The Planning of a Large-Scale Miscible Flood at Prudhoe Bay

Abstract

Summary This paper describes the technical considerations that set the major design parameters for the Prudhoe Bay Miscible Gas (PBMGP). This project was planned as a large hydrocarbon miscible flood. The basic project concept is to manufacture a miscible injectant from the field separator off-gas. This injectant is compressed and delivered to the EOR project area for injection in a water-alternating-gas (WAG) mode. The miscible gas supply will vary, generally increasing with time. Over the first 10 years of the project, an average supply of 200 MMscf/D [5.66×106 m3/d] is anticipated. The gas processing plant and the factors affecting miscible gas supply are described. The EOR project area was selected by a screening process. This led to a project area definition encompassing 4.9×109 RB [779×106 m3] PV containing 2.2×109 STB [350×106 m3] original oil in place (OOIP). The planned cumulative volume of miscible gas injected will be 10% PV. Reservoir studies indicate an incremental oil recovery by miscible flooding of some 5.2% OOIP or 115×106 STB [18.3×106 m3]. Aspects of these reservoir studies are described. The Sadlerochit reservoir is both the ultimate source of the miscible solvent and the target reservoir. This introduces several reservoir/facility interaction effects. The planning of a major EOR project in the Arctic has also involved technical considerations not routinely encountered in conventional oilfield projects. Both aspects are discussed in the paper. Introduction and Background The ultimate recovery from the Sadlerochit reservoir of Prudhoe Bay is currently estimated at some 9×109 to 10×109 STB [1.4×109 to 1.6×109 m3], leaving more than 10×109 STB [1.6×109 m3] oil in place. At this projected abandonment, the Sadlerochit reservoir will still contain more oil than the original reserves for any other conventional oil reservoir (i.e., gravity greater than approximately 15°API [0.966 g/cm3]) in North America. Thus there is strong incentive to develop methods for increasing the recovery from the Prudhoe Bay field. Severe economic and operational problems face any north Alaska EOR project. Ref. 1 highlights the relatively short window of opportunity for north Alaska EOR projects before operating economics and transportation costs curtail options. Early Prudhoe EOR screening studies were started shortly after the start of field production to investigate the applicability of the principal enhanced recovery methods. The possible recovery processes considered fell into four categories:surfactant flooding,enhanced waterflood techniques,thermal processes, andmiscible gas displacement processes. Surfactant flooding is recognized as a potential EOR process. Recent reviews did not find a surfactant currently available that would be effective over the range of temperatures and salinities anticipated in the reservoir; however, laboratory studies of surfactants continue. Enhanced waterflood techniques - such as carbonated, caustic, or polymer flooding - were found to offer limited potential for improving waterflood performance. Investigations indicated, however, that the techniques are not cost-effective because of the logistics of supplying large quantities of chemicals to the remote Arctic field location. Thermal processes were eliminated because the depth and pressures of the Sadlerochhit formation make these processes economically unattractive for the light oil reservoir. Two main types of miscible gas displacement processes were considered: vaporizing, or high-pressure lean-gas drive, and condensing, or rich-gas drive. The vaporizing process is not applicable at Prudhoe Bay because Sadlerochit crude is relatively low in intermediate components and because the critical pressure at which methane, or CO2, and Sadlerochit oil become miscible is well above reservoir pressure levels. On the other hand, the condensing process is viable. With the addition of intermediate hydrocarbons, principally ethane and propane, to separator off-gas, an injectant that is miscible with Sadlerochit oil at typical reservoir conditions can be obtained.2,3 The required intermediate hydrocarbon components can be obtained from produced fluids at Prudhoe.