Evaluation of Residual Oil Saturation After Waterflood in a Carbonate Reservoir
- Mahendra K. Verma (QGPC-Onshore) | Michel Boucherit (QGPC-Onshore) | Lucienne Bouvier (Total-CFP)
- Document ID
- Society of Petroleum Engineers
- SPE Reservoir Engineering
- Publication Date
- November 1994
- Document Type
- Journal Paper
- 247 - 253
- 1994. Society of Petroleum Engineers
- 5.3.4 Reduction of Residual Oil Saturation, 5.2 Reservoir Fluid Dynamics, 5.8.7 Carbonate Reservoir, 5.2.1 Phase Behavior and PVT Measurements, 5.6.2 Core Analysis, 1.6.9 Coring, Fishing, 2.2.2 Perforating, 4.3.3 Aspaltenes, 5.7.2 Recovery Factors, 5.4.1 Waterflooding, 5.1 Reservoir Characterisation, 5.5.2 Core Analysis, 1.11 Drilling Fluids and Materials, 2.5.2 Fracturing Materials (Fluids, Proppant)
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Four different approaches, including special core analysis (SCAL), log-inject-log, thermal-decay-time (TDT) logs, and material balance, were used to narrow the range of residual oil saturation (ROS) after waterflood, Sorw, in a carbonate reservoir in Qatar to between 23% and 27%. An equation was developed that relates Sorw with connate-water saturation, Swi, and porosity.
The importance of accurately determining Sorw cannot be overemphasized. Sorw is used not only to judge waterflood performance, but also to evaluate the potential for a tertiary recovery project. It is also used to calculate sweep efficiency, which in turn leads to an estimation of the remaining reserves.
Direct field measurements show an Sorw between 5% and 40%, whereas in previous SCAL studies, Sorw ranged from 14% to 56%. In most of these earlier studies, irreducible water saturation and Sorw were measured with a Hassler cell. We thought that this was an inadequate laboratory procedure owing to the specific characteristics of the reservoir: a thick oil column (700 ft) and neutral wettability. Because of the neutral wettability of the reservoir, water needs to have sufficient energy to displace oil, unlike in a water-wet reservoir where spontaneous imbibition displaces almost all the mobile oil. Sorw is actually the result of equilibrium between capillary and gravity forces. Also, gravity forces are of high magnitude because the original oil column is > 700 ft thick. The gravity forces overcome the capillary forces to displace the oil to Sorw and therefore playa major role in sweeping the reservoir. Thus, to obtain more-realistic Sorw values, we decided to use the centrifuging technique, which simulates the reservoir displacement mechanisms more closely, in addition to the standard flooding in a Hassler cell.
This paper presents the results of Sorw determinations with four different techniques: core waterflood followed by centrifuging, log-inject-log, TDT logging, and material balance.
Several attempts have been made over the years to calculate Sorw for the Arab D reservoir, a carbonate reservoir. The first attempt was made in 1958 when an industry laboratory was contracted to determine Sorw on 12 nonpreserved plugs cut from cores in six wells. The core plugs were cleaned and restored to initial saturations with a centrifuge, then subjected to waterflood in a Hassler cell. The Sorw ranged between 14% and 33%, depending on rock type and capillary pressure, and the average Sorw was 23%. Because this experimental procedure may have rendered these plugs more water-wet then they were initially, this Sorw is considered to be the lower limit.
The second attempt was made during 1978-79, by which time the importance of wettability had been established and some wettability work on the Arab D cores had been performed. A total of 20 waterfloods were carried out by a different laboratory on seven long (400-mm) plugs from two wells with a Hassler cell. Of these, three waterfloods were performed above the bubblepoint pressure and 17 were performed below the bubblepoint pressure to study the effect of initial gas saturation. Of the 17 waterfloods, 13 were conducted on cores with low Swi and four were conducted with high Swi.
Tests were carried out either on restored plugs or on preserved plugs (only two tests) with reservoir oil and synthesized brine at reservoir conditions. The restoration procedure was designed to achieve neutral wettability. Low initial water saturations were achieved by circulation of hot methane and partial drying.
The results indicated that the techniques of restoring Swi had no influence on Sorw. However, the value of Swi itself had a considerable influence on Sorw: the lower the Swi, the higher the Sorw. The Sorw was higher for the restored plugs, which was attributed to the possible change to a more oil-wet wettability in the restored plugs. Because most of the work was on restored plugs, the Sorw results are considered to be on the high side; between 17% and 56% for tests below the bubblepoint pressure and between 38% and 60% for tests above the bubblepoint pressure.
The third attempt was made in 1982 when six preserved plugs were subjected to waterflood in a Hassler cell at surface conditions. At the end of waterflood, the plugs were washed and restored. However, the restoration process is considered inadequate because the plugs were not aged with crude oil. The Sorw values ranged between 37% and 53% for the preserved plugs and between 18% and 44% for the restored plugs.
As can be seen from the previous discussion, a large variation in the Sorw values exists, depending on the state of core plugs (washed, preserved, or restored) and the Swi value. In a detailed survey of the effects of wettability, Anderson1-3 points out that knowledge of the rock wettability is important because it helps in designing appropriate core analysis procedures. This is imperative when the reservoir is not strongly water-wet. Therefore, we decided to study the wettability first and then define the proper experimental conditions for determination of Sorw in the laboratory.
Wettability of Arab D Rock
Several attempts were made over the years to define the wettability of the Arab D reservoir. Measurements were performed on preserved plugs, taken with water-based mud free of surfactants. They all show a quasineutral wettability only slightly water-wet or slightly oil-wet, according to samples.
The Amott-Harvey method was used to confirm the neutral wettability of the Arab D reservoir. This included performing complete capillary cycles of fluid displacements on several plugs with porous-plate and centrifugation methods. A few attempts were made to compare results obtained on preserved plugs taken with water-based mud free of surfactants with those obtained on soft-cleaned, restored plugs.
The results indicate that the Arab D reservoir rock is neutral to slightly water-wet. Table 1 and Fig. 1 show all the wettability data obtained during recent SCAL on the Arab D reservoir. The data demonstrate the neutral wettability of the rock. Fig. 2 plots the wettability index data with depth. The reservoir is basically of neutral wettability with a slight tendency toward water wettability at the base of the transition zone and toward oil wettability at the upper end of the transition zone and through the oil column.
The wettability of the Arab D reservoir was correlated with crude oil characteristic through Cuiec's4 correlation; Cuiec studied the relation between wettability index and asphaltene and sulfur index of 20 oil reservoirs. The Arab D crude has 0.12 wt% asphaltene and 1.27 wt% sulfur; both values fall in Cuiec's "intermediate range," but somewhat to the water-wet side.
Comparison of the restored and preserved plugs showed no significant difference in the Amott-Harvey index. However, some effect on the value of Archie saturation exponent, n, was measured on the soft-cleaned, restored plugs. This was attributed to a redistribution of the fluids; therefore, we decided to perform the SCAL work on preserved samples.
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