Mitigating Well Failure Due to Shear through the Analysis of Elevation Monitoring Data Using ArcGIS Spatial Analyst, Hill Property, South Belridge Field, California

Ground subsidence is a key issue impacting wellbore utility of the South Belridge Diatomite. The highly porous formation is developed with a line-drive waterflood program for voidage replacement and subsidence mitigation. Subsidence is the primary cause of well failure.

Traditionally, total subsidence is used to predict well failure focusing on compressional strain in the area of greatest subsidence. However, shear, the differential between one area of subsidence and the next, is best correlated to well failure. These shear forces cause kinks in the wellbore, forcing the operator to use smaller tubing strings until the well must be abandoned.

In an effort to monitor subsidence risk, operators contract surveyors to observe the monthly changes in elevation throughout their fields to determine where the areas of greatest subsidence occur. These reports are output as a ΔZ value over a specified time interval. Displayed as a grid, these reports show areas of high subsidence, indicating potential vertical compressional failures but do not reveal any indication of shear.

In order to approximate potential shear, and thus, areas where well failures might be greatest, we have developed a method which processes a ΔZ grid (subsidence grid) using Spatial Analyst extension in ArcGIS. The ‘slope’ tool in the software package analyzes each individual grid cell value, returning values based on the magnitude of the difference between the cells surrounding it. Using this tool on a subsidence grid returns values that reflect the severity of the difference of subsidence from one portion of the field to another.

The subsidence grid is also analyzed using the Spatial Analyst ‘aspect’ tool. This tool analyzes individual grid cells and their neighbors to determine directionality. When performed on the subsidence grid, the direction that the shear is occurring is revealed.

By combining the magnitude calculated in the slope tool with the direction determined by the aspect tool, a ‘shear vector’ map can be created. This data can be displayed in ArcGIS as arrows which increase in size with increasing slope while the azimuth of the arrow shows direction.

This methodology has provided an enhanced visual approach to identify voidage replacement inadequacies. The additional information has directly assisted surveillance engineers and geoscientists to set appropriate injection targets resulting in improved well failure statistics and field performance.

AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California