The use Of An Interface To Control Stimulation and Cementing Treatments

As the energy situation becomes more critical and more domestic oil is desired, new methods of stimulation are needed. Conventional methods generally have not been successful in treating thick or massive pay sections whether in cased or open hole. This has been true for squeeze cementing as well as fracturing and acidizing treatments. In the Levelland-Sundown areas of West Texas, most of the producing formations are thick, fractured limestones of varying porosity and permeability. The problem here has been to treat the low-permeability or tight zones as well as the more permeable zones. Many diversion methods have been tried without success. These have included straddle packers, ball sealers, and the suspended-solids type of blocking agents. While all of these may force the stimulation fluid to enter the tight zone, the fluid generally will penetrate only a short distance before seeking a fracture back into the more permeable zones. As a result, fluids from later stimulation treatments have undoubtedly been injected back into the originally treated zone time after time. The same is probably true of cement when attempting to squeeze-off undesired zones such as water-producing zones or channeled zones in injection wells. A new method of diversion has been employed with a high degree of success in this area during the last two years. The method involves the use of two fluids, one "tagged" with a radioactive material and another "untagged" fluid. The "tagged" fluid normally is pumped down the annulus while the untagged fluid is pumped simultaneously down the tubing. A detection tool, run on a wire line, is used to monitor the interface between the two fluids. The interface indicates the place of entry of the two fluids into a zone. The location of the interface is controlled by means of pump rate. It can be moved up or down by varying the pump rate down the tubing or annulus or both. The interface method allows control of stimulation fluids not only at the wellbore but out in the formation as well. Figure 1 illustrates one type of interface treatment. In this example, the problem is to acidize a low-permeability zone below a zone of higher permeability. In this case radioactive water _ is pumped down the annulus while acid is pumped simultaneously down the tubing. The radioactivity detector is located in the tubing and the interface is controlled at the top of the low-permeability zone. Most acid inhibitors are capable of protecting the wire line and detection tool from damage by the acid. However, in extremely deep or hot wells, special inhibition requirements may be required by the wireline company. In Fig. 2, the problem is exactly the opposite of that in Fig. 1 and is the problem usually encountered in the Levelland-Sundown area. Here the zone has