Pattern Oversweep In Nine-Spot Waterflood Pilots

Abstract A laboratory study has been conducted on isolated and partially confined five- and nine-spot water/load patterns. The results show that both isolated patterns continue to produce oil with continued injection, but that each pattern, when surrounded by a single ring of like patterns, behaves as if it were confined provided the injection rate is sufficiently high to ensure a stabilized flood. Introduction Although the five-spot is popular in test pilots, there are occasions where the nine-spot is used so as to increase the total injection rate into the pivot and thereby decrease the duration of the test. It was therefore decided to extend the above study to the nine-spot pattern. As the Rapoport, Carpenter and Leas scaling criterion(2) was developed for the five-spot pattern and its validity for the nine-spot was not established, it was decided to vary the injection rate for both the five- and nine-spot and to compare the results. Equipment The model studied was the same one used and described in the previous study. A new injection system was designed and built. This system was modelled after the old one, but was built to permit simultaneous injection into as many as forty wells. It consisted of 41 cylinder-pistonassemblies positioned between two 3/4-inch-thick steel plates. As in the previous study, the center cylinder-piston assembly was double acting and was used toctuate the remaining 40. Injection pressures were measured by means of manometers, as was the case in the earlier study. However, in order that up to 40 injectors could be used, the manometer anks had to be modified and extended. Model & Fluid Properties Although the model and beads used in the present study were the same as in the previous work, the system was cleaned and repacked. This resulted in a pack with a porosity of 36.4 percent and a permeability of 6.60 darcys. The fluids employed had the properties shown in Table 1 and were essentially identical to those used in the previous study. Procedure The experimental procedure used was identical to that in the earlier stuoy, except that a different constant injection rate was used in each run. In each case, the range of rates was chosen so as to straddle the value required by the scaling criterion or its equivalent. Observations Flood tests were carried out on an isolated five-spot, a five-spot surrounded by eight other fivespots, an isolated nine-spot and a nine-spot surrounded by eight other nine-spots, as shown in Figure 1. Isolated five-spot tests were conducted at constant injection rates ranging from 42.9 to 514.3 cc/hr well, as is indicated in Figure 2. From the, results, it may be seen that in each case the flood oversweeps the pattern, but that an increased injection rate tends to increase the degree of confinement. In the case of the isolated ninespot, tests were conducted at five different constant injection rates in the range of 21.4 to 257.1 cc/hr well.