Influence of Production Rate on the Recovery of Oil From Horizontal Waterfloods

Abstract

Many factors affect the behavior of a waterflood. Using data from an actual reservoir, the effect of rate on recovery from sandstone reservoirs undergoing horizontal waterflooding is investigated. The study extends previous work by considering heterogeneous systems and by including the effect of capillary forces. Introduction The optimum rate of recovery from a given reservoir must be based on good conservation practice and on sound economic grounds. The optimum rate, or maximum efficient rate, for a reservoir can be defined in various ways, depending on the criterion used, For instance, optimum rate could be defined as that which gives the highest ultimate recovery or as that which gives the highest present-worth value. The optimum rates obtained using these two criteria could be considerably different. However, in defining a practical optimum rate, both economic and conservation practical optimum rate, both economic and conservation factors must be considered to arrive at a reasonable rate of recovery. Many factors affect the behavior of a waterflood. The significant factors over which the operator has little or no control are mobility ratio, heterogeneity, and wettability. Some of the important rate-dependent processes are imbibition, gravity segregation, and, in some cases, coning. These factors may all affect ultimate waterflood recoveries; however, do the rate-dependent parameters cause ultimate recovery to be rate sensitive? Recently, production rates have approached capacity in many reservoirs because of the rapidly increasing world-wide demand for crude oil. This has resulted in consider-able in-fill drilling for increased capacity, and has increased individual well rates to relatively high levels. Using existing literature, it is difficult to establish the influence of this rapid depletion rate on ultimate recovery. The rate-sensitivity question has been debated for the last 17 years. In the 1950's, laboratory models and field performance data were used to obtain some idea of the performance data were used to obtain some idea of the influence of rate on recovery. Field performance studies were inconclusive because of the difficulty in analyzing field data for predicting ultimate recovery under changing rate conditions. Laboratory-model results were difficult to correlate to field performance because of scaling problems. Most of these studies indicate that increases in rate result in less recovery for a given amount of water injected. However, these studies fail to show recovery at the economic limit. Jordan et al. concluded that higher rates were not necessary to obtain maximum recovery and speculated that, in heterogeneous systems, lower rates could possibly increase recovery because of capillary forces. There was, however, no evidence indicating that lower rates would actually increase ultimate economic recovery. More recently, Miller and Rogers investigated the rate-sensitivity problem for waterflooded sandstone reservoirs. They carried out 156 simulation runs to investigate the influence of eight performance variables on ultimate recovery by waterflooding. The performance variables investigated were (1) oil-zone thickness (5, 10, 40, 68, and 80 ft), (2) horizontal permeability (250, 750, and 1,500 md), (3) total gross fluid rates (400, 1,000, and 2,000 B/D), (4) perforated interval (12.5 and 100 percent of oil zone thickness), (5) ratio of water-zone percent of oil zone thickness), (5) ratio of water-zone thickness to oil-zone thickness (0.250, 0.875, 1.875, and 6.000), (6) kv/kh (0.1, 0.5, and 1.0), (7) mobility ratio (0.25, 1.00, 1.25, and 4.00), and (8) drainage area (10, 20, and 40 acres). JPT P. 555