Statistical Investigation of Surface Expressions in the Midway Sunset Oil Field

In the Midway Sunset Oil Field in Central California, operators inject steam into the shallow diatomite formation to lower the viscosity of heavy oil and flow it out of wells. The injected steam, however, does not always remain in the reservoir or flow out of the wells. In two zones in the study area, the steam comes out at the surface, creating sinkholes, seeps, and steam outlets. These phenomena, called "surface expressions," pose safety hazards, including death.
This study has aggregated a complete list of all reported surface expressions in the study area and their characteristics, showing that surface expressions are more widespread than a single incident. While previous earth models of the area have been proprietary, this study describes the creation of an earth model, based on a geological literature review and synthesis of available data that is now in the public domain.
The highlights from the study are in the spatial statistical analysis. The spatial analysis examines what is unique about the zones with surface expressions that may contribute to their occurrence. It is hypothesized that the surface expressions are caused by leakage of steam through old improperly abandoned wells, high injection pressures, structurally controlled flow patterns, high injection volumes, or flow along naturally occurring faults among other possible factors. Spatial statistical analysis using logistic regression and classification trees is used to explore the relationship between the surface expressions and spatial attributes. The results point to a significant spatial correlation between the surface expressions and two predictors: concentration of plugged wells and seal thickness. These predictors emphasize the possible role of plugged wells in causing surface expressions as well as the importance of having sufficient seal between the producing zone and the surface.
Initial flow simulation results also point in the direction of damaged wells as likely pathways of surface expressions. This research implies that more attention needs to be given to analyzing the abandonment procedures of plugged wells near steam injectors and re-abandoning wells if necessary. In addition, wells that inject fluids at very shallow depth near areas