Economics of Contained Nuclear Explosions Applied to Petroleum Reservoir Stimulation

Abstract

Abstract Information obtained from Plowshare's Project Gnome subsequently has been verified by studying environments of other contained nuclear explosions. The major erect of a nuclear explosion in a hydrocarbon reservoir will be to increase the apparent wellbore diameter by creating a cavity, chimney and an associated annular fractured zone. This geometry results in a complex flow regime yielding increased productivity of the reservoir fluids. A discussion of this flow regime and the economics of utilizing nuclear stimulation for low- productivity oil- and gas-bearing reservoirs are considered. Introduction For several years nuclear devices have been exploded and completely contained within underground formations. The results, quantitative and qualitative, from these tests have established the technical feasibility of utilizing nuclear explosions for stimulating hydrocarbon-bearing formations. The primary aim of Continental Oil Co.'s continuing research in this area is to evaluate the commercial possibilities of this peaceful application of nuclear energy. Providing that emplacement and device costs are not prohibitive, this compact source of tremendous energy could be a giant stride in advancing the industry's efforts to recover more of the nation's petroleum resources. Since the 1.7 kt Rainier test in 1957, there have been a number of completely contained nuclear explosions of varying size in different rock formations. These have provided a large amount of data on the environments created by the energy released. In one of the more recent tests, the Project Plowshare Gnome test in the Salado salt formation near Carlsbad, N.M., a number of reservoir rock and crude oil samples were placed near the nuclear device by the Bureau of Mines and various oil companies. The recovery and analysis of these samples showed that shock energy from the nuclear device could be used to alter and upgrade the properties of reservoir rock. Analysis of the crude oil samples showed insignificant effects by the shock energy and radiation received. Examination of a number of explosion environments has shown other features of potential benefit to the petroleum industry. When a nuclear explosion occurs in an underground formation, a cavity and chimney, surrounded by a fractured area of increased permeability is usually created. Beyond this fractured zone is the undisturbed reservoir rock. In a hydrocarbon reservoir, this broken and fractured zone surrounding a wellbore will alter the flow pattern of the hydrocarbons toward the wellbore. A flow regime is established, in which oil or gas from the original tight reservoir rock flows through highly permeable fractures, through rubble, and into a new wellbore. The stabilized flow rate is controlled by the permeability of the reservoir rock at the boundary of the fractured zone. In addition, the rubble zone serves as a supply reservoir from which the hydrocarbon is recovered. Thus, high-rate withdrawals are possible during short time intervals. In this paper we will discuss the effect of the altered flow patterns and increased flow capacities on reservoir performance. Both gas and oil reservoirs are considered. Effects on Reservoir Rock Studies of a number of contained underground nuclear explosions have yielded a rather consistent picture of the geometric features produced. The major features of cavity, chimney and fractured zone are illustrated by a schematic drawing in Fig. 1.on detonation of the device, a cavity is formed as a result of the vaporization of rock and saturating fluids. The gases contained within the cavity are initially at extremely high pressure and temperature. Compaction of the rock in the lower hemisphere, coupled with upward and lateral rock movement, results in a spheroid cavity. The gas pressure temporarily supports the overburden-thus preserving the cavity shape. Subsequent heat losses. gas leak-off through the fracture system, and vapor condensation reduce the pressure until the fractured rock above the cavity can no longer be supported. Rock collapse into the cavity forms a complex chimney-rubble zone. Most of the molten material and radioactive fission products collect in the bottom of the zone. JPT P. 1145ˆ