Injection Fall-Off Testing

If it can be determined early in the life of an injection well that there is an appreciable "skin effect" , remedial measures may be started before a full-scale pattern flood begins. Similarly, if tests show that a gradual buildup of skin is occuring with time, it is possible to take measures to free the water of plugging material. Finally, determination of the permeability of the sand around the injection well will allow estimation of the future relation between injection pressure and rate. The INJECTION FALL-OFF test is widely used to answer all these questions (See SPE MONOGRAPH VOL. 1, PRESSURE BUILDUP AND FLOW TESTS IN WELLS, PG. 72).

The INJECTION FALL-OFF test is similar to a BUILD-UP test in that the pressure change with time is monitored after flow ceases. The differences are that flow is to the well rather than from the well, and INJECTION FALL-OFF determines the mobility of the water in rock rather than gas and/or oil. Below (Fig. a) is an actual INJECTION FALL-OFF test using the SPIDR. After a stable flow period, the well was shut in and the pressure fall-off recorded. INJECTION FALL-OFF tests are performed on disposal wells, C02 injection wells, combustion gas injection, and polymer or water floods.

Prior to the development of the SPIDR, down-hole gauges were the primary means of measuring INJECTION FALL-OFF pressure. Because of the cost and risk involved with downhole gauges, many companies opted not to test at all. The SPIDR has reduced the cost and eliminated the risk of testing since it is easily connected to the well-head (Fig. b). The SPIDR is temperature compensated so ambient temperature changes do not affect the measured pressures. Bottom hole pressures are calculated from each measured well-head pressure.

(Fig. b)