MODELLING FRACTURING IN SHALES WITH FLUID ROCK INTERACTIONS

 

           Aibol NUSSIPKOZHAYEV[1],   Jean-Jacques Royer[2], Murat BAIMUHAMETOV1

     

        [1] Kazakh National Technical University after K.Satpayev

        2 ∗Universit´e de Lorraine UdL, CNRS-GeoRessources (UMR7359), ENSG-INPL, gOcad-ASGA, Vandoeuvre, France.

 

Abstract

With the last advances in uranium mining such as In-situ leaching technology the implementation of this technology into other ore mining fields nowadays is possible.

More exactly it comes to the application of above mentioned technology to the copper mine sites.   This work focuses on the creation of the structural model on gOcad of  the testing area at the Rudna mine site and on the implementation of a hydraulic fracturing process to increase the in-situ permeability. The hydraulic fracturing was performed using a wizard developed in the gOcad environment.  In-situ recover methods – an alternative way to produce metals, were explained. The rock elastic properties were estimated from laboratory assays performed on rock samples, especially Vp and Vs seismic, density and porosity. These values were interpolated in the grid model and used to simulate the fracturing. The report shows simulation results using the gOcad software. Permeability increases with injection pressure after fracturing. A predictive cubic law relating the fractured volume against injection pressure was proposed.

Keywords: Hydraulic fracturing, In-situ recovery, copper, geomodeling, rock mechanics, rock elastic properties, seismic wave, permeability

 

Introduction

 

The Polish sediment-hosted Kupferschiefer deposit is a world-class polymetallic deposit. It extends across North-central Europe from North-Westernmost Belorussia to Northern Ireland, along an east-westerly belt of more than 1.500 km (Vaughan et al., 1989). Today this World Class deposit, exploited in the South-western of Poland by KGHM Polska Miedź S.A. (Lubin, Sieroszowice-Polkowice, and Rudna and Glogów-Glęboki-Przemysłowy mines), is placed among one of the main Cu-Ag deposits in the World1 (~ 2% Cu and ~ 2 Oz/t Ag in over 1 Gt bearing metals ore 2). Lead, zinc, gold, PGE, and some critical raw materials for the EU (2011), such as rhenium, have also economic importance (KGHM, 2011). The copper production of Poland represents half of the total copper production in Europe (or a quarter of the total European copper consumption).

Hydraulic fracturing implemented to the model with the view to test the in-situ mining or in-situ recovery (ISR) method. This involves drilling of injection and recovery wells to dissolve the deposit underground and recover the copper. Sulphuric acid is injected using the injection well. Pumping from the recovery wells drives the acid from central injection well to the recovery well where it is pumped back to the surface for processing. As the acid moves through the ore body along the natural fractures system it dissolves the oxide copper within the fractures system.

General information about testing area

The Rudna mine is the largest copper ore mine in Europe and one of the largest deep copper ore mines in the world.

Rudna Mine is located in Lower Silesia, north of Polkowice city. The mine extracts mostly “Rudna” deposit, but it also develop and exploit parts of “Sieroszowice” and “Głogów Głęboki-Przemysłowy” deposits. Industrial resources of Rudna Mine (according to state on 31.12.2013) in three operated deposits are 449 million Mg of copper ore. Average grade of copper amounts to 1,95 %, average grade of silver – 59g/Mg.

Among the Polish copper deposits operated by KGHM, Rudna deposit stands out the thickest orebody - up to even over a dozen meters; average thickness of Rudna deposit is over 4 meters nowadays and over 70% of resource is over 3 meters thick. In Rudna deposit the dominant part of ore is sandstone ores – around 80% of resources, carbonate ores represent approximately 15% and Copper-bearing shale (Kupferschiefer) only 5% of total deposit mass. Copper-bearing Shale (Kupferschiefer) contain the highest  grade of copper – over 6% of Cu. The depth of copper orebody in Rudna deposit range from 844 m up to 1250 m in depth, and Głogów Głęboki-Przemysłowy deposit is up to 1385 m in depth.

Geological setting of the Kupferschiefer

The sedimentary formation of Kupferschiefer (literally copper shale) extends over the northern Europe on more than 600.000 km2 (Blundell et al., 2003). The genesis of Kupferschiefer is related to the eustatic variations of the Zechstein Sea at the end of Permian. It is composed of a thin (< 1m) shale layer containing in average about 7% organic matter, inserted between the Lower Permian terrestrial/volcanic sediments (Rotliegend and Weissliegendes) and the Upper Permian marine sediments of Zechstein (Jowett, 1986; Oszczepalski, 1999; Blundell et al., 2003; Gouin, 2008; Borg et al., 2012; Hartsch, 2013)

                     

 

 

 

 

 

 

 

 

 

 

 

Figure 1.  – (a) - 3D model of the Lubin region, Poland. The thick Zechstein evaporates confine the fluids at the basement and interface. (b) 2D geological map of the Lubin region and mining district exploited by KGHM (www.kghm.com).

 

 

Figure 2. In-situ recovery scheme

In-Situ Recovery[3] 

ISR is an environmentally-friendly process by which copper can be extracted from the ground with minimal disturbance to the surface environment (Figure 2). ISR mining has a long history, starting with uranium mining in the 1960s.

Advantages to In-Situ Recovery Mining:

- Lower capital and operating costs

- No waste or ore moved

- No creation of open holes, waste dumps, leach pads or tailings

- Minimal visual disturbance

- Minimal noise, dust and greenhouse gas impact

- Fewer permits are required compared to other mining processes

- More cost-effective than most other conventional mining techniques, and thus doesn't require as many "pounds in the ground" to make the mine economically viable

 These advantages allow access to copper deposits not amenable to conventional mining.

 

 

 

In-situ recovery (ISR) is a non-invasive mining method whereby boreholes or “injection wells” are drilled into an ore-body, through which a dilute solution is pumped to dissolve the target minerals or metals. The solution moves through the rock in a controlled manner to nearby recovery wells, where it is pumped back to the surface for processing. Differential pumping rates or natural impermeable barriers are used to control the movement of the solution through the rock. This, combined with well field design, prevent any solution from exiting the mine area. For example, an injection well is usually surrounded by a ring of recovery wells.

The pumping action of the recovery wells make sure all of the pregnant solution is collected from the injection well. For example, in ISR mining of copper oxide ores, the dissolving solution is usually weak sulfuric, the same acid used in open pit and dump or heap leach operations around the world. In effect, the bore holes or wells become the “mine access” and the leach pad is “left underground”. Processing is usually done by chemical precipitation or solvent extraction electro winning (SX-EW).

 

Building of 3D reservoir model and hydraulic fracturing application

The following is the representation of creation of the structural model of small testing area which is in Kupferschiefer deposits in the Lubin region. The model was constructed with the purpose of performing hydraulic fracturing on following testing area (Figure 3).

Figure 3. 3D model of testing area

 

The fracturing process in gOcad provided by a special wizard developed in the gOcad environment. A gOcad Wizard has been written by (Cosson and Chaumont, 2013) for simulating fractures in a medium represented by a Gridded gOcad object. The mechanical properties of the rock massif are stored in each cell, together with the induced pressure.

So, after applying the fracturing has been obtained the results. For better comprehension and to have a good imagination how the fracturing process affects on permeability, the fracturing process has been simulated several times with different values of water initial pressure (Table 3).

 

Table 1. Initial injection pressure values

 

Experiment

Water initial

Pressure (inn GPa)

1

5

2

10

3

15

4

20

5

25

 

Volume of fracked medium

 


During hydraulic fracturing, when the initial injection pressure Pinj increases, as expected the volume of fracked rocks Vf  according to a quadratic form passing through the origin against the initial pressure according to (Figure 4):

 

(1)

Discussion:

The fact that the fractured volume Vf reaches a maximum despite that the pressure is increasing can be related to the Dupuit-Forchheimer radial analytical formula used to depict the pressure P(r) variation in the vicinity of the well. As shown in Eq.(1), when the distance r to the well increases, the pressure in the medium is varying according to P0 / ln(r). So the resulting pressure might not sufficient to fracture the medium when the distance is too big. In other words, after a given pressure sill Pmax which depends on the nature of the rock, the fractured volume remains quite constant. 

Figure 4 illustrates the results of the hydraulic fracturing with the initial pressures (5; 10; 15; 20; 25; (GPa)), respectively. Figure 4 shows the dependence of fracked volume on injection pressure.

Figure 4. Dependence of fracked volume on injection pressure

The maximum fractured volume Vmax depends on the rock type; it seems that more elastic/ductile rocks such as shale with smaller Young’s modulus E ~ 2.6 GPa, but with higher tensile strength at T = 2.7GPa the fractured volume is lower compared to more stiff/elastic rocks such as sandstones with higher Young’s modulus at E ~ 5.9 GPa with tensile strength at T = 1.8GPa. More experiments must be carried out to confirm these observations. 

As shown in Figure 5, it becomes clear that the smallest fractured volume corresponds to the lowest pressure value. The volume of fracked medium increases with the initial pressure; the largest cracked volume corresponding to the highest initial pressure values. The volumes of fractured medium against the initial pressure are reported in Table 2.

   

 

    

Figure 5. Resulting fractures induced by stimulation at different values of injection pressure

 

Table 2. Values of fracked volume in accordance with injection pressure

 

Water initial pressure, GPa

Volume of fracked medium, m3

 

sandstone

 

shale

 

5

304.2

248.4

10

448.0

355.0

15

507.0

355.0

20

512.2

355.0

25

512.4

355.0

 

Permeability of fractures

Before fracturing, the measured permeability values were about 0.6 mD for sandstone and 0.02 mD for shale. The above theoretical results show that they can increase to values as high as from 0.2 to 6 mD assuming an increase factor of 10, and 2 to 60 mD for an increase factor of 100. Fracturing really increases the permeability of blocks. To be more precise a flow model should be applied.

Conclusions

This work investigates the way of implementation of ecologically friendly method which is called In-Situ Recovery in Rudna copper mine site (Poland). This method is already extensively used in uranium mines of Kazakhstan, USA, Australia and etc. The main advantages of In-Situ recovery (ISR) method are: no wastes, no tailings, no underground excavation, no open-pit and etc.

Considered testing mine Rudna is located in Poland, which is the is the largest copper ore mine in Europe and one of the largest deep copper ore mines in the world. As the one of the optimal conditions to use ISR is that the host rock should be fractured enough to let the solution to flow through it. Permeability of rocks in Rudna mine is very low to implement the ISR. As the solution of this problem the artificial way of creating fractures, i.e. hydraulic fracturing was proposed.

Firstly, the testing site I was modeled in 3D using gOcad, rock properties being simulated on Sgrid. Fracturing was obtained using a gOcad wizard and assuming a transverse isotropic medium, elastic properties values being measured in laboratory. Elastic rock properties were deduced from the seismic Vp and Vs velocity; the permeability of the rock before fracturing is very low at 0.7 and 0.02 mD for sandstone ans shale, respectively, increasing to 60 and 0,2 mD after fracturing. The volume of fractured rock increases with the injection pressure Pinj according to a quadratic form, like the surface of the cracked medium

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[3] From http://www.excelsiormining.com/index.php/in-situ-recovery