**5. Methodology**

In-situ stress measurement using the hydrofracture method was carried out both during pre mining and post mining stages. Three boreholes were drilled, one each from 184 ML, 124 ML and 64 ML, for post mining stress determination.

The in-situ stress measurement was carried out by using HTPF (Hydraulic Tests on Pre existing Fracture) as introduced by Cornet et al. 1986]. The advantages of HTPF method are

**i.** The boreholes are not required to be oriented along one of the principal stress direction like in classical methods

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extent. In the northern part of the belt the simplest structures are represented by Khetri anticlines and synclines with increasing intensity of deformation. The simple structure passes westward into overturned Kolihan syncline which is slightly compressed in the north.

The southern part of the belt is separated from the central part by a major transverse fault. The southern part of the fault is marked by anticlines and synclines. The asymmetrically over‐ turned Kolihan syncline which is locally recumbent occupies a narrow zone. It plunged towards the SW and in the southern part the limbs are low dipping but gradually steepen northwards. The syncline is defined by the younger quartzites of the Ajabgarh series of reverse

In the scheme of mining with respect to Kolihan Copper Mine the following methods have

In the sub level open stoping method, sub levels are developed at vertical intervals of 18-20 m with a crown level at 9 m below uppermost levels. The size of the stope block is 30 m along

In the blast hole stoping method a drill level is prepared below the crown pillar of 9 m. The size of the stope block is 30 m along the strike, which includes 16.6 m stope and 13.4 m Rib

The mine extends from 486 ML to 0 ML with the surface RL of 486 m. Mining up to 306 ML is complete and presently it is active at 246 ML and 184 ML. Mine development has to commence

In-situ stress measurement using the hydrofracture method was carried out both during pre mining and post mining stages. Three boreholes were drilled, one each from 184 ML, 124 ML

The in-situ stress measurement was carried out by using HTPF (Hydraulic Tests on Pre existing

**i.** The boreholes are not required to be oriented along one of the principal stress

Fracture) as introduced by Cornet et al. 1986]. The advantages of HTPF method are

In the central part of the belt the formations show as anticline structures.

faulting (Dasgupta 1965).

918 Effective and Sustainable Hydraulic Fracturing

**i.** Sub-level Open Stoping method

strike which consists of 20 m of stope and 10 m of Rib Pillar.

and 64 ML, for post mining stress determination.

direction like in classical methods

Pillar. The proposed stopes will be developed at the lower levels.

**ii.** Blast Hole stoping Method

**4. Mining status**

at lower level soon.

**5. Methodology**

been adopted:

**Figure 2.** Status of Mining activities in Kolihan mine (ML= Meter level which indicates altitude from mean sea level)

**ii.** A new induced fracture is not essentially required to be created for stress evaluation. Stress can be evaluated both from preexisting/induced fractures

A schematic diagram showing set up of the hydrofracture system assembly is shown in Fig.3.

The straddle packer assembly (Hydrofrac assembly Fig 4) was used for fracture initiation/ opening and further extension. The straddle packer assembly consisted of a test interval of length 200 mm and two 250 mm steel reinforced packer (42 mm dia, burst pressure = 70 MPa) units attached at either end of the test interval. In the case of hydrofrac experiments in the 48 mm diameter boreholes at the present Project, the straddle packer unit was operated by 1500 mm long and 32 mm diameter tubes (dual line packer inflation + injection unit combined in one). The maximum injection rate of the electrically driven pump was 10 lit /min using water for pressurisation. All the events of injection were recorded in continuous real time digital mode.

**Figure 3.** Schematic diagram of Hydrofrac Experiment Set-up

After all the hydraulic fracturing tests were conducted in all the boreholes, an impression packer tool with a soft rubber skin together with a magnetic single shot orientation device was run into the holes to obtain information on the orientation of the induced or opened fracture traces at the borehole wall.

Two data analyses programmes were used in the analyses. They are called Plane and Gensim.

**6. Stress evaluation procedures and results**

**i.** Presence of anisotropic rock.

**Figure 4.** Hydrofracture equipment used

**ii.** Presence of mining induced stress.

The in-situ stress measurement were made from inside two vertical and one horizontal boreholes drilled from three levels. Tests were conducted with the following situations:

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921

Due to the above aspects a medium to large scatter in fracture orientation data were noticed which negated the use of classical simple hydrofrac hypothesis suggested by Hubert and Wills (1957). Therefore data analysis required a more sophisticated meth‐

The *software Plane* incorporates the impression data with the compass data as input parameters and gives the strike, dip and dip direction (fracture orientation data) as the output.

*The Software Gensim* computes the stress field on the basis of measured shut in pres‐ sure and fracture orientation data. The vertical stress is assumed to be a principal stress and its magnitude is taken as equal to the weight of the overburden. The powerful Gensim programme requires only the shut in pressure and the orientation of an induced or pre-existing fracture

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**Figure 4.** Hydrofracture equipment used

After all the hydraulic fracturing tests were conducted in all the boreholes, an impression packer tool with a soft rubber skin together with a magnetic single shot orientation device was run into the holes to obtain information on the orientation of the induced or opened fracture

**Pump Control Unit (P & F)**

**High Pressure**

**Water Reservoir**

**Pump**

**Rubber Packers**

**Quick connecting high pressure tubing**

**Data Acquisition Systems**

**Digital**

**(Compute r)** 

**Analog (Strip Chart Recorder)** 

Two data analyses programmes were used in the analyses. They are called Plane and

The *software Plane* incorporates the impression data with the compass data as input parameters

*The Software Gensim* computes the stress field on the basis of measured shut in pres‐ sure and fracture orientation data. The vertical stress is assumed to be a principal stress and its magnitude is taken as equal to the weight of the overburden. The powerful Gensim programme requires only the shut in pressure and the orientation of an induced

and gives the strike, dip and dip direction (fracture orientation data) as the output.

traces at the borehole wall.

**Figure 3.** Schematic diagram of Hydrofrac Experiment Set-up

or pre-existing fracture

Gensim.

**Bore hole 76 mm**

920 Effective and Sustainable Hydraulic Fracturing

**Hydrofrac Tool**

### **6. Stress evaluation procedures and results**

The in-situ stress measurement were made from inside two vertical and one horizontal boreholes drilled from three levels. Tests were conducted with the following situations:


Due to the above aspects a medium to large scatter in fracture orientation data were noticed which negated the use of classical simple hydrofrac hypothesis suggested by Hubert and Wills (1957). Therefore data analysis required a more sophisticated meth‐ od, namely the interpretation of measured normal stress acting across arbitrary orient‐ ed fracture planes.

In this method the shut-in pressure Psi is used to measure the normal stress component under the assumption that the vertical stress is a principal stress axis and the vertical stress magnitude σV is equal to the weight of the overburden.

The analysis program *GENSIM* was used to calculate the magnitude and the direction of principal stresses on the basis of the following equation:

$$
\sigma\_{\rm h} = (\text{P}\_{\rm si} \cdot \text{n}^2.\sigma\_{\rm V}) / \langle \text{m}^2 + 1^2.\sigma\_{\rm H} \theta\_{\rm h} \rangle \tag{1}
$$

**Stresses Pre – Mining Stage**

Stress gradient (σH) 0.031 Z +1.5968

Stress gradient (σh) 0.0145 Z +2.3892

**7. Numerical modeling**

of the model is shown in figure 5

Maximum Horizontal principal Stress (σH) orientation

**(486 mL to 184mL)**

R² = 0.91

R² = 0.93

**Table 3.** Comparison between pre and post mining stress gradient

**Figure 5.** Major principal stress contour of the modeled stope.

N 100 to N 200 N 850 to

**Post Mining Stage (184 mL to 0 mL)**

Estimation of the Impact of Mining on Stresses by Actual Measurements in Pre and Post Mining Stages…

0.0048 Z + 21.379 R² = 0.7627

0.01437 Z + 8.412 R² = 0.9862

N 900

A numerical modeling was carried out using the boundary element method to understand post mining induced stresses vis a vis mining. The initial stresses gradient of the pre mining stage was used with gravity loading as the surface topography is hilly. Three observation points were monitored for stress change in mining, due to excavation effects. The stress contour

The results of the stress output as revealed by the numerical model are given in Table 4.

**Remarks**

mining

mining

Rotation of horizontal stress orientation due to stoping

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Change in stress gradient due to

Change in stress gradient due to

Where, l, m, n is the cosines of the direction of the induced fracture plane related to the principal stress axis.

The calculations involve obtaining the best fit based on using all shut-in pressure data derived from the measurements in the boreholes and varying the ratio σH/σ<sup>h</sup> and the strike direction of σH.

**Principal Stresses** σV MPa σH MPa σh MPa Rock Cover Depth m 6.97 8.4 5.6 203 7.88 8.89 5.93 268 10.7 12.65 7.7 364

The pre-mining and post mining stress tensors as revealed are given in tables 1 and 2

**Table 1.** Pre mining stress tensor as revealed by hydrofrac stress


**Table 2.** Post mining stress tensor as revealed by hydrofrac stress

Table 3 shows the comparison of pre and post mining stress gradient


**Table 3.** Comparison between pre and post mining stress gradient
