Management of Sour Gas by Underground Injection - Assessment, Challenges and Recommendations

Abstract

Many of the world's Mega-fields (> 1 billion barrels of reserves) contain sour gas, a blend of natural gas and hydrogen sulfide (H₂S), either alone or in combination with carbon dioxide (CO₂). H₂S gas is extremely toxic, the combination of H₂S and CO₂ (Acid Gas - AG), can be highly corrosive, the elemental sulphur reacts with water to form acid rain, and CO₂is now recognized as a significant green house gas. Where there is a demand for the natural gas, and capacity to separate the components, the H₂S and CO₂ can be separated out. However, these components must be managed in a cost-effect way and according to regulatory requirements to maximize recovery of hydrocarbons and minimize AG safety and environmental impacts. To date, the CO₂ components have typically been vented to the atmosphere, and sulphur has been produced for industrial uses. Novel step changes are needed to handle the large sour gas volumes to be produced by the mega-fields under development in the Caspian Sea and Middle East regions

Underground injection and sequestration of E&P associated streams(such as produced water, drill cuttings slurries, and production/completion return fluids) have been practiced for many years by the industry; sour gas injection is a relatively new technology. This paper reviews comparative economic and risk analyses of various H₂S management options available to developing middle East and Caspian fields, considering:the impact on recovery (including rate, ultimate total volume, engineering capacity), the health and safety risks, impact on environment, reputation consequences, implementation (including construction and operation complexity), and regulatory Issues (based on current North American standards, recent judicial actions, and Kyoto Protocols). Emphasis is given to H₂S gas injection in improved oil recovery (IOR) projects/schemes because:

• It is an environmentally sound solution that manages both H₂S and other green house gases.

• It provides for permanent, reliable storage and eliminates current taxation (in foreign countries) or future liability associated with emissions and/or surface storage of sulphur. For example, one operator in the Caspian Sea was recently fined several million dollars US because of sulphur storage above ground.

• Compared with other options, H₂S injection can reduce acid production, dust generation and avoid adverse impact on agriculture and aquaculture, thus it improve the operator's image and reputation with local community.

• It is the easiest sulphur-handling method to implement, as it uses established technologies, without requiring sulphur-separation plant and equipment.

• H₂S mixture has better sweeping efficiency than CO₂ or sweet gas alone, therefore H₂S injection may increase recoverable hydrocarbon in an EOR or IOR schemes.

• It has the opportunity for favorable economics, both in terms of operating cost (lower energy, no storage operations) and by improving production through use in IOR schemes.

Introduction

Recent activities in the Caspian Sea and the Middle East deal with reservoirs that typically contain oil, water and high concentrations of sour gas. The successful production of the oil will require careful management of surface resources (land, water, and power), subsurface resources (hydrocarbons), and E&P associated streams (produced water, non-saleable products, and other wastes). To meet regulatory requirements and satisfy public expectations on waste emissions, the key elements of a robust associated stream management strategy should achieve the following principles:

1. Maintain the integrity of safety in the operations by minimizing the risks to human health and environmental damage (PROTECT)

2. Maximize value of the development through informed choices (PRESERVE)

3. Convert waste into resource (REUSE/RECYCLE)

4. Minimize the production of waste (REDUCE)