Chapter 1
Acid Gas Injection: Engineering Steady State in a Dynamic World

Jim Maddocks

Gas Liquids Engineering Ltd. Calgary, Alberta, Canada

Corresponding author: jmaddocks@gasliquids.com

Abstract

Acid gas injection (AGI), while widely used throughout the energy industry, still has significant learning and development opportunities, particularly as flows and pressures increase, and design engineers begin to stretch the limits of conventional AGI design thinking.

This paper and presentation is intended to provide some background and thoughts on the nature of acid gas injection, the uncertainties, the possibilities, and the myriad of issues that can be present during a typical design cycle. With a significant number of rapidly changing inter-related process variables, the engineering/design team needs to consider acid gas injection as a very transient and dynamic process. Adapting the design process and altering the way we think about, approach, design, fabricate and finally operate acid gas injection systems requires some innovative critical thinking skills, some learning on the custom nature of the systems involved, and finally the realization that the AGI system will ultimately have to adapt to the environment.

Keywords: Acid gas injection, compression, water content

1.1 Introduction

Acid gas is composed of a mixture of H2S and/or CO2 and usually water vapour. Acid gas, a byproduct of gas treating systems, is often considered to be a simplistic binary mixture of H2S and CO2. There are frequently other contaminants including methane, BTEXs, amine, and other hydrocarbon components. Carbon capture streams are typically pure CO2 although there are other contaminants co-captured with the carbon dioxide.

The very nature of acid gas compression and injection means that these AGI "systems" are nothing more than the glorified "garbage trucks" of gas processing. The design team has a very limited ability to manage the inputs to the process, a very limited ability to control the outputs, and the system barely even gets to talk to the upstream "garbage generator". Basically, it's desirable that this "garbage truck" simply do its job, quietly, efficiently, and as cheaply/painlessly as possible with minimal operator intervention and low maintenance.

1.2 Steady-State Processes

As engineers, we're trained to understand, develop, and then control steady-state processes. We do this hundreds of thousands of times with pumps, compressors, turbines, heaters, coolers, and a multitude of other processes. This is one way of simplifying a design problem. In many cases, we don't know anything other than steady-state and we proceed blindly assuming that the process will operate in "steady-state". Nothing could be further from the truth.

Every process, every moment, and every single second of our day is filled with processes in constant transition. Everything from our furnace, to our cars, to the internet, is filled with constant change. Nothing is steady-state. The temperature of your house is constantly changing even with the best "smart" thermostat. The speed of your car is always changing. Your heart rate goes up and down according to external and internal stimuli, energy and forces. Even a simple system like cruise control on your vehicle is constantly experiencing micro-changes in inputs (like hills, curves, vehicle mass, and wind) and the system control output is expected to compensate for all these changes. It does this (usually seamlessly) and the user seldom notices the minor variations in speed.

Every part of our world is constantly in a state of dynamic transition. The weather is never constant; our ambient is always warming or cooling slightly, moisture content is changing, and the system is adapting. The belief in steady-state performance is a misnomer.

In order to understand and evaluate the dynamic nature of this process, it's important to establish a different way of understanding this process.

Part of this learning process involves the use of "critical thinking skills". These skills are defined as:

Critical thinking is that mode of thinking - about any subject, content, or problem - in which the thinker improves the quality of his or her thinking by skillfully analyzing, assessing, and reconstructing it. Critical thinking is self-directed, self-disciplined, self-monitored, and self-corrective thinking. It presupposes assent to rigorous standards of excellence and mindful command of their use. It entails effective communication and problem-solving abilities, as well as a commitment to overcome our native egocentrism and sociocentrism [1].

This style of thinking forces us to reconsider our basic assumptions in a design process - it's an essential tool in the design of an acid gas system.

As engineers and designers, we're expected to make steady-state approximations because they allow us to essentially draw a design line in the sand. Obviously, if a client stated that the compressor would see a flow varying from 30 to 60 e3m3/day of acid gas, we'd likely establish a "steady-state" design flow condition of 60 e3m3/day and then find a way to adapt to the low flow case. However, even recognizing that this high flow case is unsteady and dynamic, there are dozens of other non-steady-state variables in the system. Many are unnoticeable, some actually cancel each other out, and finally, some compound themselves into potentially serious process issues. Many of these changes take place over seconds, hours, days, or even months as the machines and processes begin to experience wear. Even the frequency of the change itself is changing. In order to fully understand, design, and control a process, we have to know that our understanding of steady-state is at best, a semi-educated guess.

1.3 Basic Process Requirements

1.3.1 Process Needs

The acid gas streams are often captured at low pressure (40-80 kPa[g]) from either a gas treating facility or a carbon capture system. Carbon dioxide (or CO2) gathered from EOR systems may be captured at moderately higher pressures (170 kPa[g]) and pure makeup CO2 supply pressures are often higher. As the AGI equipment and injection process is often downstream of many other larger process units, the acid gas system is expected to handle everything extracted in the amine unit or recovered from the reservoir. This means that flows, composition, temperature, and often pressures are highly variable and can change quickly without notice (and often without apparent reason). In order to prevent process upsets, shutdowns, and potentially regulatory non-compliance, it's important that the acid gas injection system be able to adapt quickly (and with stability) to the changes.

1.4 Process Input Variabilities

1.4.1 Compositional Variances

While an acid gas injection system is usually designed as a steady-state process, it is far from a steady-state operation. Typically, the EPC engineering team requests a primary plant feedstock analysis from a different business unit within the owner's company. In many cases, this feedstock is a known parameter and the owner company has a high degree of confidence in the analysis. However, in many newer developments, particularly in shale gas and tight gas developments, the field or reservoir has insufficient flowing history. In some cases, the Owner development engineering teams "take their best shot" at the anticipated composition. In order to provide for maximum regulatory and design flexibility, these teams often inflate or exaggerate the H2S fraction in the feed gas to reduce the capital risk of under-design. In many cases, they aren't aware of the risk of having a large design allowance. The end result is that the composition of feed gas to the facility is often dramatically different than anticipated. As the field development matures, the composition can also evolve - in some cases, this may mean:

• A changing hydrocarbon content with altered gas dew points and heavy components like C5+;
• H2S and CO2 can vary in either direction - sometimes rapidly;
• New and unexpected components may appear including oxygen, elemental sulphur, mercaptans, toluene, benzenes, and COS.

In some cases, the feedstock may contain other unexpected contaminants. Various wellhead treating chemicals and production chemicals like wax dispersants, sulphur solvents, triazine, hydrate inhibitors including methanol, and asphaltenes solvents can be troublesome for amine plants and can trigger operational upsets and foaming events. These foaming events are often random and unpredictable and can:

• Alter the pickup of H2S and CO2 by shifting system kinetics and mass transfer
• Potentially alter the co-adsorption of hydrocarbons
• Result in significant (and rapid) flow variances as the foam breaks and re-forms

Often, these changes are simultaneous and rapid, making the prediction of acid gas compressor steady-state feedstock a challenging issue.

Redeployment of facilities often means that the industry is installing equipment in a service that may not be an ideal fit for the system. In other cases, the owner companies may need to significantly customize or even swap the solvent in the process. These seemingly minor process changes can have a dramatic effect on how the system performs.

In the natural gas midstreaming industry,...