Designing a Steamflood Pilot in the Thick Monarch Sand of the Midway-Sunset Field

Abstract

This paper describes the design and development of a steamflood pilot consisting of six inverted five-spot patterns in Section 26C of the Midway-Sunsetfield. Steam injection will be in the 330-ft Monarch sand. A steamflood simulation study indicated a potential of 60- to 70-percent oil recovery and defined the relative importance of various steamflood parameters. Introduction The reservoir characteristics and production history of Section 26C of Midway-Sunset field make it a favorable candidate for thermal recovery. Current well productivity under cyclic steaming is only about 0.05 BOPD/ft. The estimated ultimate recovery by this method is only 15 percent because of its continuously decreasing percent because of its continuously decreasing effectiveness. Underground combustion is thought to be undesirable for this section because of the reservoir's large thickness and low dip. Based on its predicted and actual performance in other heavy oil reservoirs, performance in other heavy oil reservoirs, steamflooding is the most promising thermal recovery technique for this property. The high capital and operating costs of steamflooding made a pilot desirable to evaluate the process in this field. Other reasons were uncertainties about the geological structure, sand continuity, and the role of gravity override in steamflooding a very thick reservoir. This paper describes the reservoir and its geology, the production history under cyclic steaming, pilot project production history under cyclic steaming, pilot project details, and the results of a simulation study to optimize steam injection and predict performance. Reservoir Characteristics Geology The structural feature associated with the various productive zones in Section 26C of Midway-Sunset field is a productive zones in Section 26C of Midway-Sunset field is a large southeasterly plunging nose. The main productive sands lie on the southwest flank and plunge, where the dip is about 10 deg. . Little production comes from the northeast flank, where the dip is up to 50 deg. . The two areas are separated by a thrust fault that offsets the oil-water contact. A northeasterly trending sand channel in the northern half of the section is developed and productive mainly over the northwest quarter. No production comes from the southwest comer of the section because of sand truncation. The Monarch sand, considered the main productive zone, is of Miocene age and is at an average depth of 1,300 ft. Its thickness varies from 0 to 600 ft and averages 350 ft in the main productive area. Well logs show that the sand is vertically continuous, without any significant shale breaks. A typical induction-electric log is shown in Fig. 1. However, recent cores contain a number of diatomite beds from 0.25 to 6 in. thick. Their areal extent and ability to restrict vertical fluid movement are not known. Rock and Fluid Properties The reservoir rock is an unconsolidated, poorly sorted sand with very fine to very coarse grains. The productive interval consists of turbidite beds generally 2 to 5 ft thick that exhibit normal upward fining of grain size, and that usually are separated by thin, low-permeability diatomite beds. Average porosity is 27 percent and air permeability is 520 md under reservoir conditions. The over-all net-to-gross ratio for the zone is 0.74. The rock is saturated with a 14 deg. API oil whose viscosity is a strong function of temperature (Fig. 2). Current oil saturation in the main part of the reservoir is 59 percent but only 36 percent in the depleted portions near the top. JPT P. 1559

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