Mature Fractured Reservoirs Can Be Repurposed for Gas Storage
Authors: Carpenter Chris 
Published: March 2019

This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper SPE 190134, “Securing California Gas Supplies by Storage in Mature Fractured Monterey Reservoirs Offshore California,” by Iraj Ershaghi, SPE, Donald L. Paul, SPE, and Saran Kaba, SPE, University of Southern California, prepared for the 2018 SPE Western Regional Meeting, Garden Grove, California, USA, 22–27 April. The paper has not been peer reviewed.
This paper discusses studies conducted on two California offshore fields that may be abandoned in the near future. These studies examined the feasibility of repurposing these fields for offshore gas storage by using their reservoir voidage and existing pipeline facilities. These storage sites could offer a significant alternative to the current onshore sites located in highly populated urban areas of the state.
Introduction
California has been a major oil-producing state since the turn of the 20th century. However, in terms of current natural gas production, California’s indigenous production meets only approximately 10% of state demand.
Gas imports are delivered on interstate pipelines from sources in the southwestern and Rocky Mountain regions of the US and western Canada. The majority of gas delivered to California is consumed directly or pumped into onshore storage fields. California currently has 14 onshore gas-storage fields used to stabilize the supply system. The total current onshore storage capacity is approximately 600 Bcf, with the working capacity being approximately 375 Bcf.
Background and Field Characteristics
A year-long study of issues related to gas-storage fields in California concluded that significant investment would be needed for additional pipelines and gas-compression systems if all onshore storage facilities were to be shut down. While new regulations are in place to reduce the risk of current onshore storage fields, alternatives need to be considered that would be reliable for expanded demand. The study concluded that there is no method for storing electricity that can replace the need for gas to meet the winter peak. (Note that approximately 50% of gas is for nonpower uses.) Uncertainties are also associated with renewable sources as a substitute for gas for generating electricity. The study indicated that Californian consumption of natural gas currently is approximately 6.6 Bcf/D. However, the study points out that there may be a drop in demand for natural gas. Several issues loom on the horizon, one being the increasing liability associated with operating gas-storage fields in urban areas. Another is that, if some or all these onshore sites are decommissioned, insufficient storage capacity exists to address increased use of natural gas in California.
An alternative exists in the form of offshore storage fields. Semidepleted offshore fields present an opportunity for this purpose. Some of these oil-producing operations may be reaching the abandonment stage during the next decade. Total abandonments may result in the removal of offshore platforms and plugging of existing wells, denying Californians further access to nearby offshore reservoirs.