A Case History of Reservoir Subsidence and Wellbore Damage Management in the South Belridge Diatomite Field
Authors: Dale B.A. , Narahara G.M. , Stevens R.M. 
Published: December 2007

Abstract
Production of fluids from the shallow, thick, and low-strength Diatomite reservoir in the South Belridge field has resulted in reservoir compaction, surface subsidence, and numerous well failures. The most severe subsidence problems occurred while the field was under primary recovery, prior to implementation of waterfloods for pressure support. Consequently, a significant number of wells in the field have incurred casing damage, which proportionately limits the utility of these wellbores for continued production and routine wellwork operations. The nature of casing damage varies significantly depending on the location in the field, however casing shear ("kinks") generally produces the most severe problems in retaining full wellbore utility since it limits tool passage (e.g., packers, scrapers, etc.) and is not economically repairable using conventional milling tools.
Since the mid 1980s, surveillance activities have been in place to monitor and assess the magnitude and progression of surface subsidence, reservoir compaction, and wellbore damage in Section 19 of the South Belridge field. Surveillance activities include the use of ground elevation markers and a variety of production logs — several of which involve novel application and log interpretation. These data subsequently were used to develop and verify hybrid geomechanics and wellbore geometric models for assessing casing damage, identifying well operability and wellbore utility limits, and forecasting remaining wellbore life from which a reservoir management plan and wellbore management operational strategies were established. A key ingredient in the implementation of this plan and operational strategies involved a cooperative tool development with a major oil tool service company to repair casing "kinked" wellbores, as an alternative to re-drilling wells.
This paper provides a case history describing the various synergistic reservoir and wellbore management activities designed to effectively mitigate subsidence-induced operational problems and the accompanying field benefits resulting from their implementation.

Introduction
The South Belridge Field is located in Kern County, California in the San Joaquin Valley. The field is about 45 miles northwest of Bakersfield, California. Exxon operations are located in Section 19 which is nearly in the center of the productive portion of the Diatomite reservoir. Of the 504 acres operated by Exxon (three quarters of Section 19), the productive Diatomite comprise approximately 150 acres.
Reservoir Geology. The Diatomite reservoir is a biogenic siliceous deposit, with a top of reservoir at approximately 1200 ft. The productive interval can be as great as 1800 ft, with the upper 800 ft referred to as the Opal A and the lower 1000 ft referred to as the Opal CT. The Opal CT is distinguishable by a higher degree of diagenesis. The average total porosity for the Opal A is about 58% and for the Opal CT is about 42%. The estimated average liquid permeability for the Opal A and CT is 0.2 md and 0.1 md, respectively. The oil gravity is about 290 API.
The Diatomite zones are overlain with productive Tulare sands. P. 101