Chapter 1 - Stress and Stress Transformation

1.01 Introduction to Geomechanics (14 min.) Sample Lesson
1.02 Stress and Stress Transformation (19 min.) Quiz: 1.02 Stress and Stress Transformation
1.03 Tectonic Stress (14 min.) Quiz: 1.03 Tectonic Stress
1.04 Mohr's Circle in 2D (16 min.) Quiz: 1.04 Mohr's Circle in 2D
1.05 Mohr's Circle in 3D (11 min.) Quiz: 1.05 Mohr's Circle in 3D
1.06a Fault Stress Analysis Examples - Part 1 (12 min.)
1.06b Fault Stress Analysis Examples - Part 2 (10 min.)

Chapter 2 - Strain and Deformation

2.01a Introduction to Strain and Deformation - Part 1 (10 min.) Quiz: 2.01a Introduction to Strain and Deformation - Part 1
2.01b Introduction to Strain and Deformation - Part 2 (10 min.) Quiz: 2.01b Introduction to Strain and Deformation - Part 2
2.02 Calculations for Typical Rock Types (10 min.) Quiz: 2.02 Calculations for Typical Rock Types
2.03 Stress Calculation Example - Duvernay (17 min.)
2.04 Stress Calculation Example - Montney (9 min.)

Introduction to Geomechanics / Chapter 1 - Stress and Stress Transformation

Lesson 1.01 Introduction to Geomechanics

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Transcript

01. Lesson 1.01: Introduction to Geomechanics 02. What is geomechanics?03. Geomechanics in Petroleum Engineering 04. Elements of Geomechanics

01. Lesson 1.01: Introduction to Geomechanics

Hello everyone. My name is Erfan Sarvar Amini and I'm ageomechanics specialist with GLJ. I'm very excited to teach the fundamentals of geomechanics with SAGA. But before I start to dive into the course, I want to give you a quick background about who I am, what I do, and how I was introduced to the field of geomechanics.
So I started my geomechanics career first as an independent consultant, perhaps about 5 - 6 years ago and I had this opportunity to work on a variety of geomechanics problems, from waterflood, CO₂ sequestration, solid waste disposal, and hydraulic fracturing. And my job was primarily to build new mathematical workflows to better understand these processes, probably with more focus on the field applications at the field level. And after that I joined GLJ, and our focus right now has shifted a little bit more toward the frac geomechanics and classical reservoir engineering problems and how to combine them, because I really think that these two go hand in hand and they really work very well together. We all have heard about field integrated studies and how to combine geomechanics with reservoir engineering.
However, by background, I was a reservoir engineer, both undergraduate and graduate degree in reservoir engineering. And as most of us know, reservoir engineers, they do not have a very systematic training in the field of geomechanics. That's probably pretty unfortunate, maybe a little bit in material engineering courses, maybe a little bit in a static/dynamic, but that's pretty much it. So the situation was very much the same for me. So I didn't have a lot of training until I started to do my Ph.D., where I had more opportunities to work on geomechanical-related problems. And because of my background, I primarily focused on developing analytical and semi-analytical solutions for simple frac models. And again, that's hardly related to the field of geomechanics, that was mostly reservoir engineering. So that connection was not very clear for me until I started to do a couple of years of post-doctoral study at the University of Waterloo, where I had more experience and exposure to the classical computational mechanics problems and first hand, I see how other disciplines, for example, civil engineers, mechanical engineers, essentially approached exactly the same problem in a much more rigorous way and in a different way, for example, how they analyzed the stress and the strain and how they model the fractures in all kinds of materials, for example, in composite materials, in concrete, or even in polycrystalline materials and other types of materials and how we can equally use the same type of approach for rocks to build a much better and more rigorous model. But obviously, that required a lot of effort, including a very in-depth understanding of stress and strain analysis or classical elasticity theory and even plasticity theory, but it wasn't really until then that connection became very clear for us.
Therefore as someone who has been on both sides, I clearly see the benefit and advantage for geoscientists, for engineers, petroleum reservoir engineers, to get some training and hopefully that enables us to really understand the weaknesses and strengths of the models. What distinguish bad from good models and also the underlying assumptions and even down the road, to build our own customized model which is mostly suited for our type of problem and I do think that journey should start with fundamentals.

02. What is geomechanics?

Now, let's go back and look at the definition of the geomechanics and some of the applications of the geomechanics and hopefully, that will clarify things a little bit further. So again, like a lot of us including myself, we were introduced to the field of geomechanics primarily because of the boom in the shale industry. So as we all know, operators, they have been able to save, if not billions, millions of dollars through smart utilization of geomechanics, primarily either through smart characterization of the rock masses, for example, to determine the best landing intervals or identify the geomechanical sweet spot to increase the efficiency of hydraulic fracturing or through modeling of hydraulic fracturing, either through a simple model or complex model all of which they have helped the industry to optimize the process or even lower the number of field trial studies because that's essentially something that the industry does as a standard practice to understand this very, very complex process. So what I also have noticed is that geomechanics is becoming even more popular in industry. And there's actually more demand and request for geomechanics, including the integrated studies, and I think the reason is because of the commodity prices. They have motivated a lot of operators to go back and revise their completions to even further optimize this process to make this whole thing more economical. So, we are going to hear a lot about geomechanics down the road in the next few years to come.
But I also want you to know that really geomechanics in shale is only one of many, many applications of geomechanics out there. In fact, geomechanics is a very, very old science and has been out there forever. But let's begin with taking a look at the definition of geomechanics. What's the dictionary definition of the geomechanics? So, geomechanics is a science which consists of 2 terms: the "geo" which means Earth and "mechanics". So, it's the science which studies the mechanics of soil and rock.
So, I could think of primarily 2 main disciplines when I think about geomechanics, for example, soil mechanics and then rock mechanics. So, the soil mechanics is really the inception of geomechanics. So a lot of theories that we know to date and we often regularly use them in other disciplines like given in rock mechanics at petroleum engineering, they all come from the soil mechanics. So geomechanics in soil mechanics primarily deals with the study of mechanics of the soil, either in a small scale, for example, in a laboratory testing, testing soil samples or even in large scale, for example, mitigation of the risk associated with large scale problems such as landslides or mudslides, the picture on the bottom left. So that's soil mechanics. And then we have rock mechanics, which geomechanic mostly deals with characterization or the mechanical characterization of the rock masses and primarily applied in 2 different disciplines. One is geotechnical engineering, for example, for the design of tunnels, mine pit foundations, or even bridges. So really, geomechanics encompasses a big portion of the geomechanic career in geotechnical engineering and geotechnical engineers almost work on geomechanics on a daily basis. So it's not very surprising that a lot of geomechanics specialists, in fact, they come from geotechnical backgrounds, and even they work in the petroleum engineering in the petroleum industry.

03. Geomechanics in Petroleum Engineering

And then, we have rock mechanics in petroleum engineering. Obviously, the application of geomechanics in petroleum engineering is very, very broad. For example, it has been used forever for mitigation of the risks associated with borehole stability, with sand production, for induced seismicity or for CO₂ sequestration, waste disposal, liquid and solid. It has been used in steam-assisted gravity drainage or CSS. And it has also been used, for example, in hydraulic fracturing for process optimization. So really, the application of geomechanics in petroleum engineering is also very, very, very broad. So, of course it's not expected for us to be specialized in all of those disciplines because apparently, they look very, very different.

04. Elements of Geomechanics

So I was very lucky to work across all these rich disciplines, from soil mechanics to petroleum engineering problems. And what I have learned over the years, if you go back and look at all of these applications, despite the really major differences from soil mechanics to geotechnical to petroleum engineering, they typically share 3 common or fundamental elements across. As if we take out that very different nature in the applications, what remains is going to be very, very, very similar. So what are those 3 fundamental geomechanical elements? One is stress, the other one is deformation, and the other one is failure. And I really like to always think of these components or elements as a component of the package, which a geomechanics study has to almost go through in a sequence to be able to complete that geomechanical task. So let's go briefly through each of those elements separately and clarify what they mean.
So let's start first with stress. So stress is probably one of the most fundamental concepts in mechanics. Stress is really nothing but intermolecular forces in material or rock which have resulted from reaction to an external force. Essentially, there's action, there's reaction, and that's the 3rd law of motion. So stress and forces are highly connected. So in geomechanics, it's very, very important to understand the source of external force. And that's essentially what determines and identifies all these applications, from soil mechanic to geotechnical to petroleum engineering problems.
So I have listed 4 top sources of external force that we often encounter in geomechanics. For example, top on that list is body force. So essentially, body force is probably 70% of the source of external forces in geomechanics. What is the body force? It's a force that acts throughout the volume. For example, in the design of tunnels or a bridge or even foundation or stability of salt caverns, the source of external force is body force.
So the second type of external forces are tectonic forces. The tectonic forces are resulted from movements of the plate, for example, when we investigate the stability of the boreholes, the main source of external force are tectonic forces.
And then we have another type, which is the diffusion loading. For example, if you are pumping fluid from a nearby well, the source of external force is diffusion loading. Even hydraulic fracturing, which involves a tremendous volume of liquid and fluid and sand, also is a type of diffusion loading.
The other type, which is the last one, is also change of temperature. For example, in geothermal or in cyclic steamstimulation.
So now, when the materials, they are subjected to external forces, in fact, that's not without consequences. So in fact, what happens is that the materials they start to accumulate the stresses and as a result, they have to change in shape or they have to change their volume. Essentially, that involves transformation from one reference to another and that's called the deformation. Now the way the material reacts to an external force and accumulated stress and deforms, it really depends on the property of the material. For example, the way that metals or rocks, they react to an external force is not really the same as the way that, let's say, a sponge reacts to a given external force. So that really depends upon the property of the material.
So now, as the material, they start to accumulate the strain or deformation then that strain or deformation when it exceeds a certain threshold, then the flaws and cracks, they start to initiate and then grow from a certain point until this failure either becomes large enough so the material will progressively fail or even catastrophically fail. And that encompasses the failure essentially.
So these are the 3 geomechanical elements that we often have to go through, almost like elements of the package in sequence to be able to complete a geomechanical task. So if you are doing a geomechanical study, you have to be able to identify each of those elements separately or maybe sometimes one of the elements has been worked out in the background. However, if you don't see that or it doesn't exist, so we need to question the legitimacy of that geomechanics approach.
So now, what we going to do next, we're going to go through each of those elements separately and we are going to talk about the relevant topics as we see appropriate under each of those categories.