Workover Well Operations to Restore Full Reservoir Access in an Underground Natural-Gas-Storage Cavern: A Case Study for Developing New Technology in Dual-String Section Milling Methods

Abstract

Both above and under the ground, hydocarbon storage is subject to many stringent compliance standards regarding health, environment, and ground water safety. Various issues—such as zoning regulations, future expansion needs, secondary and tertiary containments, design efficiency for storage and transfer, and financial viability of projects—determine the storage installation method. Underground storage maximizes real estate usage, reduces fire hazards, protects against weather events, and enables storing large amounts of hydrocarbons. The principal methods for underground storage include salt caverns, depleted reservoirs, and acquifers. Each method has its own application advantages and physical properties, such as retention capability and permeability. These factors, along with the economics of site establishment and maintenance as well as deliverability rates, regulate the suitability of that method for particular applications. For example, salt caverns provide very high withdrawal and injection rates relative to their working gas capacity. When the operator locates a suitable salt dome or salt bed, the site is excavated to form a cavern. This excavation process is usually accomplished by pumping or injecting highly pressurized water into the salt layer to dissolve the salt and create a cavern cavity; this method is referred to as cavern leaching or solution mining. After this process, the operator can remove the brine and commence operations.

However, if this leaching phenomenon occurs in existing natural gas storage caverns, the leached area behind the intermediate casing becomes an inaccessible part of the reservoir. To access this area, the lower section of this casing must be removed. Traditional section milling can achieve this; but if the inside diameter (ID) of the inner string is smaller than the outside diameter (OD) of the servicing tool, existing technology cannot be used. Situations such as this require new technology to pass through the restricted ID and open a section in the next-larger-size string.