Wellbore Transmission Of Electrical Power

Abstract

Abstract In-situ electrical heating of viscous oils depends on the passage of large electrical currents through a subsurface formation. It is essential to the design of such a process to know the percentage of power applied at the wellhead which actually heats the object formation below. This in turn requires that there exist some convenient and reliable method for predicting the power loss which occurs in the wellbore. Although the determination of the power loss is straightforward for a DC system, it is often desirable to use readily available AC current to minimize corrosion. On the other hand, use of AC current can lead to a significant increase in transmission power loss in the presence of steel tubular goods, primarily because of magnetic hysteresis and eddy current losses. It is the purpose of this paper to present an experimental method for determining the wellbore transmission power loss in AC systems. Theory related to AC power loss is discussed, and general laboratory experiments are described which determine the losses in complex wellbore systems. Power loss data obtained from a shallow well test installation are presented for two distinct modes of operation: one in which copper cables are used to conduct power down hole, and a second which utilizes the steel pipe as the conductor. Additional laboratory experiments are then described which were designed to duplicate the data taken in the shallow wells. The laboratory results are shown to be in good agreement with power measurements made in the two 500-foot wells of the shallow well installation, demonstrating that the method can predict power loss for field-scale systems. Introduction Electrical heating of subsurface formations has frequently been proposed as a means of enhancing oil recovery(1,2,3) In particular, electrical heating has been proposed(4) as a means for in-situ heating of a subsurface oil sand formation in order to mobilize the viscous oil contained therein. A schematic for this process appears in Figure 1 and is discussed in detail in Reference 4. Basically, the concept is to conduct large AC currents down a wellbore, through the object formation, and back up an adjacent well. Formation brine provides the conductive medium in most of the formations, and "bound" water in the shales accounts for their (high) conductivity. These currents heat the formation by virtue of resistance (Joule) heating, creating a mobile fluid path between the wells where little or no flow was possible before heating. Once the viscous oil has been mobilized, it can be recovered by a more conventional fluid displacement process. Note that although heat conduction will occur and must be taken into account, this is not the principal means by which fluid mobility is achieved. Also note that Joule heating occurs throughout the formation from the moment power is applied so that the critical area midway between the electrode wells is being heated and mobilized from the outset.