gas ref_purpose, procedure, install

7/31/2019 GAS REF_purpose, Procedure, Install

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GAS REFERENCING PURPOSE, PROCEDURES and INSTALLATION

Gas Referencing was developed and patented by Mr. Randall Amen, from Estes Park,Colorado. The U.S. patent was granted in 1994, and as many as 54 international patentsare pending or awarded. The technology was first disclosed publicly at the Houston

Geological Society workshop on Advanced Hydrocarbon Detection Technology in 1992,presented jointly by Mr. Amen and Mr. Wayne Greb, a consulting wellsite geologist from

Amarillo, Texas. The Gas Referencing technology has been successfully used since 1989in a number of onshore areas in the Michigan, Permian, and Williston basins, in SouthTexas, and in offshore California and in the Gulf of Mexico.

PURPOSE: Gas Referencing is used to improve formation evaluation, drillingoptimization, well completion, and safety. It makes mud-gas measurements moremeaningful by maintaining a small, but precise concentration of a select reference fluid inthe mud and monitoring that fluid at the same place in the circulating system asindigenous formation gases. Gas Referencing is used in four ways:

1) Meaningful volumetric gas-in-mud measurements aredetermined from chromatographic measurements using a reference gasadded to the drilling fluid. Resulting measurements are in volumetric unitssuch as in3 gas/ bbl. mud or percent of mud.

2) Meaningful volumet ric gas-in- form ation calculations are madefrom gas-in-mud, drill time, pump rate, and hole (bit) size. Resultingmeasurements are in volumetric units such as f t 3 gas/ f t 3 formation orpercent of form ation.

3) On-l ine Qualit y Assurance continually checks the system and dataquality. If the reference gas is not being measured, something is wrong withthe detection system.

4) Convenient Lag Measurement is easy, routine, and non-disruptive.

PROCEDURES: A non-indigenous reference gas such as acetylene or propylene iscontinuously injected into the drilling fluid, at the suction line of the pumps. The amountis small and designed to result in a nominal concentration of0.01% by volume atsurface conditions. At the bit, formation fluids are released into the drilling fluidcontaining the reference gas. At the surface, the reference gas and formation gases areextracted in proportion to their respective concentrations and submitted to a gaschromatograph for analysis. Most of the remaining reference gas is expelled from thedrilling fluid along with formation gases by surface mud conditioning equipment (shaleshakers, pit stirrers, hydrocyclones, degassers, etc.) allowing continuous reference gasinjection at the suction line without buildup or recycling.


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Since the concentration of reference gas in mud is known along with its measurement atthe chromatograph, gas-in-mud can be determined by normalizing the formation gaseswith the reference gas. This assumes that the reference gas is acted upon in a way similarto the formation gases in the extraction and measurement process.

Gas-in-formation calculations can be made knowing the volume of mud required to drillan interval of formation, the volume of gas in the volume of mud, and the volume offormation in the interval drilled.

System and data integrity are assured since the reference gas is always present tomeasure. If the system stops measuring the reference gas consistently, diagnostics canbe initiated to identify and correct the problem.

By momentarily increasing or decreasing the reference gas injection rate, a pulse or traceris created which can be used to measure lag. This does not interfere with the drillingoperation and it will not damage downhole tools.

Volumet ric gas-in-m ud from chromat ograph measurements and reference gasamount :

The formation gas measurements at the chromatograph can be converted to gas-in-mudby dividing the amount of the component formation gas (i.e., Methane in ppm) by theamount of reference gas (in ppm) and multiplying by the concentration of reference gasin the mud. Conversion is made by the logging computer for each chromatogram sincechanges in mud properties, gas extractor efficiency, evacuation rate, etc. continually affectthe amount of gas in the gas stream being submitted to the chromatograph for analysis.

Example: Suppose the chromatograph result shows 100 times moremethane (12,500ppm) in the gas sample than reference gas (125ppm).With the reference gas concentration in the mud being maintained at aknown amount (i.e., 0.01% by volume at surface conditions), the amount ofmethane being 100 times that of the reference gas is 1% by volumemethane-in-mud at surface conditions.

Volumetric gas-in-format ion f rom gas-in-mud, dri l l t ime, pump r ate, and bitsize: Gas-in-formation can be calculated knowing the volume of drilled formation, theamount of mud circulated during the time to drill the interval volume, and the amount of

the gas-in-mud minus the background gas-in-mud. This calculation assumes that theformation is drilled overbalanced and that no flow or production from the formation intothe wellbore occurs. Measured gases must be liberated only.

Example:An 8.75" drill bit cuts 721.6 in3 of formation per linear foot.


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Drilled at a rate of 2 min./foot at a pump rate of 5 bbls./minute, 10 bbls. arecirculated while one foot of formation is drilled.With methane-in-mud at 1% and a hypothetical background gas of0.75%...

(1.0% - 0.75% ) x 10 bbls. = 0.025 bbls. = 242.55 in.3

242.55 in.3 methane from 721.6 in.3 formation = 33.6% methane-in-mudat surface conditions.

If the reservoir pressure and temperature are known, this volume at the surface can beconverted to a volume in the reservoir or a percent gas saturated porosity of thereservoir.

On-l ine Qualit y Assurance of system and data int egrit y: An important feature ofGas Referencing is its inherent capability to continually monitor and assure system anddata integrity. Decision-making is fundamentally affected by data quality and oftendictates whether right decisions are made. Mud gas measurements are affected by severalcomplex factors which often vary during the course of drilling a well. These factors includepenetration rate, pump rate, mud properties, extractor efficiency, evacuation rate andmeasurement system performance. Lost or partial returns change the immersion depthof the gas extractor thereby affecting its efficiency. The amount of gas in the mud alsochanges the extractor efficiency thereby changing the measurements. Partial degradationor complete system failure such as broken or frozen sample lines, extractor plugged withcuttings, filter leaks, instrument fluctuations, etc. all affect the resulting measurements.Unfortunately, these changes in the measurement system integrity are often subtle and

difficult to recognize.

Gas Referencing solves this problem by providing built-in quality control as itcontinuously injects a small, but precisely known amount of reference gas into the mud.

Any change in system performance, whether it be subtle, substantial, or sudden isimmediately and clearly revealed as changes occur in chromatograph measurements of thereference gas.

Decision-making is improved when interpreters are able to distinguish between valid andquestionable data.

Lag Measurement s: Gas Referencing provides a convenient and non-disruptive meansto make lag measurements. By momentarily increasing (or decreasing) the injection rateof the reference gas, a tag or tracer is introduced into the drilling fluid. This eliminates anypossibility of damage to downhole tools and can be made without having to wait for aconnection. The tracer can be used to follow a sweep to clearly determine when it hasreturned to the surface. This technique has facilitated locating washouts in the drill stringwhen the tracer returned much earlier than expected.


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Gas Referencing is easy to install and requires very little hardware. Onthe rig, a inch NPT injection port is installed at an accessible but out-of-the-way locationon each mud pumps suction line between the pits and the charging pump. Where thecharge pump(s) are supplied by only one suction line, only one injection port is necessary.This port can simply be drilled and tapped or a collar can be welded to the suction line and

then drilled through. The port is preferably on or near the top of the suction line. Ifnecessary, it can be installed on the bottom but is more likely to require maintenance dueto being plugged with solid materials from the mud. The potential problem is exacerbatedby heavier muds with higher solids contents. A inch NPT ball valve is then added tocomplete the installation of the injection port(s). Pressure at the injection port is due tothe hydrostatic head of the mud in the mud pit. This is typically less than five (5) or six(6) psi.

Polyethylene tubing (0.25" x 0.17" x 150 psi) or reinforced acetylene welding hoseconnects the injection port(s) to the flow controller and supply. The lines should runvertically from the inj ection port to a height above the top of t he mud pit beforegoing to the flow controller. Although a check valve is installed in the logging unit, thishelps keep the lines from filling with mud more than a few feet from the injection port.The lines are routed to the logging unit and attached to bulkhead fittings on an enclosedflow-control panel outside the logging unit. In operation, the flow in these lines is bothvery low volume (on the order of 3-5 cubic feet of reference gas per day) and very lowpressure. This is approximately 200 times less than that of an ordinary welding torch. Iffor any reason these lines were cut, it would take about three to four months to completelyempty a full acetylene tank. During use, t he integrit y of t he lines should be routinelytested by simply closing t he ball valve at the injection port (s), w aiting a fewminutes, and checking for flow in t he logging unit . There will not be any flow if the

lines are intact.

The reference gas (acetylene or propylene) is supplied from one of two tanks locateddirectly outside the logging unit and must be properly rack mounted for safety. When thefirst tank is consumed, use the second tank and order another tank to replace the emptyone. An acetylene tank holds approximately 360 cubic feet of acetylene and should lastone to four months depending on injection rate and number of lag measurements.Propylene lasts about three times as long as acetylene. The tank valve is opened witheither a handle or quarter-inch wrench. The regulator is installed on the tank valve andset to 15 psi for acetylene and 100 psi for propylene. Be sure to test for leaks each timethe regulator is installed using a weak soap solution. Use the tank valve and regulator

to shut off t he system.

In the logging unit the logging computer actuates solenoid valves which control thereference gas injection. Each solenoid valve directs the reference gas to the suction linesof the pumps that are in use at the time. These solenoid valves are located on the flowcontrol panel. A gas mass flow controller also located on the flow control panel, takes


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pump rate information from the pump stroke counters, and controls the rate of referencegas injection and maintains the concentration of reference gas in the drilling fluid.

I nject ion rate: The nominal injection rate is 0.01% by volume at surface conditions.This equates to about 0.97 in3/bbl or about 16 cc/bbl. This amount can be increased or

decreased depending on upon the threshold sensitivity of the instruments, as long as therate is known and constant. As the pump rate changes, signals from the pump strokecounters pass through the logging computer to the gas mass flow controller which changesthe injection rate accordingly, in order to maintain the reference fluid concentration.

Presentation: There are three gas track presentation scales. First, raw values from thechromatograph are recorded in ppm or percent. Secondly, normalized volumetric gas-in-mud values are in percent of mud or in3/bbl. Thirdly, normalized volumetric gas-in-formation at surface conditions values must be modeled to reservoir conditions (pressure,temperature, and Z-factor) then displayed in percent gas in formation or ft3 gas/ ft3

formation.

gas ref_purpose, procedure, install

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