The patent's assignee is
News editors obtained the following quote from the background information supplied by the inventors: "The past decade has witnessed dramatic changes in the oil and gas industry with the advent of deep-water exploration and production. Deep water exploration and production favours the formation of solid ice-like materials known as gas hydrates or clathrates. Clathrates are formed when polar molecules such as Water (H.sub.2O) align through hydrogen bonding effects under the conditions of high pressure and (often low) temperatures typical of such deepwater locations to form hollow cage like structures that can trap and hold Carbon Dioxide (CO.sub.2), Methane (CH.sub.4) or other gases. These solid materials remain until they are subject to a change in their formation conditions (e.g. lower pressure or a higher temperature) that causes them to dissociate and release the trapped gases back to the surrounding atmosphere. There are several types of hydrate structure e.g. sl and sll that can form depending on the conditions of temperature, pressure and hydrocarbons present. It is known that sl hydrates form with lower molecular weight hydrocarbons such as Methane (CH.sub.4), and that sll hydrates form preferentially with the presence of heavier hydrocarbons such as Propane (C.sub.3H.sub.8) or n-Butane/Iso-Butane (C.sub.4H.sub.10) making deepwater exploration where these are prevalent in the production streams potentially even more hazardous.
"One of the major challenges within deepwater field development is to ensure unimpeded flow of hydrocarbons to the host platform or processing facilities; the early detection of the formation and managing the remediation of solids such as hydrate, wax, asphaltene and scale is key to the viability of developing deepwater prospects.
"One of the problems other than blockage is the movement of the hydrate plugs in the pipeline at high velocity, which can cause rupture in the pipeline. Any blockage in an oil/gas pipeline due to hydrate is a serious threat to capital equipment and personnel safety. A number of strategies exist to inhibit or stop hydrate formation within transfer line or process facilities and one traditional approach is to remove or change one of the elements that favours hydrate formation such as temperature or pressure.
"Examples of such strategies include thermal insulation or the external heating of transfer lines, water removal from natural gas using glycol dehydration systems, lowering operating pressure (mainly for removing blockage) or chemical approaches such as adding inhibitor materials to the system. Although often effective in reducing the formation of solids or treating the problem after the event, they increase
"In support of these strategies, attempts have been made to detect early hydrate formation and a conference paper published by Tohidi et al. (
"An alternative approach to measuring electrical properties was proposed by Tohidi et al. and is revealed in patent application, WO2006/054076. The method is based on identification of water memory by freezing point measurements. One drawback of this method is that freezing point measurements have a highly stochastic nature requiring a certain number of measurements to achieve the desired reliability/probability for hydrate early warning. Moreover, this freezing-point-based hydrate memory could be easily weakened even fully destroyed by the presence of certain hydrate inhibition additives.
"There is no method or apparatus available for detecting either the onset or the early formation of hydrates in practice that could be used to inform the existing hydrate reduction strategies briefly outlined above--such a system would potentially reduce the need for high
As a supplement to the background information on this patent application, NewsRx correspondents also obtained the inventors' summary information for this patent application: "The present invention provides a method of detecting clathrate formation, in particular hydrate formation, the method comprising: monitoring a gas phase of interest, wherein the gas phase is a mixture of gases comprising at least one clathrate (e.g. hydrate) forming gas and the monitoring includes determining the concentration of at least one component of the mixture of gases; or determining a ratio of concentration between two components of the mixture, at least one of which is the said clathrate (e.g. hydrate) forming gas; and detecting a change in the determined concentration or in the determined ratio of concentration, relative to an earlier determination or relative to a pre-established base level concentration or base level range of concentration.
"The monitoring of the gas phase may be continuous. For example continuous, typically automatic, sampling of a gas phase. Continuous monitoring may be by measurement at regular intervals (e.g. with an arrangement that takes samples at regular intervals and directs them into the analytical instrument) or even by means of constant analysis of the gas phase, for example by continuously measuring the gas phase by means of a suitable analytical instrument such as a Gas Properties Transmitter as discussed hereafter. Alternatively sampling may be intermittent with samples taken as thought necessary, for instance if a process upset has occurred and there is special concern that hydrates may be formed.
"The following discussion of the invention is described in terms of hydrate formation. It will be appreciated that the formation of other clathrate materials may be detected in the same fashion, where a liquid other than water together with clathrate forming gases can form a clathrate structure.
"The gas phase may be for example a gas phase associated with a hydrocarbon production stream from a gas and/or oil production well or collection of wells. In general a production stream will include three phases, gas/oil/water leading to the potential for hydrate formation. The gases present will typically include hydrate forming hydrocarbons (methane, propane, butanes etc) as well as e.g. carbon dioxide and nitrogen.
"The gas phase monitored may be the naturally occurring gas phase in the production stream or may be a gas phase evolved from the aqueous phase or dissolved in the aqueous phase or even a gas phase evolved from or dissolved in the oil stream. As discussed below and with reference to specific embodiments the three phases are normally separated for processing/use of the gas and oil in a separator unit that can provide a convenient location for monitoring gases of one or more of the phases. Whilst gases dissolved in an aqueous phase or an oil phase may be conveniently monitored by causing the gases to evolve from the liquid phase e.g. by reducing pressure and/or by heating; it will be appreciated that dissolved gases in a liquid phase may be analysed directly, for example by hplc techniques. In general when sampling a hydrocarbon production stream process according to the methods described herein sampling points may be located at any location where an early indication of hydrate formation is useful. For example at a wellhead or a slugcatcher.
"A change in a concentration or in a ratio of concentration may be observed by an operative who obtains the results of monitoring and makes calculations to check for a change in a value (concentration or concentration ratio) as appropriate. Advantageously a change is detected automatically and the result provided to an operative. For example the results of monitoring are compared with previous results or a base line level in a computer associated with the monitoring equipment. An alarm indication may be provided to alert an operative to a change, typically when a change is more than a predetermined minimum value.
"Advantageously, especially where a hydrocarbon production stream is being monitored a ratio of concentration is measured, typically a ratio of concentration between two hydrate forming gases as discussed hereafter. Alternatively the ratio of concentrations between one hydrate forming component and another non hydrate forming component may be measured. This method can be notably sensitive especially when monitoring a gas evolved from or dissolved in an aqueous phase.
"Most hydrocarbon systems have propane and butanes in varying amounts, promoting sll hydrate formation. Other gases such as hydrogen sulphide (H2S) also have a high affinity towards hydrate formation. As sll hydrates form they preferentially trap higher molecular weight hydrocarbons in their structures. This results in a reduction in the concentration of such compounds in the gas phase. On the other hand, the dissociation of hydrates (due to an increase in the system temperature and/or a reduction in the system pressure in their path to the production unit) results in a temporary increase in the concentration of these heavier compounds in the aqueous phase. Monitoring can be carried out on the gas content in or evolved from that aqueous phase. Using this behaviour we have found that we can detect whether hydrates are forming by directly measuring the changes in the composition or the ratio of the concentration of light (low molecular weight): heavy (high molecular weight) hydrate forming components present in the gas phase and/or the gas phase released from the produced water phase (i.e., the composition of gas present in the aqueous phase), as hydrates form, deep underwater or in transport lines. This technique can be used for detecting early signs of hydrate formation and as an early warning system against blockage due to hydrate formation.
"Very lean gaseous systems (rich in methane and low in higher molecular weight hydrocarbons) form sl hydrates and a similar approach can be used for detecting initial signs of hydrate formation in such systems.
"A further advantage of this method is that by knowing the speed of travel of the 2 respective gas and aqueous phases in the pipe or transfer line and the time lag between them arriving at a suitable detector it is possible to estimate the distance of the hydrate formation/blockage point from the detector.
"The present invention therefore provides a method for detecting the early formation of hydrates, the method involving: Analysing a gas phase of interest to establish a 'Base level' concentration profile for each component present and/or ratios of different components and where appropriate establishing upper and lower limits for each component at the 'Base level' Continuously or intermittently monitoring the concentrations of the components present and/or ratios of different components in the gas phase of interest Recording the concentrations of each of the components present and/or ratios of different components in the gas phase of interest ('Measured level') Comparing the 'Measured level' vs. the 'Base level' concentration profile and/or ratios of different components data Reporting any changes in the 'Measured level' vs 'Base Level' concentration profile and/or ratios of different components data Deciding on the significance of any change with respect to the probability of hydrate formation and Providing an automated 'warning indicator' or similar notification to an agent that hydrates are forming enabling appropriate action to be taken
"The hydrate former can be any organic or inorganic molecule capable of being trapped within a hydrate structure as it forms or being released from a hydrate structure as it dissociates.
"The gas phase of interest contains at least 2 different components; at least 1 of these is a hydrate former.
"The gas phase of interest is typically a mixture of more than 2 components; at least 1 of these is a hydrate former.
"The gas phase of interest ideally contains a mixture of hydrate former molecules of between 1 and 'n' hydrate formers present in a range of concentrations; n is a multiplier with a value greater than zero (a whole number).
"The gas phase of interest may be evolved from the dissociation of a hydrate material or released from an aqueous phase upon depressurisation; the hydrate material may be of any type.
"The gas phase of interest may additionally contain one or more of Nitrogen (N.sub.2), Carbon Dioxide (CO.sub.2), Ammonia (NH.sub.3), Water (H.sub.2O), Carbon Monoxide (CO), Hydrogen Sulphide (H.sub.2S), Hydrogen (H.sub.2), Oxygen (O.sub.2) or other gases.
"The hydrate former molecules preferably contain Carbon (C) and Hydrogen (H.sub.2) and may be saturated, unsaturated or cyclic in nature; examples of saturated hydrate former molecules of this type include Methane (CH.sub.4), Ethane (C.sub.2H.sub.6), Propane (C.sub.3H.sub.8), n-Butane and Iso-Butane (C.sub.4H.sub.10), Pentane isomers (C.sub.6H.sub.12) etc. with a general molecular formula, C.sub.nH.sub.2n+2, where C is Carbon, H is Hydrogen and n is a multiplier with a value greater than zero (a whole number).
"The hydrate former may be free of Carbon (C); examples of such formers include Ammonia (NH.sub.3), Hydrogen (H.sub.2) or Hydrogen Sulphide (H.sub.2S).
"The hydrate former may be free of Hydrogen (H); an example of such a material is Carbon Dioxide, CO.sub.2.
"The hydrate former may be from the family of Noble gases; an example gas of this type is Argon, Ar.
"Optionally the methods of detecting formation of clathrates such hydrates described herein may include the step of adding of one or more gases that form clathrates or hydrates to a gas phase being monitored. These tracer compounds may be, for example, the various hydrate forming compounds discussed above, where they are not already present or present in sufficient quantity to permit operation of the method with the analytical equipment being employed. Alternatively or additionally other compounds such as SF.sub.6 or fluorocarbons such as R152a (1,1-difluoroethane) that are not normally found in oil production streams may be employed as tracer compounds. Tracer compounds can also include the odorants such as dimethyl sulphide and t-butyl mercaptan that are normally added to gas supply systems to allow easy detection of leaks. Certain compounds (e.g., H.sub.2S) can be detected and monitored at very low, for example ppm or ppb levels. Thus they can be employed economically as tracers in the method. The tracer compounds may be hydrate formers or non hydrate forming.
"The concentration or change in concentration of the hydrate former(s) in the gas phase of interest is determined and recorded using a suitably accurate analytical technique capable of distinguishing the components of a mixture including at least 1 hydrate former species from each other. Gas chromatography is one such suitably robust technique offering repeatability of results and high accuracy that can be used in-line (with a suitable sampling loop), at-line or off-line in a laboratory situation. An alternative instrument is a 'Gas Properties Transmitter (GasPT)' that is commonly used in gas transportation & processing technology for measuring various properties of gas mixtures. A GasPT instrument can provide a sufficiently accurate estimate of gas component concentrations for the purposes of the present invention, by carrying out measurements of various physical properties of a gas mixture. These measurements of physical properties such as thermal conductivity and speed of sound are used to infer gas phase concentrations assuming four components (methane, propane, carbon dioxide and nitrogen). As illustrated hereafter by an example, the concentrations determined by such an approach can be used to indicate hydrate formation. Other instruments can be used in the same way to take measurements of physical properties of a gas phase (such as speed of sound, thermal properties, electrical properties) to determine concentrations by similar estimating techniques, where a gas phase composition is inferred.
"The concentrations of the hydrate formers in the gas phase of interest are optionally measured and recorded on a continuous or intermittent basis to establish a 'base level' or 'steady state condition' over a period of time. Establishing a 'base level' is preferred as it will show natural variation in gas composition with time (background noise) but is not essential as sudden changes in concentration between subsequent measurements can be indicative of hydrate formation. It is also possible to predict expected (base level) concentration and then measure actual concentrations for comparison against the predicted level. For example the composition of gas released from a separator unit in an oilfield production stream can be predicted on the basis of the feed composition and the pressure and temperature conditions. Gas released from an aqueous phase produced at the separator will also have a predictable composition. These predictions can provide a baseline level.
"During operation, the concentration profiles of the hydrate formers are measured and recorded continuously or intermittently and compared to those of the 'base level' or a recent measurement and any changes in concentration profile noted.
"A change in the concentration and/or the ratio of the measured hydrate former concentration to each other or a non hydrate former in relation to the baseline or steady state condition is indicative of the onset of hydrate formation, e.g. for a mixture of Hydrocarbon formers/components: A decrease in the Low carbon number hydrate formers e.g. Methane (CH.sub.4) in the gas phase is indicative that Type sl hydrate is preferentially forming A decrease in higher carbon number hydrate formers (e.g. Propane & Butanes) in the gas phase is indicative that type sll hydrates are forming.
"The information on the change in ratio of hydrate formers to each other can inform or provide a warning to an agent that hydrates are forming and that remedial action is required.
"The change in ratio can be sudden or gradual; sudden changes in concentration could be indicative of potentially catastrophic equipment failures such as thermal insulation loss, heater failure or of process issues such as flow rate changes, inhibitor injection pump dosing problems etc. on a transfer line causing rapid hydrate formation.
"The method for detecting hydrate formation can be implemented in different approaches according to the quantity of hydrate that forms in relation to the gas flow rate: (1) If more than 5 Barrels of water is (could be) converted into gas hydrates per 1 MMSCF (MMSCF or Million Standard Cubic Feet is a volume unit in the petroleum industry) of gas transported (also assuming the background noise is not too high), direct analysis of the composition of the gas phase can be applied to identify early hydrate formation, following the procedures described previously. (2) If less than 5 Barrels of water is (could be) converted into gas hydrates per 1 MMSCF of gas transported, the gas released from aqueous phase after hydrate dissociation can be analysed. This option can be used when the change in the gas phase composition is not sufficiently large for ready detection e.g. due to background noise.
"Option 2, above, is also applicable at higher hydrate formation rates. However either approach may be useful in some circumstances irrespective of amount of hydrate being formed provided suitably sensitive measurement techniques are employed. Advantageously both the options above are employed.
"The methods described herein may be carried out manually, with an operator carrying out the monitoring using suitable analytical equipment and then acting on the results obtained, or partially automatically, or fully automatically. For example the analytical equipment may be programmed to calculate when a change in concentration, from a baseline level or from an earlier measurement exceeds a predetermined level and signal an alarm. Alternatively a fully automatic system may send an alarm signal to a control system which takes a predetermined remedial action (process adjustment) or emergency action such as a shut down procedure.
"The present invention provides an apparatus for carrying out a method of detecting clathrate formation, in particular hydrate formation, the apparatus comprising: means for monitoring a gas phase of interest that comprises at least one clathrate forming (e.g. hydrate forming) gas, the monitoring including determining the concentration of at least one component of the mixture of gases; or determining a ratio of concentration between two components of the mixture, at least one of which is the said clathrate forming (e.g. hydrate forming) gas; means for detecting a change in the determined concentration or in the determined ratio of concentration, relative to an earlier determination or relative to a pre-established base level concentration or base level range of concentration; and means for generating an alarm signal as a result of the detected change in the determined concentration or in the determined ratio of concentration.
"Typically the apparatus includes: a sampling device (e.g. a valved port in a pipe or vessel), to obtain from a process stream a gas sample and/or a water sample containing dissolved gases; optionally an apparatus for reducing pressure of a water sample to release dissolved gases into a gas phase; analytical apparatus (e.g. a gas chromatograph or a GasPT device) to determine gas concentration or ratio of concentration; and a processor e. g. a PC or a processor within the analytical apparatus, to carry out calculations of concentration or concentration ratio and compare with earlier results or a base level value or range and to generate an alarm signal. The alarm signal can be output to a visual display (e.g. a screen or a light) and/or an audible alarm sounder (e.g. buzzer or siren).
"The present invention provides a system for controlling a hydrocarbon production stream process, the system comprising: means for monitoring a gas phase from the process to detect hydrate formation in the hydrocarbon production stream in accordance with the methods described herein; and a control system operable in response to an alarm signal generated as a result of the monitoring to carry out adjustment of process conditions and/or shut down procedures. The adjustment of process conditions can include a change in injection rate of hydrate inhibitors
"Typically the system will also provide an alarm to a process operator, for example to allow manual override.
"The present invention provides a method for estimating the position of hydrate formation in a pipeline carrying both a gas phase and an aqueous phase, wherein the gas phase is a mixture of gases including at least one hydrate forming gas, the method comprising: determining the velocities of the gas phase and the aqueous phase in the pipeline; monitoring at a sample position, both the gas phase and the aqueous phase, to detect formation of hydrate by the methods described herein; and estimating the distance of the hydrate formation position from the sample point based on the time difference between detection of an indication of hydrate formation in the gas phase and in the aqueous phase and the determined velocities of the respective phases in the pipeline.
"The velocities of the gas phase and of the aqueous phase can be determined on the basis of the flow rates of the respective phase, pipe dimensions and geometry, physical properties of the phases
BRIEF DESCRIPTION OF THE DRAWINGS
"Various aspects of the invention will now be described by way of example only and with reference to the following drawings, of which:
"FIG. 1 shows a simplified schematic diagram of a method of detecting hydrate formation;
"FIG. 2 shows a schematic diagram of an apparatus for testing hydrate formation; and
"FIG. 3 shows schematically a simplified Gas/Oil/Water separator system."
For additional information on this patent application, see: Tohidi,
Keywords for this news article include: Gases, Alkanes, Methane, Propane, Elements, Hydrogen, Chemistry, Hydrocarbons, Carbon Dioxide, Carbon Monoxide, Inorganic Chemicals,
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