Research Article. Experimental Analysis of Laser Drilling Impacts on Rock Properties

Transcription

1 International Journal of Petroleum & Geoscience Engineering (IJPGE) 1 (2): 106- ISSN Academic Research Online Publisher Research Article Experimental Analysis of Laser Drilling Impacts on Rock Properties M. Soleymani a,*, M. Bakhtbidar a, Ezzatallah Kazemzadeh a a Research Institute of Petroleum Industry, Iran * Corresponding author address: bakhtbidarm@ripi.ir ARTICLE INFO Accepted: 7 July 2013 Keywords: Laser Drilling Permeability A b s t r a c t The petroleum industry welcomes new technologies that not only increase production rate effectively but also minimize the formation damage. The energy required to remove a unit volume of rock, namely the specific energy (SE), is a critical rock property data that can be used to determine both the technical and economic feasibility of laser oil and gas well drilling. When a laser beam is applied on a rock, it can remove the rock by thermal spallation, melting, or vaporization depending on the applied laser energy and the way the energy is applied. Our experiments showed the most efficient rock removal mechanism would be the one that require the minimum energy to remove a unit volume of rock. This paper will present study results on using a pulsed CO2 laser to drill through the given rock samples. Preliminary test shows that CO2 laser can drill the rock as efficiently as the other types of high power lasers and the permeability of the rock lased by pulsed CO2 laser beam increases up to 150% compared to non-lased rocks due to clay dehydration and microfractures induced by the high temperature gradient and phase transformation volume expansion generated in the rock while lasing. Academic Research Online Publisher. All rights reserved. 1. Introduction In 1997, the first research regarding use of the laser technology in petroleum industry funded by Gas Research Institute (GRI) started by using high power MIRACL and COIL lasers. Studies showed that today's lasers could penetrate all rock types. The second laser drilling study in 2001 funded by the National Energy Technology Laboratory (NETL) under Department of Energy USA cooperative agreement. This study indicated the minimum amount of energy required to penetrate rock as well as the laser parameters that would penetrate most efficiently for each lithology tested: Sandstone, Shale, and Limestone. In 2009, ARPA-E gave Foro Energy Co. a $9.14 million grant to develop a low-contact drilling technology to aid development of geothermal energy, which often must be extracted from very hard volcanic rocks. The agency s goal was to reduce drilling costs by up to a factor of 10, helping to make carbon-free geothermal energy competitive with fossil-fuel

2 energy generation. With private funding, Foro investigated the same technology for oil and gas wells drilling. Finally in 2012 Foro Energy was succeeded in build the first laser drilling machine. The research team in Iran started laser/rock interaction studies since Laser drilling experiments performed using combinations of laser input parameters on several rock lithologies to determine specific cause and effect relationships. Optimized variables were identified and demonstrations conducted to show the capability of lasers to cut tunnels of at least 12-in. deep into sandstone and limestone as well as demonstrated on Bakhtbidar et al (2011) research. One notable advantage resulting from laser perforation on Limestone is the improvement of neartunnel fluid flow characteristics. Measured permeability increased between 80 percent and 190 percent along the tunnel face of a perforation demonstration on a 12-in. sample of Limestone. 2. Stage of Laser Drilling When laser is contacted with surface of stone, under following stages stone is drilled respectively: spallation, melting and evaporating which reported in Batarseh et al.(2003). Upon contact of laser to surface of stone, laser shows the following reactions: beams are reflected, beams are distributed, beams are absorbed as resulted in experiments. Tests indicate that reflected, distributed and absorbed beams have low effect on stone, in fact mechanism that results in spallation and finally drilling rock, is absorbing laser beams as demonstrated in Parker et al (2003). Using laser in rock having high heat transfer coefficient results in evaporating accumulated crystal water with solution mineral materials at stone, expansion of stone and finally fractures made in structure of stone. In different tests it is used from laser working with nitrogen gas. Reason of using nitrogen gas is burning discharged gases during stone drilling and discharging vapor. This gas results in cleaning crushes. In rotating drilling it is used from transferring fluids from bottom of well to surface of well, meanwhile research in the field of laser drilling by presence of drilling fluid is continued by Graves and O Brien (2004). 107 P age

3 2.1. Spallation and Melting Zone Identified When a high power laser beam strikes the surface of a rock, energy will be reflected, scattered and absorbed. The absorbed energy is that which is transferred to the sample, and is responsible for breaking and cutting rock. Depending on the sample composition and properties, absorbed energy will be consumed by various mechanisms, including dehydration, vaporization, grain expansion, melting, pore expansion, decomposition, and other factors. Lasers utilize three methods of rock destruction; spallation, melting, and vaporizing; and can be controlled to the extent of the application of these parameters. Since the temperature of the exposed rock sample remains below the melt temperature of the quartz grains, the primary rock removal method is spallation. The spallation temperatures in limestone have been documented as ranging between C. Should local temperatures rise and phase changes occur in the rock minerals, such as melting and vaporization, absorbed energy is redirected away from the rock cutting process. In the case of Limestone, the mechanism of laser rock interaction with Limestone is different from Sandston due to the chemical composition different. The physical and chemical changes in limestone were different due to mineralogy and chemical composition. Thermal dissociation takes place when limestone interacts with the laser, producing carbon dioxide (CO2), (Equation 1). No melting was observed in limestone, due to the thermal dissociation of CaCO3. CaCO3 CaO + CO2 (1) 3. Method and Discussion Over 18 core samples were exposed to laser energy in laboratory settings. Rocks types lased include Sandstone, Limestone, and Shale (Figure 1). 108 P age

4 Fig. 1. Perforated rock samples. Left to right: Limestone, Shale, and Sandstone. Laser CO 2 system uses from its 100% power by rate of radiation nearly 10mm/s. In this test it was used from spiral mechanism for drilling in which stone was drilled for 30sec with diameter of 1/2 inch. Figure 2 shows the laser machine which have been used for drilling rock samples in this research before and after lasing operation. Laser parameters that control the laser/rock interaction are: laser type, laser power, lasing mode, and wavelength. However, the physical and chemical properties of the rocks also play a major role in determining the nature of the laser/rock interaction. In petroleum engineering applications, certain rock properties are critical for determining flow rate, storage capacity, and overall effectiveness of enhanced oil recovery methods. Fig. 1. Laser CO 2 machine. General Rock Properties Microscopic properties, such as mineralogy, clay content, and microfractures, were determined using a scanning electron microscope with the energy dispersive system (SEM- 109 P age

5 EDS), x-ray diffraction (XRD), and thin sections. Melting temperatures of these rocks were measured using differential thermal analysis (DTA). The Pressure Decay Profile Permeameter was used to characterize the rocks before and after lasing. The PDPK measures point permeability at ambient conditions, Klinkenberg slip factor and the non-darcy flow coefficient. The PDPK is reliable down to a permeability of md and experience has shown it to be repeatable and accurate. This non-destructive, unsteady-state test can measure permeability on irregular shapes, therefore, it an excellent tool to analyze permeability before and after beam exposure. The sandstone core samples consist of grains bound by cementation, and have higher values of porosity and permeability than the limestone core samples. 4. Result 4.1. Change in physical properties of rock Evaluation of rock properties includes a number of physical and chemical analyses at both microscopic (petrographic thin sections) and macroscopic (core samples) scales. Some of the analytical techniques used in this work are standard methods and are actively applied in many engineering disciplines. In the case of some of the analytical methods used (Simultaneous Thermal Analysis) this study represent the first attempt to apply them to evaluate reservoir rock properties. The analysis of the lased rocks showed that porosity (Table 1) and permeability (Table 2). Table.1: Comparison of porosity before and after lasing for three groups of samples. Sample Porosity Before Lased Porosity After Lased Increase % Sandstone Sandstone Sandstone Limestone Limestone Limestone Shale Shale Shale P age

6 Table.2: Comparison of permeability before and after lasing for three groups of samples. Sample Permeability Before Lased Permeability After Lased Increase % Sandstone Sandstone Sandstone Limestone Limestone Limestone Shale Shale Shale These changes are due to dehydrating and disassociating the clay and other minerals and generation of laser-induced cracks, both micro and macro Identifying rock removal zones for limestone The zone change of the rock depends on the laser parameters and the melting temperature of the minerals in the sample. The melting temperature of the rock sample increases with the percentage of quartz in the rock. As the melting temperature of the whole rock increases, rock destruction decreases. Applying this concept to SE, the higher the percentage of quartz, the higher the energy consumed in melting and vaporizing. An example of zone changes as a function of rock type and SE is presented in Figure P age

7 Spallation Zone Transition Zone Melting Zone Fig. 3. Zones identification in limestone sample. The sample used was limestone. The laser power was increased from 40 to 240 W. All other parameters remained the same for this series. In this plot there are two mechanisms clearly observed: the zone on the left is a spalling zone, which occurs at a lower average power, and a melting zone is on the right. A transition zone identifies a region between average powers of 100 W and 125 W where the spalling zone changes to a melting zone. We observed that the lowest specific energy is obtained in the spalling zone just prior to the onset of melting. A possible sequence is that at low laser powers, a considerable fraction of the energy is be consumed by thermal expansion, fracture formation and mineral and gas decomposition, leaving little energy left to effectively remove rock material. As the average power increases, heat transfer and additional reactions occur, removing material more effectively. As the average power increased further, the minerals begin to melt, energy is used for melting instead of removing material and higher SE values resulted. Once melting occurred, the layer of high intense plasma created. Our results demonstrated drilling such the intense plasma need to consume additional energy, and SE values increased further. 5. Conclusion The following conclusions can be drawn from this research: i. Our experiments demonstrated, when a laser beam interacts with a core sample, heat transfer takes place from grain-to-grain by the more direct method of conduction, and across the void space by convection. By applying stress to the limestone sample, more grains are in 112 P age

8 contact with one another, the void space is reduced, and heat is more efficiently transferred by conduction. ii. The alteration in the rock properties can be summarized under two categories. The first one is an increase in porosity and permeability and a reduction in strength by developing macro, as well micro fractures. The second category is changing the matrix properties due to the temperature induced by the laser. Most of the samples lased showed the presence of clays. Clay molecules contain water; consequently, at high temperatures this water is vaporized causing fractures in the samples, on the other hand at lower temperature, clays will collapse and decrease in size leading to higher pore size. iii. This was shown in the case of sandstones. In some shales, depending on organic content, a thick melted material (sheath) was formed, which reduces the laser energy transferred into the rock sample so the changes were not as dramatic. If a sheath was not formed, the porosity and permeability were enhanced. iv. In the case of limestone, the samples tended to break down in smaller fragments, due to the sudden heat caused by lasers and low thermal conductivity of the limestone, which results in thermal heterogeneity and expansion. Acknowledgment Authors would like to special thanks from Upstream Division of Research Institute of Petroleum Industry for its help likewise supports of this research project. Reference 1. Bakhtbidar, M., Ghorbankhani, M., Alimohammadi, M., KazemiAsfeh, M. R., Laboratory experiments investigations of effect of laser energy on variety rock types, SPE no.50475, 73rd EAGE/SPE EUROPEC Conference & Exhibition, (2011). 2. Batarseh, S., Xu, Z., Parker, R.A., Gahan, B.C., Graves, R.M., Figueroa, H., Skinner, N., Specific energy for pulsed laser rock drilling, Journal of Laser Applications, (2003), P age

9 3. Graves, R. M., and Bailo T., Spectral Signatures and Optic Coefficients of Surface and Reservoir Rocks at COIL, CO2, and Nd:YAG Laser Wavelengths, Proceedings of SPIE, (2004), Parker, R., Xu, Z., Reed, C.B., Graves, R., Gahan, B.C., Batarseh, S., Drilling large diameter holes in rocks using multiple laser beams, ICALEO conference, (2003), LIA Tian, Z., Lu L., 2000, Determining Petrophysical Parameters of Rock during Phase Displacement with CT Technique, SPE no.64768, SPE International Oil and Gas Conference and Exhibition, (2000). 114 P age