DASを使ったタイムラプスVSPの利点と課題

抄録

In the conventional time-lapse VSP, a sensor array using geophone or hydrophone is permanently or temporarily installed in a well under high temperature and high pressure environment, and the baseline data is recorded over several hours to several days. Sensor arrays become more expensive in proportion to the number of sensors, and only very limited sensors can record without aging in high temperature environments above 150 °C. Therefore, it is possible to observe only in the section within the operating temperature range of the sensor array. Starting from the day when the baseline data was recorded, the data should be recorded again after fixed elapsed time of six months or one year, and then compared with the baseline data in order to observe the movement of fluid in the formation. If the sensor fails, the data for that section will not be available until the sensor is replaced. When using the VSP downhole tool, it can be difficult to place the sensor at exactly the same position as the baseline, and subtle sensor misalignment can affect time-lapse data sometimes. Permanent installation of the sensor is ideal from the perspective of data comparison. In the case of DAS, an optical fiber that acts as sensor is permanently installed in the well. Generally, an optical fiber with temperature rating of 200 °C is passed through a thin stainless steel tube called Control Line and installed on the outside or inside of the casing. Fiber optics are much cheaper than traditional sensor arrays, almost no aging, and can cover almost any section of the well. Taking advantage of these, time-lapse VSP using DAS has begun to be actively used as a practical method in recent years. There are potential issues need to be considered for successful DAS time-lapse VSP to be explained.