USE OF BOREHOLE TEMPERATURE LOGS TO UNDERSTAND AQUIFER GEOCHEMISTRY AND HYDROGEOLOGY IN AND AROUND CALIFORNIA OIL FIELDS

The abundance of heavy oil in the San Joaquin Valley of California has led to widespread use of enhanced oil recovery techniques such as steam flood and cyclic steam that rely on steam injection. The injected steam transfers heat to the aquifer (reservoir), reducing the viscosity of petroleum, and eventually condenses as a fluid. Since steam injection occurs at shallow depth (usually less than 2000 ft in California), the resultant water becomes part of the shallow groundwater system.

One useful data set for understanding the geochemistry and hydrogeology of these thermal fields are temperature measurements made during uncased borehole geophysical logging in oil and gas wells. In some cases, only bottom hole temperature or maximum temperature are recorded; in others, a continuous temperature log is measured as a function of depth. Continuous temperature logs are also available from cased well integrity tests in underground injection control wells; however, it is not clear how well these represent the temperature field outside the wellbore.

This temperature data can be used to understand two important aspects of the groundwater system around these fields:

First, salinity mapping based on borehole geophysics is an important tool in understanding the distribution of usable groundwater in California oil fields. However, in thermal fields, the increased temperatures due to steam injection decreases measured borehole resistivity, requiring a temperature-dependent correction when using resistivity-based salinity calculation methods. The borehole temperature dataset offers a way to model the temperature field needed to make this correction.

Second, once the steam condenses, the resulting water becomes part of the regional groundwater system. It is not clear how far this water migrates, either horizontally or vertically. Since rock and water are poor conductors of heat, temperature perturbations outside the immediate zone of injection may indicate that presence of far-travelled water that originated as injected steam. Borehole temperature records, especially those at field boundaries or outside of fields, can be used to understand these migration pathways.

Both aspects are explored in case studies from oil fields using data from online records managed by the California Division of Oil, Gas and Geothermal Resources.