The Effect of Fracturing Fluids Types on Reservoir Fracture Geometry, Production, and Net Present Value

U nconventional reservoirs are distributed over a broad area and only generate commercial quantities of hydrocarbon using specialized technology. Tight reservoirs are reservoirs with permeability less than 0.1md. One of the leading oil fields in Iraq is Halfaya field. The tight reservoir is Saadi. It is a carbonate reservoir with poor petrophysical characteristics, moderate pores, and limited pore throats, so production from these reservoirs require hydraulic fracturing. The hydraulic fracturing can be carried out using fracturing fluid. The selection of fracturing fluids should be selected as the first step in fracturing design. The objective of this research is to find out the effect of fracturing fluids on the shape of the fracture formed. Three designs were constructed based on the fracturing fluids used to build fracturing models, where three kinds of fracturing fluids were used slickwater, guar, and hybrid (combination of slickwater and guar) with a pump rate 31.5 bpm. In the first design, the fracture's height extends up and down the Saadi formation due to the presence of fragile layers around Saadi formation, so fluids are produced from these three formations. In the second design, the fracture's height is limited within the Saadi formation from the top but extends to the Tanuma from below, and therefore the production will be from the Saadi and Tanuma reservoirs. Three hybrid treatments were analysed: 30% guar & 70% slickwater, 50% guar & 50% slickwater, and 70% guar & 30% slickwater. The results show that the third design (50% guar & 50% slickwater) give the best oil production and NPV.


INTRODUCTION
Halfaya oil field is one of the leading oil fields in the Maysan province, in the southeast of Iraq.It is situated 35 kilometers from downtown Amara City.Saadi is a carbonate petroleum reservoir with poor petrophysical characteristics, moderate pores, and limited pore throats (Tali and Farman, 2021; Jasim et al, 2020).The Saadi Formation's thickness ranges from 120 to 130 meters on average.Each layer's thickness values are around 51 m for Saadi-A, 28 m for Sadi-B1, 30 m for Sadi-B2, and 20 m for Sadi-B3.The petrophysical examination of the Saadi reservoir reveals that it has poor petrophysics properties.Saadi is characterized by moderate to high porosity (15-20%) and low permeability of (0.05-1.3 md), therefore causing an insufficient liquid supply (Al-Sudani, 2022; Cui et al., 2018).Tight reservoirs are defined as those reservoirs with extra low permeability and are present in the world and Iraq.They contain large oil reserves but can be only extracted after the application of necessary stimulation techniques, such as hydraulic fracturing.It is applied in oil reservoirs to produce oil (Liang et al., 2015).(Hubbert and Willis, 1957) found that in situ stresses control hydraulic fracture initiation and propagation: the minimum stress depends primarily on where the fracture is initiated, and the maximum stress dominates which direction the fracture propagates (Hubbert and Willis, 1957).The width of the fracture is related to the viscosity and pumping rate of the fracture used (Perkins and Kern, 1961).The hydraulic fracturing process includes two fluid pumping steps: pad and slurry.Through the pad step, the formation is broken by injecting just liquid into the zone of interest, at this stage there is usually propagation of fractures has occurred.Some chemicals are used with frac fluids to boost viscosity.Afterwards, the slurry stage, in which the liquid is combined with the proppant and injected into the fracture to prevent closure.This combination widens the fracture, thus facilitating the transfer of proppant to the formed fracture( Kreipl and Kreipl, 2017).The purpose of fracturing fluids is (1) to suspend and transport proppant into the formed fracture by using liquids with sufficient viscosity.(2) Fracture cleaning after the fracturing treatment is finished, it is chemically decomposing to low viscosity, allowing a large portion of the fluid to flow to the surface (Jennings, 1996).The fracturing fluids viscosity is an important factor in fracture creation, as many complex fractures form with narrow openings when fluid's viscosity is low (Warpinski et al., 2005;Cipolla et al., 2008).Rickman et al.,2008 observed that the length of the fracture increases when the volume of the fracturing fluid increases (Rickman et al., 2008).(Economides et al., 2010) evaluated the productivity of created fractures in vertical and horizontal wells for formation with permeability (0.001 md -500 md).They reached to the conclusion that in oil reservoirs with permeability lower than 10 md, a vertical fractured well may be the most economical choice.(Lebas et al., 2013) demonstrated that for transporting proppant through the fractures, cross-linked gels outperform slick water, due to high viscosity.(Inui et al., 2014) studied the fracture geometry by using fracturing fluids of high and low viscosity such as slick water and cross-linked gel.They found that slick water generates complex fractures with multiple branches, and the width is narrower than gel (Taleghani and Olson,2013).Saadi reservoir is a tight petroleum reservoir with poor petrophysical characteristics, moderate pores, limited pore throats, and ultra-low permeability (Jasim et al., 2020; Al-rubaye and Hamd-allah, 2020).Hydraulic fracturing is the capability of hydrocarbons to flow more easily from the reservoir to the wellbore.Hydraulic fracturing treatments are carried out to improve well performance and to achieve economic growth from improved production rates and the ultimate reserve recovery.To create hydraulic fractures, hydraulic fracturing fluid must be injected into the well.As the fracturing fluid is considered one of the most important requirements in this process to ensure its success.The purpose of it is to create fractures of sufficient length and width to place and transport the proppant from the surface to the formed fracture.This treatment is performed with high pressure and rate (Gaurina-Međimurec et al., 2021).Halfaya oil field is an important field in Iraq, located in Missan governorate, 35km south of Amara city (Farouk and Al-Haleem, 2022).The tight reservoirs have a focus in oil industry because they contain huge reserves of oil, and they impact on oil production.These reserves have only extracted by using an important technique namely hydraulic fracturing, that can create fractures and increase production.For this a combination of fracturing fluids are used to evaluate their effect on fracture geometry dimensions and production (Hasan and Hamd-Allah,2023).
The key point of this work is to set the height of the fracture within the Saadi reservoir.Three types of fluids such as slickwater, guar, and hybrid were used and compared economically by NPV, and the accumulated oil is estimated.

SPECIFICATION OF FRACTURE'S GEOMETRIES
3D model used to calculate hydraulic fracture geometries, it is concerned with: -Hydraulic Fracture Height: The height of a hydraulic fracture can be estimated using the following equation known as the Perkins-Kern-Nordgren (PKN) model (Perkins and Kern, 1965): ℎ  = 0.00623 * /( *  * ()) ( 1) where hf is the fracture height (ft), Q is the total injected fluid volume (Ib/gal), K is the rock permeability (md), w is the proppant concentration (Ib/ft 2 ), p is the net pressure (psi).
-Hydraulic Fracture Length: The length of a hydraulic fracture can be estimated using the following equation known as the Geertsma-de Klerk (GDK) model (Geertsma and De Klerk, 1969): where: L is the fracture length (ft), E is the young's modulus of rock (psi).
-Hydraulic Fracture Width: The width of a hydraulic fracture can be estimated using the following equation known as the Carter-

Economics
Net Cash Flow (NCF) is an approach used to determine profit of hydraulic fracturing process (adopted for increasing oil production).The Net Present Value (NPV) is the difference between the present value of cash inflows and the present value of cash outflows over some time.The net present value of fractured wells for number of years is estimated as:

RESULTS AND DISCUSSION
Hydraulic fracturing models were constructed by selecting three types of fracturing fluids, which include slickwater, guar, and hybrid (combination of slickwater and guar) with a pumping rate of 31.5 bpm.These fluids were selected depending on high NPV and best fracture height which gave the highest NPV and best fracture height confinement within Saadi Formation.

Design 1 (Guar100%)
In this design, 53476 gallons of cumulative pad volume, 55871 gallons of cumulative slurry volume, and 72013 lbs of cumulative proppant are injected into the perforation.The propagation of the fracture is in the Saadi formation with lateral direction.It's clear that the fracture height is not confined within the Saadi Formation, but extends higher toward the Hartha formation and lower downward Tanuma Formation.Due to the presence of highly brittle rocks above and below the Saadi formation, hydrocarbons may be produced from these three formations as shown in Fig. 3. Also the fracture grows and spreads within the Saadi formation, and it has a degree of symmetry on the other side of the well.The value of the maximum fracture's width is 0.127in.In this design, 53354.4gallons of cumulative pad volume, 56404.5 gallons of cumulative slurry volume, and 72013 lbs of cumulative proppant are injected into the perforation.Each fracturing fluid was optimized, and these quantities were selected because they gave the best results (best fracture height within Saadi formation and highest NPV).It is clear that there is no significant barrier in the pay zones between the Saadi and Tanuma formations, so the fracture will extend into Tanuma and cannot be controlled as shown in Fig. 4.However, for the top part, the fracture remains only within the Saadi formation, that means not extending to the Hartha formation and the production is going to be from these two formations (Saadi and Tanuma).The fracture is asymmetric on both sides of the well.The dimensions of the created fracture are; length 257m, height 96 m, and maximum width 0.114in.Three hybrid treatments were analysed within the formations: 30% guar with 70% slickwater, 50% guar with 50% slickwater, and 70% guar with 30% slickwater.Hybrid-frac design was used to investigate the impact of reducing the amount of guar in the treatment design.Using less guar in the treatment can lower the cost of well stimulation 4.3.1 Design 3a (30% guar with 70% slickwater) In this design, 52727.2gallons of cumulative pad volume, 55739.5 gallons of cumulative slurry volume, and 72013 lbs of cumulative proppant are injected into the perforation.The created fracture has a maximum width equal to 0.154in.Fig. 5 shows that the fracture's width is higher than design 3b and 3c because of the fracture's propagation in the lateral direction.

Design 3b (50% guar and 50% slickwater)
In this design, 52989.6 gallons of cumulative pad volume, 56057.3gallons of cumulative slurry volume, and 72013 lbs of cumulative proppant are injected into the perforation.The length of the fracture is 274m, the height is 186m, and the maximum width is 0.148.Fig. 6 demonstrates that the fracture extends greatly to the top of Hartha and the bottom of Tanuma.Since the guar can create long fracture height by overcoming the stress between layers.In this design 53454.3gallons of cumulative pad volume, 56505gallons of cumulative slurry volume, and 72013 lbs of cumulative proppant are injected into the perforation.The length of the fracture is 269m, height is 172m, and the maximum width is 0.107in.Fig. 7 shows the fracture's height propagate laterally and extend to Hartha and Tanuma, and the production from three formations.

Production Forecast and NPV using Fracturing Fluids (Guar, Slickwater, Hybrid)
Fracturing fluids and proppant units cost assumed at 1$/gallons, and 0.4 $/Ib, respectively.Monthly well cost is assumed at 1500 $/month.Also, the oil price is assumed at 60$.The fracturing treatment cost is assumed to be 400000$.The net present value for each design is calculated by assuming the discount rate at 15 %.So, the NPV increased as cumulative production increase.Oil rate for design one (100% guar), design two (slick water 100%), and hybrid (3a, 3b, and 3c) 5. CONCLUSIONS Design one that uses Guar generates wider, longer, and higher fractures than design two of Slickwater because of its low viscosity.The fracture formed when using guar is able to penetrate the layers above and below the Saadi formation through overcoming the stress between Saadi and the formations above and below it.Also, according to the high viscosity of the guar, Guar is able to transport the proppant (combination of fluid and sand) into the wellbore to prevent the fracture from closing, because the proppant resists the stress on both sides of the fracture, allowing a large amount of fluids to pass into the well and thus increasing the reservoir conductivity.The fracture will be within almost the Saadi formation, when using slickwater.The created fractures will be narrower, containing less proppant, because the proppant settle away down in the fractures and thus less conductive.The design that uses hybrid containing little guar like (30% guar and 70%slickwater), causes less production.Finally, based on treatment economics and oil production, the design that uses hybrid fluids system containing 50% guar and slickwater seems to be the greatest effective hydraulic fracturing fluid intended for the examined location, since it gives cumulative oil production 8011.67Mbbl and NPV is 171.22.As the increase in production may be due to high fracture height, thus increasing NPV.

NOMENCLATURE
viscosity fracturing fluids like slickwater which contains chemicals and guar should be used (Zicen and Yanling, 2018).The viscosity of the slickwater is suitable for the reservoir requirements from the beginning of the hydraulic fracturing process as shown in Fig.1.µo is the value at which the viscosity plateau occurs.The viscosity of slick water at zero-shear rate µo equal 0.8.This figure shows that the viscosity of slickwater remains constant and independent on the amount of shear rate and behave as Newtonian fluid.3.1 Fracturing Fluid ViscosityViscosity describes the internal friction of a moving fluid.The physical features of the fracturing fluids are crucial for the efficient implementation of hydraulic fracturing HF treatment.One of the key factors affecting fracture designing is fluid viscosity.High viscosity widens the crack, promoting proppant transfer, and lowers loss of fluid throughout formation.Nevertheless, low fluid viscosity produces a narrower fracture width.The majority of complicated fracturing fluids behave in a non-Newtonian manner.When it comes to fluids that are shear-thin, the viscosity drops as the shear rate increases.Since neither infinite (for low shear rates) nor zero viscosity (for high shear rates) can be attained in practice, this pattern only applies to a certain range of shear rates for real fluids.Consequently, Newtonian plateaus are obtained for both low and high shear rates.The viscosity of the base solvent is correlated with the high shear rate plateau(Wrobel et al., 2021; Jennings,1996).

Figure 1 .
Figure 1.Variation of viscosity of slickwater with different shear rate

Figure 2 .
Figure 2. Viscosity of guar at different shear rate

Fig. 8
shows the results of the production and economic evaluations for two design.Design 1 (guar100%) provides an average oil rate of 718 barrels per day with plateau duration 4044 days (11 years), cumulative oil production 7860 Mbbl with NPV 172 MM$ from the software.The design 2 (slickwater 100%) consistently produces 198 bbl/day of oil on average over the course of production with plateau duration 593 days (1.6 year), the cumulative oil production 2170Mbbl and NPV 81 MM$ as shown in Fig.9.
considered the best design, since it gives the highest NPV with an average oil rate of 731.659 bbl/day.That means this design is superior to other ones economically.