P2-2-23

Experimental and numerical analysis of using laser for perforation operation in sedimentary rock reservoirs

Mohsen Bazargan1,2

, Agust Gudmundsson1, Philip Meredith

2

1Royal Holloway University of London, Egham, UK,

2University College London, London, UK

Introduction

Economical production volume in oil and gas industry means the maximum use of each well. Some applications are increasing well productivity, such as hydraulic fracturing, acid treatment, thermal treatment, horizontal drilling, etc. In addition, each of these methods has some disadvantages which may effect on the reservoir properties. The effect of each one of these methods is often limited by connections between wells and producing formation, which may be caused by low permeability, heterogeneity of reservoirs or insufficient length of each perforation.

In upstream oil and gas industry, perforation operation is including a range of process to creating effective tunnels through the cemented behind the steel casing and rock texture to let the formation fluid flow into the well. During the past decades the petroleum companies has been applying the wellbore perforation method introduced in the 19 century which is using explosive shaped charges. This conventional method crushes the rock matrix and produces fine grain particles which might plug or decrease the pore size which are leads to formation damage and hence remarkable permeability reduction in the rock formation. These disadvantages can increases the wellbore skin and consequently decrease production. Petroleum companies has been looking for a decades to find non-explosive and non-damage methods that not only create perforation holes effectively but also minimize among of formation damage.

Considerable research has been shown that laser system can be reliable source of cutting and penetrating in different material. In this case, studies around the development of the theoretical and practical laser drilling models are usually includes many detailed from investigations. The carbon dioxide (CO2) laser is the powerhouse for high tech industrial cutting and welding of metals and many other materials. Small CO2 lasers are used for marking of metal, wood, and composites, and in medicine and surgery. Even a 'small' CO2 laser produces 10s of watts of beam power and the largest are in the 100 kW range. Its output is at 10.6 um (10,600 nm) or mid-IR. This wavelength is about 10 to 30 times longer than the other lasers under discussion and often considered a source of a heat beam than a light beam.

Recent advances in high power laser technology provide an apparatus to replace the conventional perforation methods in oil and gas wells. High power lasers are capable of cutting and removing rocks textures efficiently and they might be considered as one of the appropriate substitutions for current shaped charge perforation methods. According to its advantages the conventional shaped charge methods that one the important one is increasing permeability considerably and no need to have costly re-perforation operations to decreasing new formation damage named by perforation skin. Laser perforation is gone along with heat flux generation. As the temperature increases during laser induced operation, thermal energy accumulates the matrix expansion. This expansion generate thermal stresses induce the rock texture. Furthermore, thermal stresses exceeds the rock strength, thermal fractures will form in texture that mainly depend on rock thermal properties, pore size distribution, applied thermal stresses and confining and pore pressures. In this paper, the results of experimental studies on implementation of high power laser in perforation and fracture initiation in oil and gas wells is presented. Also, numerically analyzing of generating these thermal fractures during thermal perforation will facilitate hydraulic fracturing operation.

Numerical Modelling

Within one 2D system under transient condition, without material properties changing or in-situ hear generation, equation (1) can show reliable output:

2

2

2

21

y

T

x

T

t

T

∂

∂+

∂

∂=

∂

∂

α

(1)

Owing the fact that temperature with change by time and in specimen body during heat transfer, then above equation will discrete by place and temperature. Dynamic modeling will be done by time discrete:

tpt ∆= (2)

Here t∆ is time period.

Finite difference equation for T related to t is like equation (10), in this equation results can be

more reliable when t∆ decrease.

t

TT

t

Tp

nm

p

nm

nm ∆

−≈

∆

∆+

,

1

,

,

(3)

By placing equation (10) in equation (8) by assuming yx ∆=∆ (that can let us use rectangle

meshes) then temperature distribution in p+1 can be detect from equation (11).

)()(

1,

1

1,,1,12

1

,

p

nm

p

nm

p

nm

p

nm

p

nm TTTTx

tT +++

∆

∆=

−

+

+−+

+ α

(4)

Results and Discussion

Laser produced plasma because it’s one of the fifth interactions in materials, on the surface of samples, causes absorption and scattering laser energy. Therefore, the laser drilling operation can be controlled by reducing the density of the charged system. Number of researchers studied the characteristics and behaviour of spatter and related the in ounce of different assist gases on the mechanism of ejecting the materials or cutting removal during laser drilling. The quantitative of the characterization of spatter geometry is so important when laser drilling occurred really close to hole and it could lead to an increase in the spatter bonding strength as a result of the overlapping is between adjacent holes. The deposition characterization of spatter.

Fig1. XRD tests of Limestone after laser irradiation test.

Increased efficiency of the laser beam will show their results in increasing vaporization and reduced crummy material. In limestone as it shows in Fig1. After laser irradiation, crummy material is not proper limestone which is CaCO3 anymore. It changes to CaO follow by eq5.

CaCO3 + Heat ==> CaO + CO2 (5)

Reflected and scattered beam have losses while it is the absorbed beam that is responsible for rock heating, expansion, melting and destruction. As it shown in Fig2 the absorbed energy is utilized for fusion or melting, vaporization or spalling of the rock. It has been found that rock spalling is the most effective and important, the desirable mode for rock expansion and exchanging the rock surface. These fractures around the laser irritated zone can be very helpful initial cracks for hydraulic fracturing (HF). During spall process the rock absorbs heat resulting in development of cracks within the rock which are caused by beam transmission. Then the rock weakens and breaks away. High intensity laser energy on the rock concentrates locally on the rock surface area and causes the melting temperature to increase simultaneously. This temperature

induces a local thermal stress in subsurface that is enough to spall the rock. Spall process requires pretty less specific energy and rock removal occurred when the specific energy is high and make vaporization happening. Also the smooth edge, stability of the hole walls and boundary shape and conditions of laser perforation zone might lead to easier HF operation later on.

Fig2. Laser irradiation experiment method, samples before, during and after laser tests

Otherwise the laser energy will break and remove rock without significant melting Fig2. Laser rock spallation is a rock removal process that utilizes laser induced thermal stress to fracture and cause a break through the rock by creating small fragments before melting of the rock. At low desired for the laser to work within the spall process zone and as close to the transition zone because there is a small narrow line of difference between melting part of the rock and warm part of that. Hence, efficiency of lasers in rock to generate fractures with removing the rock samples during penetration. This process is surely make some changing on permeability and porosity of specimens, weather caused by pore fluids absorption or design of tests.

The foundations of perforating techniques are created to make flow path from hydrocarbons to be production well. That path had been pass throw the casing, cement, skin zone, to catch reservoir to improving the rate of production.

Fig3. Laser perforation modelling based on finite difference method code in MATLAB

One of the problems of conventional perforation methods such as gun shoot Jet is their damage zone. Mainly because these tools add additional skin to the overall skin factor number and also reduces reservoir rock inside effective property like porosity and permeability. In addition, their rate of penetration is not high enough to in most of cases to pass through the whole skinned zone and provide better production rate of the oil or gas produce well. In this part of the paper we focused on modelling of laser perforation operation. In Fig3. We show how 10X10 centimetres cubic limestone sample with be perforated by laser CO2. Fig3 shows the cubic from the top view and how laser will start from the middle of the surface on left side boundary and penetrated to the other side. This model was prepared in MATLAB based on heat transfer and thermal coefficients of Limestone. In this model laser as the only source of heat start to irradiating on the rock specimen and when each element each their vaporization temperature, laser heat source will move from the element to the next element and this process continues until the whole sample were perforated. This models had been match with the time of operation with experimental samples to show how long does it actually going to take to penetrate the 10 centimetres Limestone and this experimental results from Fig2 were matched with Fig3 to verify this model.

Conclusion

For thermally induced rock samples, it has been observed with experimental data, that the expansion of rock structure had generated fractures in rock body.

The mechanism of laser perforation is based on heat transfer and removing rock structures, by the time and the power of laser.

The energy requirement is different between spalling or cutting the rock and melting or vaporizing. Particularly in the limestone samples cause it seemed to spall through a thermal distribution mechanism rather than breaking or melting with constant temperature induced.

References

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Acknowledgements

The Author is sincerely thanking Sharif University of technology, Tehran University and Tarbiat Modares University for their research grant support. And also Weld on Sweden company and Amir Kabir University for their contribution.