The injection of sand-entrained radioisotopes at the latest time before fracturing fluid enters the wellbore minimizes personnel exposure to radiation, contamination of surface equipment, and hazards of transporting radioactive material. This paper presents design and operational details of the devices used for wellhead injection of radioisotopes, transportation considerations, disposal of leftover radioactive sand, closed-loop control of injection, and a comparison of the injection plan with the actual injection.
The use of better practices in handling radioisotopes is becoming more important than it has been in the past. Since the early days of using radioisotopes for oilfield applications, radioisotopes have been added manually and/or mechanically to the upstream side of high-pressure downhole pumps. This process may include the mixing tank on the blending equipment or a self-contained, low-pressure (below 100 psi [690 kPa]) injection unit. Injection units are commonly used before or after the blending equipment. Adding the radioisotope in the mixing tank is the easiest method with respect to equipment requirements, but it can lead to contamination of the pumping equipment, manifolding, and well location and to increased personnel exposure at the mixing tank. Many of the contamination and exposure problems cam be minimized greatly by injecting the radioisotope near the wellhead. The radioisotope can be concentrated when injecting in the high-pressure side, thus reducing the physical volume of the radioisotope. This paper describes a high-pressure injection system for adding a radio-isotope to the fracturing fluid.
Fig. 1 illustrates a typical equipment layout used in a fracturing treatment including radioisotopes. As shown, there are four points where radioisotopes may be added on the surface:
before the mixing tank on the blending equipment,
at the mixing tank on the blending equipment,
between the blending equipment and the high-pressure downhole pumps, and
after the high-pressure downhole pumps and before the wellhead.
For Points 1 through 3, radioisotope-laden proppant usually is contained in 1-qt [0.946-L] metal cans. Each can commonly contains 3 lbm [1.361 kg] of radioisotope-laden proppant on which a prescribed volume of a radioisotope, commonly expressed in millicuries, is fused. If a large volume of radioisotope is required, then several cans are needed to complete the injection process. For Point 4, radioisotopes are contained and transported in special cylinders. A brief description of the methods used at Points 1 through 4 follows.
Point 1-Before the Mixing Tank on the Blending Equipment. This addition point is the least desirable of the four methods overall. At this point, essentially all the pumping equipment and the associated manifold become contaminated. Because a person usually is near the injection point, this person receives a relatively high exposure compared with that received at Point 4, assuming local operation and little shielding. Radioisotope injection equipment generally is skid-mounted and requires a pump to inject the radioisotope into the clean fracturing fluid. If a failure occurs downstream of this point, not only is the contamination of the equipment increased, but also the well location is contaminated.
Point 2-At the Mixing Tank on the Blending Equipment. This addition point is not much better than Point 1, except that the inlet side of the mixing tank is not contaminated. In the past, this point was the most common and popular for adding radioisotopes to fracturing fluid manually. The operator usually must open each can and shake the radioisotope into the mixing tank at a uniform rate. Therefore, the potential for high personnel exposure is possible if the operator does not use tongs. The general blending equipment and well location are contaminated by radioisotopes that miss the mixing tank during addition. If any failure occurs downstream of the mixing tank, then contamination of the well location and surrounding equipment and exposure of personnel are increased.
Point 3-Between the Blending Equipment and the High-Pressure Downhole Pumps. This addition point is currently the most common for injection of radioisotopes. This method usually incorporates the same injection unit and uses the same operation, in every respect, as the method used at Point 1. The main difference is that the blending equipment and most of the manifold between the blending equipment and the high-pressure downhole pumps are not contaminated. The operator still receives a relatively high exposure if the unit is operated locally or little shielding is used. If any failure occurs downstream of this point, contamination of the pumping equipment, high-pressure manifolding, and well location increases.
Point 4-After the High-Pressure Pumps and Just Before the Wellhead. This addition point is the best for injection of radioisotopes. Contamination of the fracturing equipment is eliminated. and personnel exposure is minimized. The injection process is near the wellhead; therefore, contamination of the high-pressure manifolding is reduced. If a failure does occur, personnel should not be contaminated because they should not be near the wellhead during treatment. Also, contamination of the well location is limited to a specific area, rather than spread over most of the location, as is possible with Points 1 through 3. The remainder of this paper addresses this method in more detail.
Low-Pressure vs. High-Pressure Injection Systems Until recently, there was no need for injection of a radioisotope on the high-pressure side of the downhole pumps. Today, contamination of equipment and well locations and personnel exposure are major considerations in the planning and execution of radioisotope tagging operations. One of the first methods was to have personnel meter the radioisotope into the blending tub. Then, methods of injecting a radioisotope into the stream before the downhole pumps were introduced. The currently used method of injecting the radioisotope as near the wellhead as possible is necessary to minimize the potential exposure problems. A brief description of a typical method of low-pressure injection system in current use follows, and the major advantages and disadvantages of each method are listed. Finally, an examination of the differences in these two methods illustrates why the radioisotope should be injected near the wellhead.