This paper deals mainly with the distribution of oil and water in a reservoir with gradual change in rock charateristics, from a viewpoint of reservoir geology. In this paper, water-wet sandy reservoirs under hydrostatic condition are considered. (Reservoirs under hydrodynamic condition will be discussed in the near future.) Before taking up the main subject, it is necessary to make the concept of free water surface clear. In a sand reservoir of uniform rock characteristics, a cave of supercapillary size is assumed. An oil-water interface in the cave is the free water surface (Fig. 1A). If oil and water have a phase continuity in the reservoir respectively, and if the displacement pressure of the reservoir sand is not zero, relationships between water saturation and height, and between fluid pressures and height, are shown in Fig. 1B and 1C respectively. However, the shape of capillary pressure curve changes with the change of reservoir rock characteristics, as shown in Fig. 2. Fig. 3 shows a generalized relationship between relative permeability curves and the oil-water transition zone. This figure is drawn taking into consideration the free water surface and displacement pressure. In case where there is a lateral and gradual change in reservoir rock characteristics, Fig. 4 was proposed by J. J. Arps (1964). I have revised this figure taking displacement pressure into consideration (Fig. 5). In any event, as a thick reservoir rock changes laterally and gradually from well-sorted coarse-grained clean sand to poorly-sorted fine-grained shaly sandstone, oilwater transition zone will certainly get thicker and be shifted upwards above the free water surface. Based upon these considerations I discuss the distributions of oil and water in a permeability trap. Fig. 6 shows a gently dipping reservoir, in which well-sorted coase-grained clean sand changes gradually into mudstone in the updip direction, via poorly-sorted fine-grained shaly sandstone. When oil is trapped in this reservoir, a sharp oil-water contact is formed on the downdip side. Above this contact oil reservoir with a small amount of interstitial water exists. Further up the dip, water saturation is considered to be getting larger, forming a wide oil-water transition zone. Provided that the first exploratory well (Well 1) was encountered with the transition zone, the second well (Well 2) will be drilled on the updip side, so long as the geometry of the sandstone body and its lithologic characteristics are unknown. But, Well 2 will be a dry hole. Well 3, located on the downdip side, will produce oil. I think such a type of oil pool as shown in Fig. 6 is likely to be often overlooked, and many such pools may be awaiting discovery. How to find out this type of pools by the minimum number of wells is an important practical problem. Sedimentological analysis of cores, high density analysis of continuous dipmeter logs, shape analysis of S.P, curves, etc. in connection with the sedimentological and geohistorical studies on the surrounding area seem to be most effective.
We discovered a new gas-condensate reservoir named Fujikawa III Layer in Fujikawa gas field (Niigata Prefecture) last spring (1971). The production test of the discovery well, Fujikawa SK-16D, was planned to evaluate the reserves and its physical properties of this reservoir. The build-up tests and the drawdown test for a long period, 3 months, were performed. This report is the results of the interpretation of these production tests. We have found this reservoir is a rather small size reservoir and the distance from the well to the nearest boundary of the reservoir is about 400m.
Brines taken from inland natural gas wells are inordinarily different from other ones in their chemical behaviors. As these brines have long been oxygen-free conditions, they would soon be changed with the air in their gualities. In the Naruto gas field, we chose a well for this experiment and sampled the brine with various methods such as using various species of bottles and various sampling procedures. Spending 20 days, we analyzed them periodically for 13 items, examining whether they were changed or not. Remarkable changes were observed in ferrous and ferric ion contents, while no changes were observed in most of the items only when those brines were kept properly during these 20 days. In conclusion from our experiments, brines should be analyzed as soon as they are sampled. And they should be kept free from the air in brown-coloured glass bottles if they could not be analyzed soon after sampled.