoir conditions experiments have been performed to alter wettability of outcrop chalk and produce a porous rock which mimics the reservoir rock during laboratory studies (Ref. 5). In this paper we describe reproducible ways of altering the wettability in low-permeable outcrop chalk.
The methods most often used to measure wettability in core plugs are the Amott test (Ref. 19) and the United States Bureaus of Mines (USBM) method (Ref. 20). In this paper the Amott wettability index for water together with the imbibition rate will be used to characterize the wettability of the cores. Both the Amott test and the USBM method only give one parameter to characterize the average wettability of the cores. Both methods have serious weaknesses with respect to discriminating between different wettabilities in a certain range of wettability (Ref. 21). Lately a new test has been proposed where the early imbibition rate was used to determine the core wettability (Ref. 9).
The Rørdal chalk used in this study was obtained from the Portland cement factory in A°lborg, Denmark. Core data for the outcrop chalk is found in Table 1. The rock formation is Maastrichan age and consists mainly of cocolitt deposits (Ref. 22) with about 99% calcite and 1% quartz. The brine permeability and porosity for the Rørdal chalk cores ranged from 1-4 mD and 45-48%, respectively. All the core samples were drilled in the same direction from large chalk blocks to obtain analogous material and to ensure the same orientation relative to bedding planes or laminations. The chalk cores were dried at 90 °C for at least seven days before being used.
The composition of the brine was 5 wt. % NaCl+5 wt. % CaCl2. CaCl2 was added to the brine to minimize dissolution of the chalk. Sodium azide, 0.01 wt. %, was added to prevent bacterial growth. The density and viscosity of the brine were g/cm3 and 1.09 cP at 20 °C, respectively. The brine was filtered through a 0.45 µm paper filter membrane. The salts used in the brine were: NaCl obtained from Phil Inc. with a purity of 99.5%, CaCl2 obtained from Phil Inc. with a purity of 99.5%. Sodium azide had a purity of 99.5%. The materials were used as received. The physical properties of the fluids are summarized in Table 2.
A North Sea stock tank crude oil was used to alter the wettability of the Rørdal chalk cores by aging; i.e., submersing cores in the crude oil at elevated temperature for various length of time. Crude oil composition was measured to 0.90 wt. % asphaltenes, 53 wt. % saturated hydrocarbons, 35 wt. % aromatics, and 12 wt. % nitrogen-sulphur-oxygen containing components (NSO). The acid number was measured at 0.094 and the base number at 1.79.
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