Studies on Pilot Water Flooding

The purpose of this work was to learn what in formation could be obtained from various types of pilot water floods and to attempt to find the optimum pilot pattern for a reservoir which had previously been depleted by a solution gas drive. The study was made in the laboratory with mathematical methods, a dynamic analog and a potentiometric analog. Results were tested against the field histories of a number of pilot water floods. At a reasonable value of cumulative injection, the total production rate for the one-in factor five-spot should reach about 6.5 per cent of injection rate, and for a four-injector five-spot, about 9 per cent. Accurate estimates of ultimate recovery cannot be made on the basis of such small production rates. However, with a pilot composed of nine injectors and 16 producers, the production rate is approximately 50 per cent of injection rate at a reasonable value of cumulative injection. Some information for extended performance predictions high be obtained from such a large pilot. These conclusions were drawn on the basis of results obtained for unit mobility ratio, and a study using the potentiometric analog was made of the effect of other mobility ratios to determine the range of applicability of these predictions. For the four-injector, five-spot pilot with Mw, o = 1/4, the ratio of production to injection rate (before water breakthrough) is about twice that for Mw, o = 1; with Mw, o = 4, it is about two-thirds; and with Mw, o = 10, it is about one-third. For high mobility ratios, it was found that the production rate increased considerably as water-cut increased. These results can be used to modify, qualitatively, the interpretations based on curves for the unit mobility ratio cases. It was found that the maximum ratio of production rate to injection rate observed in field pilot floods was of the same order as that predicted by these methods. The time required to reach this maximum did not generally agree with the time predicted for a homogeneous reservoir. The difference between predicted and observed time of response gives an indication of the permeability profile and of the condition of the producing wells.