The purpose of the project is to develop a prototype to establish the technical feasibility of the next generation of drilling and completion technology using high-power lasers.

Gas Technology Institute (GTI), Des Plaines, Illinois
Halliburton Energy Services, Houston, TX
Petroleos de Venezuela-Intevep, SA (PDVSA) (2000–2003), Caracas, Venezuela

This project builds on earlier experimental work (1997-2000) that proved the feasibility of using high-power lasers for oil and gas applications. These experiments, conducted at ANL in cooperation with the Colorado School of Mines, showed that sufficient energy could be generated using a 1.6 kW pulsed Nd:YAG laser beam to remove rock from sandstone, shale, and limestone—but at a high cost.

Results
The results of this study are an extension of the GRI-funded work in two ways:

1. The test plan was developed to measure the amount of energy required to remove material under various laser conditions, and not how quickly a hole could be made into a rock sample (penetration rate). Focus is on trying to minimize the secondary effects that absorb so much of the laser power.

2. By focusing on establishing an absolute specific energy for each sample, it became clear that there is a measurable difference in this value for the different lithologies, sometimes by an order of magnitude. The GRI study did not show this difference conclusively. Instead of making deep, narrow holes in the samples, the hole diameters created by the laser beam were larger than the depth. This, in combination with a coaxial purge gas nozzle, meant that the exsolved gases and spalled particles, the cause of much of the energy robbing secondary effects, were removed quickly enough such that the laser beam was continuously hitting newly exposed rock surface.

The results may not provide perfect measures of the absolute specific energy, but the researchers are confident that the SE’s determined in this study are very close to the intrinsic SE for each sample.

The plan for this study included a series of tests with CW laser beams to determine the absolute SE under the same conditions of the GRI study. It became clear that the CO2 laser at Argonne National Laboratory, under CW conditions, went from merely scorching the rock to melting it without a discernable spalling zone. Based on these preliminary tests, the experiment plan was modified to focus on the pulsing capabilities of the ND:YAG laser. A series of linear tests were done where the power density was changed along the length of the sample. The tests were performed on the Berea gray sandstone, a shale and a limestone. For each combination of peak power, pulse width and repetition rate, a spalling only zone was clearly visible. The power density of that zone was used as the starting point of the test matrix developed for each lithology.

Limestone is the only lithology the absolute SE of which is practically the same as the SE range determined in the GRI study (20-50 kJ/cc). It appears that the hole is made by thermal degradation (CaCO3 to CaO and CO2) instead of breaking bonds between grains or within mineral crystals as is seen in sands and shales, so there is no melting and no secondary effects to cloud the results. Reservoir rocks can be removed using a high power laser beam through thermal spalling, melting, or vaporizing. Thermal spallation is the most efficient rock removal mechanism requiring the lowest specific energy. The laser beam 128 irradiance required for producing the thermal spallation zones are around 920 W/cm2 for Berea gray sandstone and 784 W/cm2 for shale.

The absolute SE required for laser removal of rock material was obtained in this study by carefully controlling the laser beam irradiance and exposure time and avoiding most of the secondary effects. As laser power increased, two rock removal zones, spallation and melting, were identified. In the sample data the lowest SE occurred at the point just prior to melting.

Increasing beam repetition rate within the same material removal mechanism zone would increase the material removal rate due to an increase of the maximum temperature, thermal cycling frequency, and intensity of laser-driven shock wave within the rock.

In the final report the researchers presented studies of the effects of the various Nd:YAG laser parameters on the specific energy for samples of shale, limestone, and sandstone.

The major observation can be stated as follows:

• Measured SE increases very quickly with the beam exposure time indicating the effects of energy consuming secondary processes.
• Shale samples recorded the lowest specific energy values as compared with limestone and sandstone samples.
• As both pulse repetition rate and pulse width increase, the specific energy decreases, however, pulse width is a more dominant mechanism for reducing the specific energy than the pulse repetition rate.
• Two rock removal zones, spallation and melting, were identified in the shale sample data with the least required SE occurring at the point prior to melting.
• Each rock type has a set of optimal laser parameters to minimize SE as observed in the linear track tests.
• Rates of heat diffusion in rocks are easily and quickly overrun by absorbed energy transfer rates from the laser beam to the rock. As absorbed energy outpaces heat diffusion by the rock matrix, local temperatures rise to the minerals’ melting points and quickly increase SE values.
• Sandstones saturated with water cut faster with more power able to be applied before melting commenced.
• The laser is able to spall and melt rock through water.

At the beginning of the GRI study, it was felt that porosity would be an important factor in efficiency of cutting rock, as high porosity rocks would have narrower grain contacts to be broken. It was feared that shale, being non-granular, with no discernable porosity, would not cut well, or at all. The GRI study showed that the shale spalled in a manner similar to granular rocks. All lithologies were shown to 119 have similar measured SE. The behavior of shale is important, as approximately 70 per cent of rock encountered in today’s wells is shale.

Parameters that are probably important, and will be studied further, include:

• Thermal conductivity
• Reflectance
• Color (Albedo)

The physical characteristics of the rocks undoubtedly have a role in how they are affected by laser energy, such as:

• Porosity
• Permeability
• Mineralogy
• Degree of Cementation
• Compressive Strength
• Tensile Strength

Unfortunately, the size of the sample has been revealed to be a secondary effect. Often cracking from the hole to the edge was observed and, when present, affected the SE. Changes in the thickness of the sample also affected the SE. The mechanisms causing these changes are not known.

Benefits
Near-term impact is expected from the spin-off laser completion and stimulation research now being conducted in cooperation with an industry partner. Lasers present the possibility for precise straight or slotted cuts in casings and formations, and based on tests to date, it is believed that lasers may be able to enhance local permeability. Long-term impact from laser drilling will likely be profound, if certain key problems can be overcome. Drilling with lasers under conditions of weighted drilling fluids—most are opaque to lasers—to counteract gas pressures at greater depths is one of the problems.

Summary
Work performed under this contract included design and implementation of laboratory experiments to investigate the effects of high power laser energy on a variety of rock types. All previous laser/rock interaction tests were performed on samples in the lab at atmospheric pressure. To determine the effect of downhole pressure conditions, a sophisticated tri-axial cell was designed and tested. For the first time, Berea sandstone, limestone and clad core samples were lased under various combinations of confining, axial and pore pressures. Composite core samples consisted of steel cemented to rock in an effort to represent material penetrated in a cased hole. The results of this experiment will assist in the development of a downhole laser perforation or side tracking prototype tool.

To determine how this promising laser would perform under high pressure in-situ conditions, GTI performed a number of experiments with results directly comparable to previous data. Experiments were designed to investigate the effect of laser input parameters on representative reservoir rock types of sandstone and limestone. The focus of the experiments was on laser/rock interaction under confining pressure as would be the case for all drilling and completion operations. As such, the results would be applicable to drilling, perforation, and side tracking applications.

In the past, several combinations of laser and rock variables were investigated at standard conditions and reported in the literature. More recent experiments determined the technical feasibility of laser perforation on multiple samples of rock, cement and steel. The fiber laser was capable of penetrating these materials under a variety of conditions, to an appropriate depth, and with reasonable energy requirements. It was determined that fiber lasers are capable of cutting rock without causing damage to flow properties.

Furthermore, the laser perforation resulted in permeability improvements on the exposed rock surface.

(July 2007)
The Project ended and the Final Report is in review.

$3,139,082

$1,424,755

NETL – Rhonda Jacobs (rhonda.jacobs@netl.doe.gov or 918-699-2037)
Gas Technology Institute - Iraj Salehi (Iraj.salehi@gastechnology.org or 847-768-0931)

Gas Technology Institute Presentation - Improving Gas Well Drilling and Completion with High Energy Lasers [PDF-628KB]

Technical Paper [PDF-111KB]