GAS DETECTION

HYDROCARBON GAS: DETECTION AND OBJECTIVES

Reasons for gas measurements and monitoring:1. Identify toxic or harmful fluids and gases which may be released at surface2. Monitor pressure differencial between drilling and formation fluids3. Detect well bore pressure imbalance4. Identify zones of abnormal pressure5. Identify reservoir and source rocks6. Characterise reservoir fluids7. Detect mechanical problems at the bit or in the drill string

Components of the Measurement Chain: Degasser or gas trap to obtain continuous, representative samples of the gas contained in the drilling fluid H2S warning system Vacuum system (with drying and filtering equipment), for transport the gas to the Mud Logging unit Total gas or total hydrocarbon detector, for continuous, qualitative monitoring and safety Gas chromatograph, for separation and quantitative evaluation of the individual components (primarily hydrocarbons) Calibration/reference gas system to ensure repeatability and consistency of the measurements Recording equipment

Gas LineSpare Gas LineDryingAgentTotal GasDetectorChromato-graphCompressedAir SupplyVentVentCalibrationSystemRecordingEquipmentDegasserMud LoggingUnitReturn Mud FlowFrom Well(Auxiliary Gas Detection Equipment Not Shown)In-lineH S Sensor2PumpPump

Reservoir Fluid Composition:Oil and gas reservoirs contain fluids which exist in different phases and mutual solutions. The temperatures and pressures found at each depth control the phase type and solution composition.Generally, formation fluids include:

Water fresh water or brine, often containing hydrocarbons, polar gases and inert gases in solution

Oil always containing some water and with associated gas in solution

Gas free gas at the top of the reservoir (usually saturated with water), or dissolved in other fluids

For hydrocarbon bearing reservoirs five fluid types are identified by their chemical and production characteristics:1. Black oil oils which have initial producing gas-oil ratios of 2000 standard cubic feet (SCF)/stock tank barrel (STB) or less. The stock tank oil usually has a gravity below 45 API. These oils are not necessarily black, but may be greenish or dark brown. During production, the stock tank oil gravity will decrease slightly with time.2. Volatile oil generally oils with initial producing gas-oil ratios of 2000 to 3300 SCF/STB. The stock tank oil usually has a gravity of 40 API or higher, with colours ranging from brown through orange and green. The producing gas-oil ratio increases as the reservoir pressure decreases below the bubble point pressure of the oil.

3. Retrograde gas fluids with a producing gas-oil ratio of 3300 SCF/STB or higher, up to about 150.000 SCF/STB; above 50.000 SCF/STB, the quantity of retrograde liquid in the reservoir is very small. The producing gas-oil ratio will increase during production as the reservoir pressure falls below the gas dew-point pressure.

4. Wet gas this term refers to hydrocarbon liquids which condense at surface. Wet gases and retrograde gas are commonly confused, with the term condensate describing both. For a wet gas, producing gas-oil ratios remain steady throughout production.

5. Dry gas the term dry indicates that the gas does not contain enough heavy hydrocarbons to form a liquid at surface; usually some liquid water is associated with the gas.

Composition of Typical Petroleum Gases:

HydrocarbonMethane70 98%Ethane 1 10%Propanetrace 5%Butanestrace 2%Pentanestrace 1%Hexanestrace 0.5%Heptanes+trace 0.5%

NonhydrocarbonNitrogentrace 15%Carbon Dioxidetrace 5%Hydrogen Sulphidetrace 3%Heliumnone trace; some up to 5%

Most Common Gas Readings at Surface: Light Alkanes: C1, C2, C3, iC4, nC4, iC5, nC5 H2S CO2

Units of Measurement: % Gas in Air Parts per million (ppm); 10000 ppm = 1% Units - arbitrary and vary from Client to Client (usually 50 units = 1%; for Petrobras: 30 units = 1%)

Types of Detected Gases:Liberated gas: the gas originally contained in the pore spaces of the rock crushed by the drill bit only.

Produced gas: gas obtained from the undrilled part of the formation, due to drilling fluid pressure lower than formation fluid pressure.

Recycled gas: residual gas in mud, recirculated through mud system and back into borehole.

Contamination gas: gas entering mud from a source other than formation or recycling. May be from mud additives like lubricants or other oil based material.

Background gas: average or a baseline liberatated gas value. Usually low readings, more or less constant ranging from 3 to 10 units, but may reach 100 units. It is typical for a determined interval or formation.

Connection gas: short duration gas peaks occurring approximately one lag time after a pipe connection, caused by reduction of hydrostatic pressure and drill string movement during the connection

Trip gas: gas produced by swabbing during trips or air in the drill string

Mechanism of Gas Liberation:When the bit contacts the formation, gas in the pore spaces of the rock will be released into the surrounding mud. For an undisturbed formation, the volume of gas liberated by the drilling process is proportional to:1. the porosity and permeability of the rock2. the diameter of the well bore3. the rate of penetration4. the mud flow rate

Gas Ratios Analysis:The principle of gas ratios analysis is that increasing hydrocarbon fluid density in the reservoir will result in increasing gas density measured at surface. Thus gas ratios analysis is most commonly used to characterise the reservoir, and, to a limited extent may help identify the source rocks.

One Depth Plots

Triangular Diagram: (C2/CT, nC4/CT, C3/CT)1. A triangle with the apex pointing upward indicates a gas-rich zone2. A triangle with the apex pointing downward indicates an oil-rich zone3. A large triangle indicates dry gas or oil with a low gas-oil ratio4. A small triangle (or nearly invisible) indicates wet gas or oil with a low gas-oil ratio

Format of Triangle Method Gas Evaluation Chart

Defining Ratios Triangle and Determining the Homothetic Center

Pixler Plots: C1/C2, C1/C3, C1/C4, C1/C5The zones are defined as follows: non-productive gas zone primarily methane, probably dissolved in water with no free gas productive gas zone methane and heavier alkanes existineg as free gas in the reservoir, wet gases will ten to appear lower in the zone, dry gases higher productive oil zone gases derived from black or volalile oils; higher API gravity, density or heavy alkane content will result in lower values in the zone non-productive oil zone heavy residual oils, tars or waxes with little methane and poor mobility

Pixler Plot for Gas, Oil Zones

The gas ratio may be interpreted like this:1. a ratio of C1/C2 between 2 and 15 indicates an oil-bearing zone2. a ratio of C1/C2 between 15 and 65 indicates gas3. the lower the C1/C2 ratio, the richer the gas (or the lower the oil gravity)4. a C1/C2 ratio less than 2 or greater than 65 is probably non-productive5. the slope of the line connecting the plotted ratios is a qualitative permeability indicator. Steep slopes indicate tight zones.

Continuous PlotsMethod of gas analysis that can be plotted continuously on a depth based mud log.

Wetness Wh = C2 + C3 + C4 + C5 x 100 C1 + C2 + C3 + C4 + C5

Balance Bh = C1 + C2 C3+C4+C5

Character Ch = C4 + C5 C3

The wetness ratio indicates:Wh < 0.5 = very dry gasWh from 17.5 to 40 = oilWh from 0.5 to 17.5 = gasWh > 40 = residual oil

Balance ratio indicates:Bh > 100 = very dry gasWh in gas range and Bh greater than Wh = gasWh in gas range and Bh slightly greater than Wh = wet or retrograde gasWh in oil range and Bh less than Wh = oilWh greater than 40 and Bh much less than Wh = residual oil

Character ratio indicates:Ch < 0.5; the Wh and Bh interpretation is correctCh > 0.5; gas character indicated by Wh and Bh actually represents oil

Ch is only used when Wh and Bh indicate gas.

Gas Readings in Relation to Pore Pressure Prediction:Gas readings may help in pore pressure prediction and/or interpretation. Usually when reaching a transition zone or a higher pressure zone, the background gas will gradually increase as well gets imbalanced and more cuttings are generated. This will be true if mud weight remains constant.Then gradually produced gas readings will be noticed as any permeable formation is drilled if there are any fractures in the shale of the transition zone.

Also an increase in heavy gases relative to C1 and C2 (C2/C3 ratio decreasing) may indicate entry into a transition zone.

10Ratio C2/C3Total Gas %MWDepth (m)200025003000350040000.110.1110KickBut the best indication of a transition zone or hole imbalance is the connection gas, which will increase as pore pressure approaches mud weight.

GasCGCGCGCGCGCGCGCGCGCGABC

A. Positive, steady differential pressureB. Positive, decreasing differential pressureC. Negative differential pressure

Guiding Horizontal Wells With Gas Indices:At least two criteria must be reached: over balanced drilling, guidance must depend on a number of gas indices (observation of a chosen gas indicator) with respect to a minimum cut off quantity. If gas quantity > minimum cut off, then well trajectory is correct.The most representative gas indicator can be one of the components in particular, or the sum of several of them, which characterise the specific reservoir. The choice should be as simple and pertinent as possible for local production conditions. Whenever possible, light weight components should be preferred (C1, C2 or C3) or sums or ratios of them.Nowadays with the several available LWD tools, navigation inside reservoirs is a much easier processs. However, if inside the same reservoir the oil/gas and oil/water contacts are present, the gas index may be a valuable tool to show whether navigation is taking place inside the pay zone.

Gas Indices for Horizontal Wells

References:Gas Detection: Theory and Practice (Geoservices, 1995)Guiding Horizontal Wells According to Gas Indices (Geoservices, 1995)Validation of Gas Micro-indices During Drilling (Geoservices, 1997)Mud Logging Presentation During PERFORM School (2003)