In the first part of “Introduction to hydraulic fracturing" we've developed: What is hydraulic fracturing?, Which is the operation principle?, Operation of hydraulic fracturing, What form and direction can take the fracture?, Fracture orientation, Hydraulic fracture design, Selection between vertical, horizontal and multi-wells pad and Fracture fluid. Now we're going to develop:
The proppant is a component of main importance in this kind of treatment since it's responsible for maintaining the fracture open and provide a path of high conductivity for that the hydrocarbons flow inside of the well (wellbore). As result its correct choice is decisive for the well productivity in the long and short term. Currently there are a great variety of proppants being the sand the most used due to its low cost and abundance, but according to the features of the formation and depth of the same can also be used resin coated sand and special ceramics .
2- Grain size: The grain size is linked to the permeability resulting of the fracture, to greater grain size greater permeability but greater difficulties for transport it and greater possibilities of crushing it.
3-Sphericity: the sphericity improves the distribution of loads and stresses.
4-Quality: The quality is measured in terms of the impurity degree of the proppant, increasing their quality by decreasing the impurities degree, as these impact negatively on the fracture conductivity.
5-Proppant density: low density proppant facilitates its transport toward the inside of the fracture.
Figure 1: Typicall sand used in hydraulic fracturing treatment as a proppant.
As the proppant properties improve by increasing the price, for its choice is made an analysis of costs and benefits searching a compromise solution between the investment required and the technical performance of the same.
Shown below is the proppant pyramid of conductivity.
Figure2: Proppant conductivity pyramid, source: Saldungaray and Palisch 2012.
Today is common use of 30/50 and 40/70 sand and resin-coated sand, 40/80 ceramics, and 100-mesh sands of various gradations.
The surface equipment for the hydraulic fracturing is formed by a large number of equipments that are used intensively during a period of time from hours to months depending on the number of fractures to make and wells to stimulate.
This is constituted by: acidification unit, chemical additive unit, fracturing blenders, fracturing pumps, hydration systems, pumps for energized, iron truck, Data Acquisition & Control Centers (VAN), manifolds, wellhead and several equipment of transport, storage and delivery of proppant.
Figure 3: Hydraulic fracturing equipment, source: Michigan Department of Environmental Quality.
Manifolds: the manifolds used are typically 7-1/16", 5-1/8", 4-1/16" of diameters with connections flanged and maximum work pressures of 15000psi (usually).
Wellhead: is the tool through which the fluid enters to the well and is connected to the outputs of high pressure pumps. It's designed to withstand high pressures that are determined in advance for each treatment.
Chemical Additive Unit: a chemical additive unit to hold and deliver each chemical in controllable quantities in order to blend the fracturing fluid.
Hydration unit: the hydration unit is responsible of hydrating the polymers and chemicals before being sent to the blender. It generally has a residence time of five minutes.
Figure 4: Hydration unit, source: Trican Well Service, www.tricanwellservice.com
Fracturing Blender: Is the responsible of mix the chemicals, polymers and the proppant with the fracture fluid in specific quantities before being sent to the high-pressure pumps. In order to optimize its performance in large operations are used two blenders working in parallel.
Figure 5: Slurry Blender, source: Trican Well Service, www.tricanwellservice.com
Fracturing pumps: to the hydraulic fracturing is used pumps of high pressure and high flow rate, normally triplex or quintuplex, with power ranging from the 1300bhp to 2000bhp each one, being normal to use between 10 to 20 pumps during the fracture operation reaching pressures of 15000 PSI and flow rates greater than 100 barrels per minute.
Figure 6: Fracturing pump, source: Trican Well Service, www.tricanwellservice.com
Iron truck: used for transporting and rigging-up the high-pressure lines or “iron” that use manifolds to connect the various components of the fracturing spread and wellhead
Data acquisition & control centers (VAN): the unit of data acquisition and control center has modern computers from where it is monitored, recorded, controlled and change all the parameters of the operation in each phase of the fracture treatment.
Figure 7: Data acquisition & control center, source: Trican Well Service, www.tricanwellservice.com
Due to the importance of fracture treatment and its high cost the operation is monitored in real-time to ensure its successful realization.
The parameters monitored are those who determine and allow modify the dimension, extension and geometry of the fracture produced. Among them are the viscosity of the fracturing fluid, proppant concentration, proppant rate, slurry rate, fluid rate and surface injection pressure.
Figure 8: operators working during a treatment inside de VAN.
The values of these parameters have been calculated previously during the fracture design and any significant variation of these variables during the operation with the calculated previously must be analyzed and corrected immediately if it necessary.
Figure 9: Typical graph of Slurry rate, Blender Density and Treatment Pressure vs. time during an stage of hydraulic fracturing.
Together with them are often carried out three-dimensional maps of seismic induced or micro frac that allow to have an immediate view of the fracture growth in the formation helping to control, run and optimize the process.
The seismicity induced is any vibration of the Earth generated by the activity human. When hydraulic fracturing starts the rock breaks and releases a small amount of energy that can be registered by ultrasensitive geophones located in a nearby well (monitoring well) and be processed by specialized software to determine the place and the exact depth in which was generated such release of energy.
Figure 10: microseismic monitoring of a hydraulic fracture stimulation. Image courtesy of John Logel.
The growth and extension of the fracture continuous releasing small amounts of energy allowing the software to calculate the azimuth, vertical extension and complexity that is taking the fracture. This information is shown in real time to operators through a three-dimensional map. This helps to control in optimal way the operation and avoid that the fracture is spread out of the interest formation.
All this makes of vital importance to use the induced seismicity in new shale plays during the hydraulic fracturing, allowing know the real behavior of the fracture and calibrating the geomechanics parameters of the software with the information obtained. The obtaining of such parameters together with the software calibration help to understand better the behavior of the formation allowing design the following fractures with greater precision.
Figure 11: three-dimensional map obtained by induced seismicity during a hydraulic fracturing. Source: ESG Solutions.
There are basically two possible ways to contaminate an aquifer and these are by filtering of chemicals spilled from the surface (it allows to be remedied before reaching the aquifer) and/or by the introduction of fluids or contaminants from the same aquifer (fast pollution of high risk) either by through an extraction well of fresh water or by a well that goes to a greater depth through the aquifer without having the correct insulation with the same.
In the first case to protect the aquifer of drilling fluids and fracture fluids spilled from the surface these are treated under procedures established in the legal regulations in force in each country that clearly set out that way must be handled the same, the tools that should be used and the contingency actions to carry out in the event of a possible spill.
Figure 12: Scheme showing how the groundwater is contaminated in the real life. Source: U.S.E.P.A.
Before the fracture process starts the water stored in the location is fresh water so that a possible spill of the same would not produce pollution whatsoever, later when the fracture treatment has finished part of the pumped water is returned from the fractures to the surface with dissolved salts, hydrocarbons and in some cases with heavy metals. From this moment the processing and handling of the same requires greater attention and for its final disposal the legal regulations require to follow one of these procedures: be treated and reused in future fractures (APACHE has obtained good results reducing the volume of fresh water used and the operation costs, saving near $1.15 billion in the long term) or in the treatment of secondary recovery, be treated in purification facilities of water to remove their contaminants and be returned to the surface and/or be introduced into a disposal wells enabled for this purpose.
In the second case, the situation of greater risk for the aquifer security, are followed government regulations for the wells construction, which seek to eliminate any possibility of contamination and are similar between conventional and non-conventional wells. In order to gain a better understanding of the way in which the aquifers are protected during the drilling and useful life of the well we will briefly analyze its construction.
Once the position and the design of the well have been determined and have been obtained the permissions required for its construction is begun with the conditioning of the location where will be located the drilling equipment. Then starts the drilling following a program previously designed that we will divide into three parts: groundwater protection, insulation of the productive formation and fracture treatment.
Subsequently is drilled up to the first formation consolidated by below of the last aquifer in the zone (depth that is usually regulated by government entities) and is installed a second casing that is also cemented to the surface. This second pipe is often called surface casing, or security casing and meets two important functions: first is to isolate and protect the groundwaters of the region and the second is to provide a solid installation where to place the Blowout Prevention (BOP). This valve is of vital importance for the safety of the equipment and the environment since it is responsible for controlling the well in case of blowout (safety valve used to "prevent" the uncontrolled flow of liquids and gases during well drilling operations). Therefore before continuing with the well drilling is made a pressure test to check the casing integrity and it runs a log inside the casing (CBL-VDL) to check the cement quality with the purpose of ensuring the groundwater protection.
We can see then that the aquifers of the area during the fracture operation and subsequent well productive life is protected by one or two annulus of cement and at least two steel pipes, making impossible the contact of fluids that circulate through the well with the water of the aquifers.
The development and production of the shale plays as of conventional field are performed under a complex network of laws and regulations that govern each stage of the operation in order to minimize the environmental impact and make the hydrocarbons production a safe industry for the society and the environment.
Due to this in each country to be able to drill a new well and make a fracture treatment must be presented to government entities of each region a drilling permit application where are included extensive analysis of environmental impact, drilling and fracture program, chemical composition of drilling fluid, chemical composition of fracture fluid, gases emission into the atmosphere, treatment of waste generated by the operation and others. This application is analyzed by the relevant entities and must be approved by these organizations to carry out the construction of the well and subsequent fracture treatment.
Figure 13: Examples of Federal and Pennsylvania State regulations driving reliable well construction and operation in the Marcellus Play. Source: Nygaard, 2013.
In the United States for example the shale industry is regulated by federal, state and local laws and regulations. Being the U.S. Environmental Protection Agency (EPA) the responsible entity at the federal level of establish the laws and regulate the fluids injection below the surface (including solids, liquids and gases), under the SDWA, EPA sets standards for drinking water quality and with its partners implements various technical and financial programs to ensure drinking water safety. Whereas the development on federally owned land is managed primarily by the Bureau of Land Management (BLM), which is part of the Department of the Interior, and the U.S. Forest Service, which is part of the Department of Agriculture. In addition, each State has one or more regulatory agency that work together with the other organizations and these establish the local laws and regulations for its development.
Hydraulic fracturing account with 69 years of trajectory in the oil industry through which it has been shown to be a safe and useful tool in a large field of applications being its use indispensable for the development of the unconventional fields.
Its use in massive form doesn't imply a greater risk to the environment and the regional groundwaters since the possibilities of their contamination are similar between conventional and unconventional wells, being the determining factor for their protection a good cementation and quality of the surface casing. Also the water volumes used in such treatments (to develop a whole shale play) don't reach to overcome, in the best of the case, to the 10% of the water used in other industries in the same region. All this together with the progress in the legal regulations in force in each country have managed to protect and control the gases emission, the noise emission and the uses of the chemical substances making the shale industry an industry friendly for the nature.
Figure 14: Insulation used to deaden noise from drillin operations. Source: Chesapeake Energy Corporation, 2008.
In addition the hydraulic fracturing is the well stimulation technique more important in the oil industry since it has increased the world reserves estimated of oil and gas at 11% and 47% respectively, and it reached in 2015 a global market of 42,23billion dollars that will be duplicate to 81,10billion dollars for the 2024 according to the studies made by Grand View Research, Inc.
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