Method and apparatus for elemental analysis of a fluid downhole

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The present invention provides a method and apparatus for performing elemental analysis of a formation fluid downhole. The present invention provides elemental analysis of a formation fluid downhole using breakdown spectroscopy. In one aspect of the invention, a method and apparatus are provided for performing laser induced breakdown on a formation fluid sample is provided. In another aspect of the invention a method and apparatus are provided for performing spark induced breakdown spectroscopy. Plasma is induced in a fluid under test downhole. Emissions from the plasma are analyzed to determine the elemental composition of the fluid under test. Emissions include but are not limited to light in the ultraviolet, visible, and near infrared regions of the spectrum. A spectrometer is provided for elemental analysis of a fluid downhole. Elemental analysis yields information about the fluid and the formation from which the fluid originated.

The present invention relates to compositional analysis of a fluid sample downhole. More particularly, the present invention relates to the elemental analysis of samples downhole such that they may be analyzed for their constituent components via laser induced breakdown spectroscopy (LIBS), spark-induced breakdown spectroscopy (SIBS) or some similar technique of plasma generation and optical emission analysis.

There are many situations where it is necessary or desirable to obtain substantially instantaneous and/or immediate major and trace constituent analysis of a sample material. Sample materials may include geological samples, soil samples, powder metallurgy, ceramics, food, pharmaceuticals, and many other materials. There are many reasons why it would be desirable to test these materials for their composition of components. Hydrocarbon production is costly and knowing that production of a particular hydrocarbon bearing formation is not feasible due to content of undesirable elements such as sulfur may deem a formation infeasible. Compartmentalization is also a problem encounter during hydrocarbon production and the existence of such compartmentalization is valuable knowledge affecting production decisions involving millions of dollars in production expense.

There is currently no known method and apparatus for performing elemental analysis downhole. It would be useful to perform elemental analysis downhole on formation fluid to determine the characteristics of a formation fluid sample and the formation from which the fluid originated.

The present invention provides a method and apparatus for performing elemental analysis of a formation fluid downhole. The present invention provides elemental analysis of a formation fluid downhole using breakdown spectroscopy. Plasma is induced in a fluid under test downhole. Emissions from the plasma are analyzed to determine the composition of the fluid under test. Emissions include but are not limited to light in the ultraviolet, visible, and near infrared regions of the spectrum. A spectrometer is provided for elemental and compositional analysis of a fluid downhole. Compositional analysis yields information about the fluid and the formation from which the fluid originated. In one aspect of the invention, a method and apparatus for performing laser induced breakdown on a formation fluid sample is provided. In another aspect of the invention a method and apparatus are provide for performing spark induced breakdown spectroscopy. It is well known how to apply breakdown spectroscopy in air in the laboratory at room pressure. However, applying this technology downhole presents several challenges. First of all, downhole fluids are typically under tremendous pressures of 10–20 kpsi. Therefore, to apply such a technique downhole, sufficient energy must be applied over a small enough volume within a short enough period of time (for example by using a strong enough laser or a spark), so as to raise the temperature high enough (about 10,000 C) that the pressure within the plasma exceeds the pressure within the fluid. In this way, it becomes possible for a small bubble of plasma to form within the high pressure fluid. Secondly, it must be possible to detect the light coming from this bubble of plasma even though this bubble is immersed within a dark fluid (such as crude oil) that strongly absorbs the light which it emits.

The present invention is useful for analysis of formation fluid extracted from a dilled wellbore or for analysis for fluid in a monitoring while drilling operation when deployed from a drill string or coiled tubing. The term fluid is used in this specification to mean a gas, fluid or a multiphase mixture of gas, fluid and condensate or particulate suspended therein. In an alternative embodiment, the present invention may also be deployed in a pipeline for analysis of fluid transported in the pipeline. In each case, a LIBS device is provided to perform elemental analysis of a fluid associated with the deployment environment. Similarly, a spark induced spark spectroscopy (SIBS) device may be used in place of the LIBS device for elemental analysis of the fluid. Elemental analysis enables the present invention to estimate the composition of a fluid and estimate a property of the formation from which the fluid originated.

Spark induced breakdown spectroscopy (SIBS), Laser-Induced Plasma Spectroscopy (LIPS) or, as it is more often known, Laser-Induced Breakdown Spectroscopy (LIBS), is a form of atomic emission spectroscopy in which a pulsed laser is used as the excitation source. The output of a pulsed laser, such as a Q-switched Nd: YAG, is focused into or onto the surface or of the material to be analyzed. For the duration of the laser pulse, which is typically 10–20 nanoseconds, the power density at the surface of the material can exceed 1 Giga watt per cm2 using only a compact laser device and simple focusing lenses.

At these very high power densities, a fraction of a microgram of material is ejected from the surface by a process known as laser ablation and a short-lived but highly luminous plasma with instantaneous temperatures reaching 10,000° C. is formed at the surface of the material. Within this hot plasma, the ejected material is dissociated into excited ionic and atomic species. At the end of the laser pulse, the plasma quickly cools as it expands outwards at supersonic speeds. During this time the excited ions and atoms emit characteristic optical radiation as they revert to lower energy states. Detection and spectral analysis of this optical radiation using a sensitive spectrograph can be used to yield information on the elemental composition of the material.

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