The name or title of a cross section seldom fully describes its location, therefore, a small-scale key-map of the locality should be printed in one corner of the cross section. A heavy line drawn upon the key-map will indicate the position of the cross section and the wells included.
An index to all cross sectional drawings is most conveniently made upon a small-scale map of the entire field, a line being drawn thereon to indicate the location of each cross section. Each cross section may also conveniently carry an index or serial number.
A filing system for cross sections should be provided, so that they can be preserved and also readily found. Ordinary letter files or drawers may be used for this purpose if the drawings are carefully folded with the key-map on the outside.
The value of cross sections is shown by the following oil field problem, which probably could not have been solved without them. The cross-section (Fig. 20) does not conform to the specification requiring that the horizontal and vertical scales should be the same. The distortion is here justified by the necessity of confining the drawing to a single page of the book. Some details, such as names of formations, are also omitted.
The wells were drilled in the order as numbered. The strata are not bent or deformed in this particular locality. In each well the 10-in. casing was intended to shut off the water occurring above the oil sands. In well No. 3 it was landed at a depth of 1880 ft., in conformity with successful results obtained at No. 1 and No. 2. However, the sand between 1980 ft. and 2016 ft. was found to contain some oil and considerable water, hence it was necessary to use more casing to shut off this water. Since some experimenting was necessary, two strings, the 814 in. at 2039 ft. and the 6%-in. at 2098 ft., were used. Had all conditions been known beforehand only one string would have been neces sary, but when the 8%-in. casing was landed and the well drilled deeper more water was encountered and the 6%-in. casing had to be used. The final oil string of casing, 412 inches in diameter, was carried to 2351 ft., and upon testing the well was found to produce mostly water, so the bottom was plugged (using waste rope, iron lathe-cuttings and cement) up to 2265 ft. and the well again tested, when it was found to be free from water.
Since well No. 3 was one of the first wells in this locality to encounter water in the oil sands where oil was expected, careful work was necessary on the next well drilled, in order to verify conclusions drawn from evidence furnished at well No. 3. This was done an the neighboring wells, not shown on the drawing, and since similar results were found, the top sands in well No. 4. were shut off with the 10-in. casing at 2320 ft. The well was drilled to 2467 ft. only, so as not to encounter the sand which contained water in well No. 3 between the 2325 and 2351-ft. levels.
After completion, all these wells produced clean oil for about a year, then well No. 2, began to show a little water, and about three months later showed about 80 per cent. There were three possible sources from which the water could come: from other wells through the oil sands, or from above the oil sand, or from the bottom sands penetrated. The drawing shows that the possibility of bottom water was slight. The possibility of water coming from above was tested by removing the 8% in. "oil string, " putting a bridge or plug in the open hole between the bottom of the 10-in. "water string" and the top of the oil sand. After bailing the well dry above the plug no water came in, proving that the "water string" was not leaking at any point and was effectively excluding upper water. Therefore the natural presumption was that the water in well No. 2 was coming through the,oil sand, and as the same sand showed both oil and water between 1980 ft. and 2095 ft. in well No. 3 it was concluded that the water was rising along the dip of that sand as the oil was removed. Such a movement is called the encroachment of "edge water." The 8%-in. casing was again put in and cemented at 1886 ft., in the shale presumed to correspond to that found at 2091 to 2122 ft. in well No. 3, and 2318 to 2325 ft. in well No. 4. After pumping the well for about 10 days, the oil was found to contain less than one per cent. of water, proving that the water had been coming through the upper sands between 1782 and 1870 ft. After about four years the water commenced to show in well No. 1. This movement of water naturally follows the removal of oil and can not be stopped, but the particular sand affected can be kept separated from other productive sands.