Reservoir engineering aspects of tight gas reservoir evaluation

1Tight Gas ReservoirsProfessor Ole Torsæter11/11/11

2Unconventional natural gas resourcesWhat is really considered unconventional natural gas changeswith time. However, there are six main categories ofunconventional natural gas:• Deep gas• Tight gas• Gas-containing shales• Coalbed methane• Geopressurized zones• Hydrates

3Resource PyramidLarge VolumesSmall VolumesGasShalesHighQualityMediumQualityTightGasLowQuality1000 md100 md1 md0.1 mdCoalbedMethane0.001 mdIncreased DemandBetter Technology0.0001 mdContinued Drilling and Development

6Tight natural gas• Permeability less than 0.1md(1*10 -16 m 2 )• Porosity approx. 5-15%• Limestone or sandstoneformations• Not readily extractable. We mustput more effort into theextraction process than forconventional gas reservoirs; likefracturing, acidizing etc.

7Shale gas• In Devonia shales (formed in shallow seas more than350 million years ago).• Permeability in the nanoDarcy range (10 -9 D or 10 -21m 2 )• Difficult to produce due to the properties of shale andtherefore the expected recovery of the gas is low.• Shale gas is found all over the world.

8Shale gas

9Statoil1,8million acres=7300km 2From Statoil.com

10The Marcellus Shale• Organic rich shale at a depth of approx. 1000m• The gas occurs in three ways:– Within the pore space of the shale– Within fractures (vertical) and joints in the shale– Adsorbed on mineral grains and organic materials• Most of the recoverable gas is contained in the porespace.• Natural fracture system is important for wellproductivity.

11Coalbed methane• Many coal formations (seams) contain natural gas,either within the seams itself or the surrounding rock.• What was once a by-product of the coal industry isbecoming an increasingly important source ofmethane and natural gas.

12Productivity of tight gasreservoirs

13Presentation Summary• Some additional information about tight gasreservoirs• Conditions generally required for economic tight gasreservoir production• Common formation damage types occurring in tightgas reservoirs

14‘Conventional’ vs 'Unconventional'Accumulations• Characteristics of ‘Conventional’ Accumulations– Relatively high matrix permeability– Obvious seals and traps– High recovery factors• Characteristics of ‘Unconventional’ Accumulations– Regional in extent– Diffuse boundaries– Low matrix permeabilities– Low recovery factors– Productivity of a tight gas reservoir is only 25% of gas in place in the best case(average 10-15%). Gas connectivity only in 4% of porosity.From John Lee (Texas A&M University)Advances in Unconventional Resources Technology: Assessment Methodology

15What controls the ability toeconomically produce tight gasreserves?• Effective permeability to gas• Initial saturation conditions• Size of effective sand face drainage area accessed bythe completion• Reservoir pressure• Degree of liquid dropout from gas (rich vs. dry gas)

16Capillary equilibrium in gasreservoirs – High PermCapillary Pressure - PsiRelative PermeabilityFWCWater SaturationWater SaturationBrant Bennion, presentation at Canadian Well Logging Society

17Capillary equilibrium in gasreservoirs – LOW PermCapillary Pressure - PsiRelative PermeabilityFWCWater SaturationWater SaturationBrant Bennion, presentation at Canadian Well Logging SocietyJune 9, 2004

18Generally if a tight gas matrix is inequilibrium with a free watercontact,→equilibrium water saturationreduces reserves and effectivepermeability to gas below theeconomic limit for production

19Non - capillary equilibrium in gasreservoirs – LOW PermCapillary Pressure - PsiRelative PermeabilityNO FWCWater SaturationWater SaturationBrant Bennion, presentation at Canadian Well Logging Society

20For significant reserves and mobile gasproduction in very low perm gasreservoirs,a CAPILLARY SUBNORMAL watersaturation condition usually must existWater Gauge

21Subnormally water saturated tightgas reservoirs:• A gas reservoir in which the initial water saturation isless than that which would be achieved on aconventional drainage capillary pressure curve at theeffective capillary gradient of the reservoir

22Some interesting aspects of tightgas relative permeability.

23Relative permeability of tight gas reservoirsDemonstration cross-plot of drainage and imbibition gas-water relativepermeability.

24Comparison of relative permeability between conventional andunconventional gas reservoirConventional reservoirUnconventional reservoirConventional reservoir: wide range of water saturation where both water andgas can flow.Unconventional reservoir: broad range of water saturation in which neither gasnor water can flow.Shanley et al. AAPG Bulletin 2004

25Numerical studyNote crossover, where krg=krw isapproximately 67% for all permeabilities butkrg value at crossover decreases withdecreasing permeability. Dark blackhorizontal line marks the krg = 2% (0.02).The Sw region where both gas and waterhave kr < 0.02 broadens as k ik decreases.

26Capillary pressure of tight gas reservoirComparison of capillary pressure between conventional andunconventional gas reservoirShanley et al. AAPG Bulletin 2004

27Relationship between capillary pressure, relative permeability andposition within a trap.Shanley et al. AAPG Bulletin 2004

28Shanley et al. AAPG Bulletin 2004

29Fracturing of tight gas reservoirs.

31Performance of fractured wells in tight gas reservoirs.Frac technique: horizontal well withmultiple fracturesTest rate: 14,000 m 3 /h vs 4,000 m 3 /h withouttechnologySeven Fracture system ready to open sliding sleeves3.5 increase in well productivityFrom Gaz de France

32Causes of productivity loss offractured wells in tight gas formation• Rheological fluid properties (affectfracture length)• Polymer residus in the fracture (affectfracture conductivity)– with cross-linked fluids– up to 90% of conductivity loss– optimization of cleaning fluids like"breakers"• Filtration (affects reservoir properties)– fracturing fluid invasion into thereservoir (decrease of the fracturelength, reservoir damage around thefracture)• Water blocks (affect gas back flow)– decrease the gas permeability

33Water blocks investigation.Aim:• Simulation of the filtration of the fracturingfluid and evaluation of damage.Method• Cores at Swi (gas)• Fracturing fluid filtration with 200 bars oncore face• Gas backflow at various pressures• Water blocks identified by XRayacquisitionResults• Water saturation profiles• Gas return permeabilityConditions of the equipment• Direct measurement of water blocks byXRay• Temperature: 130°C, Pressure 330 barsFracturing fluid filtration200 barsGas Back flow5, 10, 15 bars

34Flow rig at Institut Francais du Petrol.x-rayfluidscore holderTmax =130°C (210°F)Pmax=330bar (4700psi)

35Workflow for improved predictionsFracturing fluid selection- Lab studyfiltration, absolute permeability damage,measurement of in situ saturation (waterblocks), petrophysics (capillary curves)Modeling at laboratory scaleReservoir and fracturedescriptionModelling parameterscake properties, absolute permeabilityreduction, relative permeability curvespermeability hysteresisNear Wellbore SimulatorWell Productivity Evaluation

36Conclusions•Tight gas reservoirs have a huge future potentialfor production.•Generally to be economic tight gas reservoirs arenormally in a subnormal water saturation condition.•Fluid trapping (water blocks) tends to be adominant damage mechanism for tight gasreservoirs.•Productivity prediction is almost impossiblewithout detailed petrophysics data measured inrepresentative conditions (stress, hysteresis,capillary pressure...)