Chapter 1 - Review Of Fracture Mechanics

1.00 - Introduction (11 min.) Sample Lesson
1.01 - Distribution Of Stresses In An Infinite Plate (24 min.) Quiz: 1.01 - Distribution Of Stresses In An Infinite Plate
1.02 - Illustration Of 3D Stress Field (22 min.) Quiz: 1.02 - Illustration Of 3D Stress Field

Chapter 2 - Stress Shadowing in Multi-stage Multi-cluster Hydraulic Fracturing

2.01 - Stress Shadowing In Multicluster Completions - Stage 1 (10 min.) Quiz: 2.01 - Stress Shadowing In Multicluster Completions - Stage 1
2.02 - Duvernay Case Study Example (27 min.)
2.03 - Stress Shadowing In A Multi-Cluster Wellbore: Subsequent Stages (7 min.) Quiz: 2.03 - Stress Shadowing In A Multi-Cluster Wellbore: Subsequent Stages
2.04 - Stage By Stage Stress Shadowing Effects: Stress Escalation Theory (17 min.) Quiz: 2.04 - Stage By Stage Stress Shadowing Effects: Stress Escalation Theory
2.05 - Late Or Dominant Fracture Growth Phase (6 min.)
2.06 - General Stress Escalation Solution (9 min.) Quiz: 2.06 - General Stress Escalation Solution

Chapter 3 - Discussion Of Fracture Half-Length

3.01 - Fracture Propagation In Leakoff & Storage Dominated Regimes (13 min.) Quiz: 3.01 - Fracture Propagation In Leakoff & Storage Dominated Regimes
3.02 - Quantifying Fracture Half-Length & Height In Duvernay Case Study (17 min.) Quiz: 3.02 - Quantifying Fracture Half-Length & Height In Duvernay Case Study
3.03 - Limiting Factors: Rock Heterogeneities Impact On Fracture Non-Uniformity (4 min.)

Fundamentals of Stress Shadowing Effects During Hydraulic Fracturing / Chapter 1 - Review Of Fracture Mechanics

Lesson 1.00 - Introduction

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Transcript

01. Lesson 1.00: Introduction 02. Optimization Process in Unconventional Rocks 03. Subsurface Uncertainties During Optimization of Unconventional Rocks 04. Field Diagnostic for Fracture Geometry

01. Lesson 1.00: Introduction

Hello everyone, and welcome to another series of geomechanics with Saga. My name is Erfan Amini and I'm a geomechanics instructor. So today we want to talk about stress shadowing effects during multi-cluster and multi-stage hydraulic fracturing. So stress shadowing is a phenomenon with profound implications on design and optimization of hydraulic fracturing jobs, essentially. Specifically in the context of multi-cluster and multi-stage frac. Today, we want to talk about the key subsurface drivers of stress shadow effects, impact on fracture geometry and the non-uniformity, and how to potentially control it, increase it, or reduce it for a frac design that aligns with its intended purposes. So we are looking for a deeper understanding of these mechanisms and its implication in optimizing the hydraulic fracturing process. So I recommend this course to everyone who is involved with design and optimization of hydraulic fracturing jobs, or anyone else that is looking to deepen their understanding of hydraulic fracturing and its nuances. So let's get started.

02. Optimization Process in Unconventional Rocks

So I'll start with a quick overview slide here to lay out a context as what we want to achieve by the end of this course. So really this flowchart tries to summarize the subsurface optimization process for a hydraulic fracturing job in unconventional rock. So as we all know the optimization process can be very complex. But in a nutshell, when we're developing unconventional plays we strive to optimize economics. Now the economics may mean different things to different people. That might be different strategies to optimize the economics. But from the subsurface perspective, this economic optimization means if we have, let's say, a block of rock, we want to optimally drain that block of rock at a lower cost. This is really the ultimate goal here. Now, the common way to do that is to drill horizontal wells, probably multi-well, from different benches of geological rock. Then each well will be completed using advanced completion techniques; multi-stage/multi-cluster fracs. So we commonly take sufficient, let's say, x number of stages. And then each stage accommodates from a single cluster up to an appropriate number of clusters that meet them. Frack and operational efficiency, that grows simultaneously during the frac'ing. And then all we need to do, we need to repeat this process for each well within the multi-well pad, either in the form of zipper fracturing (means alternating between stages of different wells or simultaneously or standalone on each well on its own until we are done.
So when it comes to optimization of the frac design, there's two or maybe three major questions that needs to be answered as a part of that process. One is the optimal cluster spacing to adequately drain that hydrocarbon between the clusters. And of course, the appropriate number of clusters per stage that meets the operational efficiency. And the third question is the optimal intra-well and inter-well spacing. So going back to optimizing the cluster spacing, ideally we know the matrix permeability to be precisely themobility. And we set the cluster spacing to adequately drain the rock between the cluster. And then we pick also an appropriate number of clusters per stage to meet the operational and efficiency, which is another economic question. Then ideally we know the fracture geometry and we set the intra-well spacing. So intra-well spacing means two wells within the same geological bench. So we set that intra-well spacing at roughly double the effectivefracture half-length. Sometimes we call it "propped fracture half-length". And then the inter-well spacing between two different benches is basically equal to half of the fracture height or equal to unit of fracture height to leave minimal undrained hydrocarbon between the walls. So now this looks very simple. But in reality, the challenge is that both matrix perm or mobility and the fracture geometries are both unknown and ambiguous and also they are variable along the lateral.

03. Subsurface Uncertainties During Optimization of Unconventional Rocks

So again, like kind of going back to two big subsurface uncertainties during the optimization of unconventional rock. Number one is the matrix permeability, which is the key parameter for optimization of the cluster spacing. Really the reason for that ambiguity is basically do to the extreme tight nature of the rock. So there are multiple ways to measure the permeability. One is the core derived perms. Approaches or methods like MICP or pressure pulse decay, or sometimes using log porosity or resistivity or even a UCS curve. Or techniques like DFIT, RTA, PTA, they are all common to estimate the perm. So now, based on my experience, when the permeability is measured using different approaches, core log or DFIT, PTA or RTA, there are always inconsistencies relating to potentially the problem scale when measured using different approaches. But in operation it's really common to use DFIT, RTA and PTA to infer the matrix permeability. However, in order to do that, we also need the fracture geometry to begin with. So often there is a big range of uncertainty to matrix permeability because of an uncertainty related to the fracture geometry.
So, the matrix perm is one and the other one is hydraulic fracturing geometry. So even when we think about a single cluster, a single stagehydraulic fracture which is initiated from the vertical wellbore, there is uncertainty as to what the fracture shape is, the fracture half-length and the fracture height. And most of that in part is due to the notoriously geomechanically heterogeneous nature of the rock. We saw some of that in the previous course when we talked about construction of the mechanical Earth model. Their heterogeneity on stresses and the pore pressure, rock stiffness and minerals and all that that makes understanding fracture shape, half-lengths and fracture heights pretty, pretty complex. So that's for a simple single stage single cluster from a vertical wellbore. Now when we go to fracturing of a horizontal wellbore, we have an additional layer of complexity on the top of that is associated with the non-uniformity in the fracture geometry, particularly in the context of multi-cluster and multi-stage fracturing. Now, some of those non-uniformities are driven by the impact of heterogeneity along the lateral. That could be related to the cross-cutting. It could be related to the change in depositional area. It could be related to full frac interaction. It could be related to depletion effects. That all would lead to non-uniformity in the fracture geometry.
The other driver itself, excluding all the heterogeneities, is stress shadowing effects. The intra-stage between simultaneous propagation of the fracture within a particular stage. Or inter-stage, the effects from the previous fractures on the subsequent stages, and also the well-to-well shadow effect. So today we really want to focus on the stress shadowing effects, intra-stage, inter-stage and well-to-well shadows. Now, really something to note here, that matrix permeability is basically outside of our control. So the rocks are either good or bad in terms of reservoir quality. But hydraulic fracturing geometry, to some extent, is under the operators control. So it's really important for us to understand the drivers of those non-uniformities and potentially the variability along the horizontal wellbore.

04. Field Diagnostic for Fracture Geometry

So field diagnostics have been very instrumental to understand the behavior of hydraulic fracturing geometry. Specifically, some of the modern field surveillance techniques like cross-well and involve fiber optics, for example the permanent distributed strain (DSS) or acoustic DAS or even DTS, they have been all helpful to understand the behavior of the hydraulic fractures, especially at the individual cluster or sleeve. So obviously we have come a very long way since the advent of these technologies. So one big issue with some of these advanced and modern field surveillance techniques, particularly the fibers, that they are, the permanent ones especially, they are very, very expensive. So the operators, they often deploy the cheaper options such as temporary fiber, like wireline fiber or disposable fiber or techniques, pressure monitoring techniques like sealed wellbore pressure monitoring. So in all of these alternative solutions, the cheaper option, a key element or the key deliverable is to monitor a volume to first response. So what is the volume to first response? So if we are frac'ing one stage here. So the volume of the fluid pumped here in a treatment well in a stage when the response is seen at an observation well. So basically, that is a volume to first response. And the volume to first to response is used to infer the fracture geometry, like fracturehalf-lengths, from the VFR. So all of this alternative solutions with a key deliverable as to volume to first response, they give insights into the growth of dominant fractures at the stage level. And not so much on the morphology at the cluster level behavior of those individual fracture sleeves. So really, one of the objectives of this course is to provide insight about the impact of a stress shadowing on the fracture geometry non-uniformity, and also specifically the fracture morphology at the cluster level, including the key drivers such as cluster count, cluster spacing, all of that using quick and robust analytical approaches that are repeatable, they're easy to execute with really minimal efforts and investments.
So here's the end of this lesson, and we are going to carry on with some fundamentals in the next lesson.