Hydraulic Fracturing – Geomechanics

1 Introduction to Hydraulic Fracturing – Geomechanics

HYDRAULIC FRACTURING – GEOMECHANICS: Fracking, as a technique, was born in the 1830s, with the use of acids to fracture the reservoir rock. In 1947, studies to change acids for water began; in 1960, the use of dynamite in its base state, nitroglycerin, was being tested on the coast of the United States. However, it was not until 1998 that hydraulic fracturing began to be used commercially, thanks to George P. Mitchell, the father of the fracking technology.

A study conducted by the IEA states that thanks to technologies such as fracking, by 2019, the United States will produce more oil than the largest producer in the world, Saudi Arabia. In 2020, that will be a fact. Despite the production benefits offered by hydraulic fracturing, whether for fracturing source or reservoir rocks, the technique’s difficulty generates high implementation costs. Because of this problem, new alternatives are sought to face the heterogeneity of the formations.

Geomechanics is a branch of geology that is shown as one of the scientific alternatives to decipher formations’ behaviour when the fracking technique is practised. Below is a detailed explanation of how geomechanics can be an essential ally when discussing hydraulic stimulation.

2 Hydraulic Fracturing – Geomechanics: Fundamentals of Fracking

Hydraulic stimulation consists of injecting a highly-viscous fracturing fluid at a pressure above the pressure of the fractured formation, intending to change the skin factor and looking for negative values. That is to say, to generate flow channels or fractures around the drainage area, placing elements in the generated channels in a packaging form that help the fracture take longer to close and generate sufficient permeability for fluid passage.

During the process, the fundamental concepts of the operation must be taken into account, which are:

• Rupture pressure is the point at which the formation fails and breaks; this point is above the rock’s elastic capacity.
• Pumping pressure: It is the pressure necessary to extend the fracture, keeping the good flow rate constant.
• Instant closure pressure (ICP): This pressure can be read when the fluid injection stops, and all friction pressures disappear; at that time, the formation returns a part of the fluid pumped to it at a certain pressure: the ICP.
• Well flow rate: It refers to the pumping time represented in the total volume of the fluid. This allows directly inferring the size of the fracture created, and it also allows identifying the number of additives, proppants, and control fluid used.

3 Fundamentals of Geomechanics

Geomechanics is closely related to the main currents of geology, classical mechanics, and mechanics of materials. Despite being widely used in the mining and civil engineering sectors, geomechanics is positioned as an excellent ally for operations in the oil and gas industry.

It is based on the calculation of the stress field in equilibrium in the subsoil. It expresses the maximum stress, the minimum stress, the overburden stress, and the shear and sliding stresses.

Soils behave under the action of loads as elastic materials, in some cases, greater deformations than the regular ones are produced, having to turn to calculations that take plasticity into account. For the oil and gas industry, it is crucial to understand the nature of the shear strength to create a simulation or prediction of the different problems that may arise under certain types of operations.

4 Geomechanical Model

The geomechanical model explicitly describes data relevant to the construction and production of oil wells; these include pore pressure, the state of the stresses, mechanical properties, and the strength of the rock for the total stratigraphic section penetrated by the wellbore.

Today, different methodologies are used to construct a geomechanical model. One of the most used is the one by geomechanical engineer Marcelo Frydman. In his methodology, he details processes such as:

Additionally, a geomechanical model can be calculated in two ways: a dynamic model, where all the calculated resources depend on indirect data, such as records or area iterations, to create data simulations, and static models, which are characterized by using core samples and direct data from the formation.

5 Geomechanics, an Ally for Fracking

Geomechanics is shown as an ally of high importance when facing the heterogeneous challenge that hydraulic fracturing represents. Each reservoir is a new world for fracture specialists. Every wellbore is a new adventure since the conditions of pressure, temperature, rock strength, and elastic rock do not necessarily maintain homogeneous behaviour. Due to these factors, using data from constructing a geomechanical model is a high-impact ally.

5.1 Data Audit

During geomechanical modelling, an inspection of the behaviour of the reservoir is performed, analyzing each wellbore. During this process, possible operational problems related to borehole stability are identified, as in turn, it is identified if the wellbore was fractured.

If the wellbore was fractured, data such as fracture pressure, type of fluid used, type of control fluid, and proppants used to maintain the fracture are detailed, as well as the closing pressure and well flow rate that the wellbore registered. The data also analyze the functionality of the fracture over time and how much the wellbore’s skin factor was achieved after the process. These data provide a background of the reservoir events.

5.2 Framework Model

The stratigraphic framework helps to understand how old the rock is, what type of rock it is, and where the work will take place. For example, rocks from Eocene ages are characterized as hard rocks and have a high degree of compaction. It is understood that the deeper, the more compact and resistant type of rock is expected in theory.

5.3 Mechanical Stratigraphy

One of the processes practised within stratigraphic mechanics is taking petrographic samples. This type of task is performed using direct core samples, which are analyzed using a petrographic microscope. This direct analysis allows going beyond a wellbore’s electrical or special records. They provide knowledge of factors such as:

• Grain selection: This parameter refers to the type of grain that predominates in the formation, the types of grains are classified into:
  • Very good: In this section, the grains are uniform, maintaining shape, size, and composition.
  • Good: Unlike the previous one, good grain differs in one of the scenarios. It can have a good composition and size, but it varies in shape.
  • Moderate: It is the balanced point of grain types. In this section, a transactional shock is commonly observed when the study area is close to an age change.
  • Poor: The poor grain is characterized by meeting one of the characteristics: size, shape, or composition, but it does not have a positive behaviour in one of the other two conditions.
  • Very poor: It is characterized by a heterogeneous composition. It is common to see it in formations that have undergone migrations or are still young in depth.

The types moderate, poor, and very poor are optimal for good fracking because they reflect less opposition to the fracture. In turn, their deformities help the adaptation of flow channels.

• Differences of porosities: With petrographic analyses, we obtain a 2D view of the flow channels that are in the formation, this type of porosity is punctual and not effective, but to have a variety of samples along the producing area of the oil well, similarities are noticed depth traces, this helps to choose more functional stages when practising fracking, and in turn, it allows predicting which area will have a better behaviour before hydraulic stimulation.
• Contact between grains: the type of contact between grains is a property rooted at the time the sediments were deposited in the area, there are different types of contact:
  • Sutured: It refers to the roundest grains in the area.
  • Concave/convex: This type of grain tends to have half-moon shapes.
  • Longitudinal: These grains are characterized by having a linear formation longer than a wide one.
  • Tangential/punctual: These grains have the main characteristic of having one part rounded and the other smooth.
  • Floating grain: This type of contact is characterized by being away from other grains. It is attached to the cementing material of the rock and is surrounded by it.

For fracking processes, knowing the type of grain helps in deciding the proppants that should be used to avoid slipping processes, size, and shape. The use of proppants of a much larger size than those of the formation would create spaces in the pore throats created with the fracture. It would facilitate the detachment of the grains of the formation by the effect of fluid friction, occupying the spaces created by the proppants, thus creating slipping over time.

5.4  Direction and Magnitude of STRESSES

The stresses operating in the sub-soil are divided into three main stresses: the overburden stress (Sv), the maximum horizontal stress (SH), and the minimum horizontal stress (Sh):

• Sv > SH > Sh: Normal regime
• SH > Sv > Sh: Transcurrent regime
• SH > Sh > Sv: Reverse regime

In turn, horizontal stresses have preferred routes in the subsoil; this translates into directions of preferential flow, porosity, and natural fractures in the study’s formation. They are opposite and obtained by the practice of 6-arm or so oriented calliper records, image logs, or sonic-oriented records, which allow noticing fractures in the formations.

This type of information allows the fracture specialist to determine the optimal direction of fracture. In a normal regime, the recommendation is to direct the fracture towards the maximum stress; only the opposition of the fracture’s minimum effect will have to be kept open.

On the other hand, the correct selection of the hand direction of the stress regime avoids tortuosity in the pore throats; this type of problem prevents the passage of proppant towards the fracture and, therefore, its functionality.

5.5 Overburden Stress

The overburden parameter is the weight generated by the layers of formations on the target formation being studied. It is obtained using density registers (RHOB), and Gardner’s equation is subsequently applied. An overburden pressure reading is obtained at each point read by the register, considering the existence of shale.

For hydraulic fracturing, the knowledge of an abnormal overburden pressure is crucial in selecting the strength of the proppant. Fracturing studies conducted in Venezuela and Argentina demonstrate an abnormal behaviour of the stresses, with the overburden stress (SV) equal to or greater than the maximum stress (SH).

Therefore, it is established as an abnormal stress regime. If the fracture is planted in a horizontal area, the overburden stress directly affects the closure of the formation; therefore, it affects the proppant’s strength pressure. In some cases, crushing effects have been evidenced on proppants that degrade their size and become part of the production.

5.6  Rock Strength and Elastic Properties

To obtain the results on the properties of the rock, dynamic tests are carried out, which are sonic logs (DOC, DT), depending on the travel times that the sound waves take and using software such as Drillworks or Skua Geolog. Profiles of mechanical properties of the rock are obtained, both elastic and resistance. These programs are based on the formulas of Horner, Eaton, and Bowers.

On the other hand, there are tests of sample destruction. These are based on the sampling of the nuclei. After tests are performed, such as:

• Unconfined pressure test (UCS): In this test, the sample is pressed vertically and unconfined, and the maximum pressure that the rock resists is read. With this test, the Young modulus, which is the elasticity of the rock, is calculated.
• Brazilian test: It consists of subjecting a cylindrical sample to diametrical compression, applying a load uniformly along two lines or opposite generatrices until the rupture is reached. These tests obtain the strength of the formation, which helps to calculate the compaction of the rock.
• Triaxial compression tests (TRX): In this test, a rock cylinder is compressed under constant containment pressure until it reaches its maximum strength. Traditionally, it is measured for a containment pressure given the maximum strength, i.e., the conditions to which the rock was in the sub-soil. We obtain the Young modulus, Poisson ratio, and Biot coefficient from this test.

Knowing the rock’s mechanical properties, it is possible to calculate the magnitude of the area’s main horizontal stresses. The minimum stress translates into the pressure required to open the rock with the fracture. To achieve a correct propagation of the fracking technique, it is necessary to know the magnitude of the maximum stress since this value is the rock’s maximum strength. After this point, the rock is no longer elastic. The fundamental objective of the fracture is achieved: opening the rock.

5.7  Failure Analysis

One of the most used failure analyses in the oil industry is the Mohr-Coulomb equation; this type of test takes factors such as Biot coefficient, Young’s and Poisson’s modules, UCS values, and TRX values. To determine the angle of internal friction and the values of the rock’s cohesion, the fracture specialist can estimate the values of collapse pressure and fracture gradient along the rock, either in a vertical or horizontal plane.

Knowing the rock’s cohesion value and degree of compaction allows the fracture specialist to establish an adequate type of fracturing and control fluid. Hard rock does not necessarily contain characteristics of high cohesion, so using a highly viscous fluid can cause problems if this characteristic is not known.

6 Hydraulic Fracturing – Geomechanics: Conclusion

Fracking technology is not a failure-safe process to full functionality. Despite this, the use of the geomechanics discipline establishes a clear view of the properties and features that the fracturing fluid will face, as well as the proppant, which will be added to provide a lasting effect on this process. A profound understanding of the rock is and always will be necessary.

7 References

• International Energy Agency, 2019, “The United States to lead global oil supply growth, while no peak in oil demand insight”,

Extracted from: https://www.iea.org/news/united-states-to-lead-global-oil-supply-growth-while-no-peak-in-oil-demand-in-sight

• Andres Ocando, 2020, “Geomechanics, the Nemesis of the NPT”

Extracted from: https://oilmanmagazine.com/article/geomechanics-the-nemesis-of-the-npt/

• Wafa Al-Kattan and N. Jasim Al-Ameri, 2012, Estimation of the Rock Mechanical Properties Using Conventional Log Data in North Rumaila Field.
• Ocando A, Osorio J, 2016, “Geomechanical characterization of the deposit B2-X-68, for fracture optimization. Lake Lagunillas, North Lake”

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