Optimizing the Distribution of Cathodic Protection Current
- Glenn R. Roberson (Standard Oil Co. of Texas) | F.W. Schremp (Chevron Oil Field Research Co.)
- Document ID
- Society of Petroleum Engineers
- Journal of Petroleum Technology
- Publication Date
- July 1970
- Document Type
- Journal Paper
- 812 - 816
- 1970. Society of Petroleum Engineers
- 4.1.9 Tanks and storage systems, 1.14 Casing and Cementing, 4.2.3 Materials and Corrosion, 4.2 Pipelines, Flowlines and Risers, 1.10.1 Drill string components and drilling tools (tubulars, jars, subs, stabilisers, reamers, etc)
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Relative currents applied for a given potential may vary because of variations in distance from well to ground bed, in resistivities between wells and ground bed, and in pipe-to-soil resistance. Here is a simple method of measurement - more economical than the potential profile - that is sensitive to the variables and that makes it possible to compensate for them.
In most instances wells are given cathodic protection on an individual basis. However, when well density is great, the interactions between cathodic protection systems and the electrochemical differences between wells cannot be overlooked. We shall discuss here various factors that affect the cathodic protection of well casings, and a procedure that can be used to optimize the distribution of cathodic protection current.
Methods of Current Distribution
Fig. 1 shows a typical control arrangement for a multiwell system. A variable rectifier generates a given total current output that is distributed according to a plan with the aid of a variable resistance at each well. The conductors may be flowlines, or they may be special conductors installed to carry the current. Fig. 2 illustrates, for each well, individual variable rectifiers that can be adjusted according to plan. In some cases two- to four-well systems are adjusted only by a central rectifier, and the current is allowed to flow to each well in inverse proportion to the resistance in that leg of the circuit.
Measurement of Current Requirements A number of methods have been used for measuring current requirements for each well. Following are some examples. 1. E-Log I on a sampling or on all wells. 2. Some variation of E-Log I. 3. Potential profile data. 4. Current density. 5. Estimated economic limit. 6. Some combination of these. Of these methods, only the potential profile has enough sensitivity to give a measure of current requirement under operating conditions.
Variations in Current Requirement
Fig. 2 shows an installation with equal spacing between the wells and their individual ground beds. In actual practice, because of topographic adaptation or selection of ideal ground-bed soil, there is usually considerable variation in wellhead to ground-bed distances. In Fig. 3 the best ground bed is located close to two wells and distant from the other three. Fig. 4 shows the location of the ground bed in an almost ideal position; but two wells are in a subsurface salt water aquifer with the ground bed, and the other wells are not. Fig. 5 shows two wells where the low resistant area of the mud pit includes the upper casing and two where it does not. According to potential theory, the change in voltage (E) with distance (x) from a spherical anode is given by:
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