Hydraulic Fracturing also known as fracking or fracing is a wells stimulation technique developed in 1947 by Stanolind Oil and aims to increase the drainage area and the performance of the same through the creation of fractures in the productive formation. It was used for the first time in a commercial way for Halliburton 17 March 1949 in Stephens County, Oklahoma and Archer County, Texas. Since then its use, technology and development have been in continuous growth becoming a daily technique of completion of wells in formations of low permeability and high pore pressure or wells where the drainage area was damaged during the drilling or termination of the same.
The combination of horizontal drilling with multi-stage hydraulic fractures was the driving force that enabled to develop the unconventional fields in the United States and the rest of the world. Due to this in the last few years there was a vertiginous growth in the use of this technique arriving to be stimulated 35000 wells per year in the U.S. and surpassing the 2.5 million of hydraulic fracture treatments made to date at the global level.
The operation principle of hydraulic fracturing is very simple: It consists in pumping a fluid (usually composed by 89% of water) to pressure inside the well (through the production casing or by the use of the tool "frac string") until this exceeds the resistance of the rock, moment from which start the fracture, subsequently continued pumping the fluid with proppant to extend the growth of the same. Once finished the treatment the proppant introduced within the fracture making impossible that the rock joins again, being formed millimeter channels of flows that allow the hydrocarbon flow easily to the wellbore.
Figure 1: Cross section showing the micro-channels created by the hydraulic fracturing process.
To better understand the operation of hydraulic fracturing we will analyze the following image where is showing a conventional well and a well fractured hydraulically; as we can see in the conventional well the hydrocarbons flow from the productive formation to the wellbore through the wall of the same and from there it is brought to the surface by the reservoir pressure or with the help of an external extraction mechanism.
Figure 2: Image showing how the hydraulic fracture increase the contact area between the well and the formation.
On the other hand in the well with hydraulic fracture the hydrocarbons in addition to flow through the wall of the well as before now they also flow from the productive formation towards the interior of the fracture (which is a millimeter channel of high permeability and porosity), from the inside of the fracture toward to the wellbore and then to surface. We see then that the main achievement of the hydraulic fracturing is to increase the contact area between the productive formation and the well thus producing an increase in the flow and volume of the hydrocarbons produced.
The growth and the direction of the hydraulic fracture are determined by the stresses to which are subjected the rock in the area of interest. At any point given the rock is subject to three main stresses: a vertical stress and two horizontal stresses.
The vertical stress is generated by the weight of the rock column that is located above the point considered, it is also known as overburden stress and its magnitude increases with depth. Horizontal stresses on the other hand vary with the direction due to tectonic components.
Figure 3: Image showing the main stresses in a shale reservoir: a vertical stress and two horizontal stresses Fracture orientation.
The Fracture always is propagated perpendicular to the direction of minimum stress present in the formation. As the growth of the vertical stress is approximately 1psi per foot we can divide the hydraulic fractures in two types, horizontal fractures and vertical fractures, according to the depth in which is located the area to fracture.
Approximately until the 2000 ft the horizontal stresses are greater than the vertical stress and the fracture propagates horizontally. From the 2000 ft the overburden stress begins to be bigger than the horizontal stresses and the fracture propagates vertically as we see in the figure.
Figure 4: Fracture orientation: horizontal fracture and vertical fracture as a function of the main stress.
Given that all shale fields are located at depths greater than 2000ft the hydraulic fractures are oriented vertically in all shale fields.
As the direction and orientation of the fractures are given by the tectonic properties of the productive formation for their design are made complex studies which examines the reservoir permeabilities, the in situ stresses distribution, the geological model of the interest area and the possible fluids losses in naturally fractured formations.
Then with all the information obtained are made simulations of hydraulic fracture with specialized software to optimize the location, size, number and fracture length to carry out, together with the selection of the fluid of fracture, additives and proppant.
Figure 5: Hydraulic fracture simulation model, source: Chesapeake 2008
Today it's normal to make from 20 to 40 stages of fractures by well with fluid volumes that can exceed the 8 million gallons and the 5 million pounds of proppant.
When the fracture design is made prior to the drilling of the well, as is common in the shale fields, also is designed the location, orientation, distribution and type of wells to drill in order to maximize the stimulated volume of the productive formation and the volume of hydrocarbons recovered according to the specific characteristics of each oilfield. According to this is selected between vertical wells, horizontal wells or multi-well pad where from a same location are drilled several horizontal wells.
Figure 6: Comparative between VERTICAL AND MULTI-WELLS PAD USED TO DEVELOP A SHALE PLAY, source: ERCB 2011.
The multi-well pad have some advantages compared to the vertical wells because for a same number of wells to be drilled they need less locations (sometimes only one), so less property permissions are required and fewer roads are done, also it decreases the times generated by disassembly, transportation and assembly of drilling equipment since in the multi-well pad when the drilling of a well is completed this is moved 20 feet and the drilling continuous. These advantages turn into a significant reduction of the operating costs and the environmental impact making the multi-well pad the drilling technique more used in the shale plays in the United States.
The fluids more used in the hydraulic fractures are constituted by approximately 99.5% of water and proppant (sands or ceramics) and 0.45% of additives. According to the characteristic of the formation to fracture can be use different types of fluids like water based fluid, foam based fluid (CO2 or N2) or oil based fluid. Due to the fact that water is an abundant resource and of low cost, the fluids water-based are the most frequent in the fracture treatments offering a wide variety of possible configurations among which we can emphasize: slickwater, linear fluids, cross-linked fluids and viscoelastic surfactant gel fluid.
Figure 8: Frequent fluid fracture composition using in a shale gas and/or a shale oil, source: US DOE Exhibit 35
Slickwater fracturing fluid: is the type of fluid more used in the fracturing of unconventional wells (over 30%) and is composed by water, sand and additives which include friction reducers, corrosion reduction, bacterial-growth control, clays inhibitor, and others. The advantages of this fracture fluid are: large reservoir volumes stimulated, better fracture containment and damage reduction generated by gels. Its disadvantages are: excessive use of water, narrower fracture widths that other fluids and a poor capacity to carry the proppant toward the interior of the fracture due to its low viscosity.
Linear fracturing fluids: Are achieved by the polymers addition to water forming a gel with higher capacities of transport that the low viscosity fluids (slickwater). Among the polymers used there are: guar, Hydroxyethyl Cellulose (HEC), Hydroxypropyl Guar (HPG), Carboxymethyl hydroxypropyl guar (CMHPG), and Carboxymethyl Hydroxyethyl cellulose (CMHEC) (EPA 2004). The advantages of this fluid are: greater transport capacity of the proppant and a minor loss of fluid in low permeability formations. Among its disadvantages are found: lose of circulation in permeable formations, form thick filter cakes on the face of lower-permeability formations impoverishing the conductivity of the fracture and the formation of less complex fractures.
Crosslinked fluids: These fluids were developed in 1964 to improve the performance of the gels without increasing their concentration through the use of borate ions. The polymers most used in this kind of fluid are guar and HPG and since the seventies have been added zirconate and titanate complexes of Guar, Hydroxypropyl Guar (HPG) and Carboxymethyl-Hydroxypropyl Guar (CMHPG) in order to enable its use at high temperatures . The Crosslinked obtained by using borate is reversible and is activated by an alteration of the pH of the fluid system, this reversible feature of the crosslinked fluids allows a clean up more effectively post-fracture, resulting in a better permeability and conductivity. Its use has showing a positive performance in formations of high and low permeability, with good transport properties and stable rheological properties in a wide range of temperatures.
Viscoelastic surfactant gel fluids (VES): these fluids used surfactants combined with inorganic salts to create orderly molecular structures, which produce an increase of the viscosity and elasticity. As a result they have high zero-shear viscosity being capable of transporting the proppant with minor loss of charges and without the requirements of viscosity of conventional fluids. These fluids also present the advantages of not needing the use of biocides, clays controller and additional surfactants to improve the flowback.
It is important to note that the fluid chemical composition to be used in each fracture must be filed with the governmental entities of each region and it has to be approved in order to be able to carry out the fracture treatment in the well. Below is shown a table with the additives more used in hydraulic fracturing fluids, their properties and common uses in everyday life:
Table 1: Fracturing fluid additives, many compounds and common uses. Source US DOE.
In the part B of "Introduction to hydraulic fracturing" we'll develop: proppant, surface equipment, monitoring of the operation in real time, well construction and aquifers protection, laws and a general conclusion.
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