KNE711 Hydraulic Engineering | Hydraulic Fracturing System

Overview of hydraulic fracture

Hydraulic fracturing is also known as hydro-fracturing or hydro-fracking. It refers to a well modified and a stimulated technique whereby a rock is broken down using liquid under high pressure (Bernabe, 2009, p. 78). This mechanism includes exertion of great pressure by the pressurized fluid which can either be water , sand or in some cases a proppant suspended by a thick agent inside a wellbore to assist in creating point which are weak during the formation of deep rock through which free flow will be noticed in brine, natural gas, petroleum (Fink, 2013, p. 346). On the other hand, the removal of the good pressure, the hydraulic fracturing small grains proppants, that is, the fracture is held open by either the sand or aluminum oxide holds (Gleeson, 2016, p. 452).

The hydraulic fracturing system is regarded as most of the controversial techniques in most countries where its proponent's advocates for the economic remunerations of expansively reachable hydrocarbons (Wu, 2018, p. 164). Some opposing powers argue that these measures are overshadowed by prospective environmental impacts which range from the surface or underground water contamination, noise pollution to public health-related consequential hazards (Wu, 2018, p. 64). Another related problem of this process is the leakage of the methane gas to the humanitarian environment and this was discovered in Pennsylvania, that is in the United States by the Environmental Defense Fund's report. There was also an increase in the seismic action due to hydraulic fracturing through the inactive faults that resulted due to deep- injection disposal of the fracturing flow-backs leading to the formation of end products for both fractured and non-fractured oil together with gas wells.                                                 

   The pressure overwhelmed those rocks that fracture at large depth as a result of the weight of rock layers lying on top and their formation cementation. This overpowering is normally important in ‘tensile’ fractures which require the ramparts of the broken rock to go contrary to this pressure. The occurrence of fracture takes place when operational stress is overwhelmed by the fluid pressure inside the rock and the minimum principal stress becomes tensile thus exceeding the tensile strength of the real materials (Chen, 2016, p. 421). The fractures formed in this way are generally oriented in a plane perpendicular to the minimum principal stress and is because of this reason that the fractures in the wellbores are used to determine the orientation of stresses (Bucher, 2013, p. 98).

  Most occurring mineral vein systems are as a result of repeated natural fracturing during the periods of high pore fluid pressure. The action of this high pore fluid is normally evident in crack-seal veins where the vein material is part of the chain of discrete fracturing events, and extra vein material is deposited on each occasion.

 The minor intrusions in the upper part of the earth’s crust such as dikes, exists in the form of fluid-filled cracks and this fluid is referred to as magma, but in sedimentary rocks, fluid at fracture tip will be in the form of steam.

Theory of Hydraulic Fracture

According to Walker (1949), the following arguments were made based on the assumptions such as; the treatment pressures should be more compared to the overburden pressure for it to cause a horizontal fracture. This means that fractures are consistently vertical. On the other hand, the people who oppose this matter shall actually dispute that the cause of horizontal fractures occurring horizontally takes place since the pressure is less than the average overstretch pressure, simply because there is lack of even distribution of overburden pressure is not evenly distributed hence shall be induced along paths of least overburden load (Spellman, 2012, p. 129). Another prominent issue coming up when overburden pressure is not distributed evenly is used in explaining the low treatment pressure. In some cases, unsatisfactory overburden distribution might be important for the shallow depths (Shojaei, 2017, p. 2237).

It is also important to note that the objectivity of the opinion that horizontal fractures are in fact produced with lower than the overburden treatment pressure will result to gamma-ray logs which gives a high reading at a single spot in the hole after the injection of radioactive sand, thus showing a horizontal fracture even though the resultant pressure was smaller than the recommended average for the overburden pressure (Adwera, 2008, p. 43).

Again during formation of a crack, it is convincing to be certain that the fracturing fluid may at times move in and open a plane fracture in regard to a normal plane which has minimal stress along with no initial fractures. Sometimes it seems possible to check on the fracture as it is first propagation comes from the wellbore by a locally transformed stress field (Tuman, 2011, p. 28). Personally, I think that induction of fracture in un-wanting orientation but following a tortuous route due to many oriented planes of weakness will have a slight selectivity in choice of path that would otherwise orient the general path with time and this, therefore, makes fractures to be extended only by the pressure which is much higher compared to the compressive stress which is in the direction of fracture displacement. Water-flood is another well-illustrated example for obtaining data during the study of pressure- parting phenomena. 

Generally, it is evident that the concern measures for pressures during the production of vertical fractures may differ with the existence of the resulting fluid pressure (Sharp, 2014, p. 56). The sustained pressure exerted before the breakdown in a thin zone would be suitable to perform a horizontal fracture whereas a short period of injection in a thick bed before a breakdown leads to a vertical fracture. In summary of this theory, it is important for one to take into account that if a horizontal fracture is realized, the hole at the bottom resulting to pressure should approximately be equal to the calculated overburden pressure (Sanford, 2010, p. 98).

Vertical stress

The vertical stress in most cases equal to the to the weight of the overburden per square and the vertical stress is quite variable at shallow depth. In the case of a vertical fracture is vertical, the treatment pressure closely equal to the least horizontal stress which generally lies below the overburden pressure (Shojaei, 2017, p. 43).       

Horizontal stress

In shallow depth, the vertical stress may be the minimum stress resulting in horizontal fractures. This takes place for highly overpressured formations and in most cases, the horizontal in-situ stress is less as compared to the vertical stress. The horizontal stress originates from two sources;

• Tectonic forces; geologic forces corresponding to the local geologic structure.
• Reaction to the overburden such that the overburden weight compacts the formation and the reaction to this creates horizontal stress

Fracture pressure

The fracture pressure is the pressure level above which drilling fluids are capable of inducing the formation of rock fractures hydraulically. The consideration of these pressure is at hydraulic fracturing which is a stimulating process that is carried out on the routine basis on oil wells. The deformation of rock is caused by fracture pressure and also it etches the formation resulting into fissures that are favorable for the passage of oil and gas. The measuring of this pressure can be done in gradient or by the density of the fluid equivalent (Bucher, 2013, p. 216).

The fracture pressure is simply defined as the amount of pressure that is needed in disintegrating the formation of rock. The measurement of pressure should be done since it will assist in knowing whether the pressure available is enough to cause a fracture. In deep oil well, the pressure of the fracture is increased due to the overburden pressure. Since the location of the rock formation is at the deep end thus tough to fracture then the pressure needs to be set really high to avoid rock resistance or problem of circulation (Bucher, 2013, p. 12).

The stress within a rock can be resolved into three principal stresses. A formation will fracture in case the borehole pressure exceeds the least of the pressure of stresses within the structure of the rock (Bucher, 2013, p. 76). The following tests are carried out in order to determine the pressure at which the rock creation fractures after exposing it to the pressure at the borehole. They include; leak-off test, limit test and creation breakdown test (Pointet, 2008, p. 54).

Fracture Propagation Models

The earliest fracture treatments were first put into the investigation by just pumping them to notice if in case the formation of fracture can take place a fracture could be formed through pumping of sand into a fracture (Mader, 2009, p. 123). This science was first published by Howard and Fast whereby they assumed in their models that the fracture width was constant everywhere thus make it easier for the engineers in finding a fracture area relying on the fluid of the fracture which constitutes the features of the formation and the fracturing fluid. The following are the well- known models for fracture propagation techniques:

(a) Two-dimensional fracture propagation model

A clear example of a two-dimensional model can be given by that of Howard and Fast where they fixed one of the dimensions, usually the height of the fracture in calculating the fracture length and breadth (Bucher, 2013, p. 678). In regard to understanding and precise sets of information, the model is applied; supposing the design engineer is capable of estimating the generated fracture height precisely (Fink, 2013, p. 568). If the fracture length is higher compared to the height of the fracture the Perkins-Kern-Nordgren (PKN) geometry is usually applied. And in case the fracture height is the one dominating the length then the Khristianovic-Geertsma-de Klerk (KGD) geometry is needed, and when it comes to designing the hydraulic fractures than the two mentioned models are successfully required (McBroom, 2013, p. 519). The actual results with predictions from models calculations are compared by this model. (Fink, 2013, p. 216). The use of exact fracture height in this type of model gives a sensible approximation of shaped fracture length and width if other parameters like creation permeability coefficient are used (Choquette, 2012, p. 68).

b) Three-dimensional fracture propagation model

Due to the invention of the high powered computers in the current world, most engineers have developed the use of the pseudo-three-dimensional(P3D) models in the fracture propagation techniques. These P3D models are considered to be the best alongside the 2D models because they are capable of computing fracture heights, length, as well as breadth, spreading through the information for pay zone together with entire layers of the rock beyond and underneath the holed intermission (Bernabe, 2009, p. 129). Dimensions and geometry of the fracture are given by this model in detailed which can result in better wells as well as designs (Bucher, 2013, p. 129). The hydraulic fracture shape and dimensions are computed using this particular model (Chen, 2016, p. 208). The key to any model, be it 3D or P3D model should pose a data which is detailed and precise which pronounces the developer layer to be fracture preserved, together with the rock found beyond or beneath the zone of interest (Howard, 2012, p. 32). In most occasions, the data set should contain information ranging from 5 to 25 layers of rocks that could affect fracture growth (Carroll, 2014, p. 219). During this programming, it is advisable for one to key the information on as numerous times as possible to enhance the model to decide the fracture height growth as a function of where the fracture is commenced in the model (Gleeson, 2016, p. 18). It is also important for one to note that he/she can only decide the fracture shape by entering the data into three to five layers of the rock rather than determining the fracture model.

Tight Gas Reservoir

The natural gas known as tight gas is acquired from reservoir rocks having low permeability. Tight gas is an environmental source of natural gas.

Characteristics of reservoirs

Are pore in structure

The main pore types are the primary pores and the secondary pores. Secondary pores are intragranular while primary pores are intergranular and have micropores in the matrix

Are porous and permeable

The porosity of Xujiahe formation is less than 6

Fracture porosity

This is a kind of secondary porosity by the tectonic fracturing of rocks. The fracture does not typically have great volume themselves but through joining the pre-existing pores assist in enhancing their permeability significantly. The granite which is non-reservoir rocks can turn into reservoir rocks and separation of rough surfaces into two surfaces resulting to fracture porosity takes place. This takes place when enough fracturing takes place. Every surface can be covered by the infilling of minerals and the whole fracture can be filled by the minerals which later convert the open fractures to healed or sealed fractures.

In a total volume of the rock, the fracture porosity is usually small since it is about 0.0001 and 0.001. The porosity related to fractures for example in granite or carbonated reservoir may acquire larger values but in actual fractures, it is still very tinny. It can be obtained accurately through processing the curves of formation micro-scanner for fracture apertures and intensity of the fracture. The approach of normal porosity calculations, the porosity, and permeability including those attributed to fracture can be found (Tuman, 2011, p. 453).

Permeability

The measuring of connectivity between the pores spaces and the ease with which the fluid flow through the connecting pores spaces of a rock with naturally occurring fractures in the formation is what is known as the fracture permeability (Bucher, 2013, p. 315). The measuring of fracture permeability is necessary so that to know hydrocarbons recovery from a reservoir or what can be produced by the reservoir. The permeability is measured traditionally in the laboratory since it is difficult to determine directly and the permeability of the fracture relies on the size of the grain, the formation type, the formation porosity and the formation kind of fluid or pressure (Beiyadi, 2016, p. 219).

Types of Drilling

 Drilling is a process of cutting that involves the use of a bit which cuts a circular hole in the cross-section of a solid material (McClure, 2013, p. 549). The exploration of oil and gas in deep formations is majorly done by the vertical and directional drilling and they are also used in exploiting geo-resources and deep thermal energy. The natural gas and crude oil contain a mixture of hydrocarbons, non-hydrocarbons, and other trace elements and their storage is in sedimentary rocks of deep formation (Nassar, 2009, p. 65). Numerous methods need to be applied to remove the gas and oil outside the deep formation.

When rock drilling a bit is usually rotated but not making a circular cutting motion. Hammering can either be from the outside the hole or within the hole (Mader, 2015, p. 129). Drifter drills are commonly applied. Drilling form outside is the top – hammer drill and drilling within is the down – the hole drill.

Characteristics of drilled holes.

• Helical feed marks in the inside part of the hole.
• Burs tend to be on the exit side if not removed.
• Sharp edge on the entrance side.

All these problems and faults can be avoided by;

• By ensuring that fluids commonly used in cooling the drill bit are cut down hence increasing the lifespan of the tool, speed, surface finish as well as aiding ejecting chips. This fluid can be applied by flooding the working area with a coolant or a lubricant or applying a spray mist.
• Spot drilling.
• Centre drilling.
• Deep hole method.

Types of drilling.

Conventional Drilling

Conventional wells are normally drilled in a vertical manner that is straight to the ground. This is the traditional example of drilling.

Horizontal Drilling

The mechanisms for example bits are driven at the bottom and drillers are able to make a sharp turn as well as horizontal drilling along a thin pay zone. The drilling of two horizontal wellbores is done above the other, about 3 meters apart (Adwera, 2008, p. 132). One usage of this is the Steam Assisted Gravity Drainage where the injection of steam is carried out into the higher of these horizontal holes and the oil is precipitated down into the lower hole by the heat, leading to increment in heavy oil production. (Ahmed, 2016, p. 123).

Slant Drilling

This is the making of drills at a perpendicular angle (commonly 30⁰ to 45⁰). This method inhibits surface environmental interference (Nelson, 2009, p. 76).

Vertical Drilling

This is an advancement from slant and horizontal drilling. Directional drilling is able to change directions and depth several times in one wellbore. The mentioned drilling method remains exclusively suitable to pay zones in the Lloydminster area which are often distributed like prairie sloughs across the underground landscape (Bernabe, 2009, p. 123).                  

CMG Software

Computer Modelling Group is a software company dealing with the production of reservoir simulation software for the oil and gas industry (McBroom, 2013, p. 32). A three reservoir simulation is offered by this company which includes an advanced Equation-of-state, a conventional black oil simulator which enhances the recovery of oil and advances process simulator (Ahmed, 2016, p. 56). How CMG simulators are being used;

• Tight reservoir modelling features
• Tight reservoir modelling features

The Computer Modelling Group starts working by following the following processes;

Working principle

The Computer Modelling Group starts operating by;

• Formation of the grid for simulation.
• Allocating porosity and permeability to the model.
• Creation of PVT data
• Creation of relative permeability data.
• Creating the initial conditions.
• Integrating routes and gaps.
• Addition of historical data to the model.
• Creation of field production history for history match.
• Addition of aquifer.
• Revising the results obtained from the simulation.
• Using Historical Data file in prediction run.

The reservoir simulation models are used by the companies dealing with gas and oil. These models are used also in areas where production forecast are needed to assist in making decisions relating to investment. These models may assist in improved oil recovery by hydraulic fracturing and the Computer Modelling software may be used in in hydraulic fracturing designing as well as improvement in oil recovery with pressure maintenance through re-injection of the gas produced or injection of water (Carroll, 2014, p. 193).

The lowering of oil viscosity is done in order to assist in improving oil recovery and the reservoir simulation is applied widely to obtain opportunities to upsurge the production of oil. Simulators such as black oil is not considering the changes in composition of hydrocarbons as the field is produced and modelling of hydrocarbons in tight matrix blocks is done by natural fracture simulation known as dual-permeability and dual porosity (Beiyadi, 2016, p. 563).

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