GAS CONDENSATE RESERVOIRS. Dr. Helmy Sayyouh Petroleum Engineering Cairo University

Transcription

1 GAS CONDENSATE RESERVOIRS Dr. Helmy Sayyouh Petroleum Engineering Cairo University

2 Introduction and Definitions Gas condensate production may be thought of as a type intermediate between oil and gas. The liquid is sometimes called by an older name distillate, and sometimes simply oil. The term wet gas is sometimes used as more or less equivalent to gas condensate. Exist in the reservoir as a vapor ( more or less ).

3 Rule of thumb : a gas condensate exists, if GOR> 5000 scf/bbl and API 50. Exceptions. Production with GOR > 100,000 scf/bbl is commonly called lean or dry gas. Most known retrograde gas-condensate reservoirs are in the range of 3000 to 8000 psi and ºF. Composition is important. Correlation between MW of C7+ and ºAPI of ST liquids.

4 No correlation between GOR and ºAPI of ST liquids. Well completion interval is important: gas cap, oil zone, both? Gas cap may contain gas-condensate or dry gas. Classification of condensate reservoirs depends upon : the composition of hydrocarbon accumulation, temperature and pressure.

5 Phase Diagram Defined for a given fluid composition. If the composition changes, diagram changes. Why retrograde?. The term retrograde is used because vaporization occurs during isothermal expansion, rather than condensation. Condensed liquid is immobile phase envelop shifts to right.

6 The over-all retrograde loss will be greater: for lower reservoir temperatures for higher abandonment pressures for greater shift of the phase diagram to the right.

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8 Calculations of in-place gas and oil Calculate avg. gas gravity, allowing for liquid. Determine oil gravity and MW. Calculate GOR, R and well fluid gravity. Use this to obtain Tc, Pc ; and hence Z. Calculate gas in place,g,from gas equation for one ac-ft, allowing for any liquid ( connate water )

9 Calculations of in place gas and oil Assume that volume fraction of gas in the formation is same as flowing fraction. Find the flowing fraction, fg fg = ( moles of gas prod.) / ( moles of gas prod. + moles of liquid prod.) Initial gas in place = G. fg Initial oil in place = initial gas in place / R Total daily gas-condensate production = daily gas / fg For daily voidage, convert the above to reservoir conditions.

10 Example Calculations of gas and oil in place, per acre-ft. Initial press., psia = 2740 psia Reservoir temp., = 215 º F Porosity = 25 % Initial water sat., = 30 % Daily tank oil = 242 bbl Oil gravity = 48 º API Daily separator gas = 3100 Mcf Separator gas gravity = 0.65 Daily tank gas = 120 Mcf Tank gas gravity = 1.2

11 Performance of Volumetric G-C Reservoirs The content of a PVT cell is expanded from the initial volume to a larger volume at a pressure below the initial. Time is allowed for equilibrium between gas and retrograde liquid. Gas is removed at such a rate to maintain constant pressure in the cell. The volume of this gas is measured at cell pressure and temperature. The volume of retrograde liquid is measured.

12 Performance of Volumetric G-C Reservoirs The cycle expansion to a next lower pressure followed by the removal of a second increment of gas is repeated down to any selected abandonment pressure. Each increment of gas removed is analyzed to find its composition. The volume of each increment of produced gas is measured at Pat. The gas deviation factor is determined.

13 Example Calculating the volumetric depletion performance of a retrograde gas-condensate reservoir based on the given laboratory tests : Initial pressure = 2960 psia Abandon. pressure = 500 psia Reservoir temp. = 196 ºF Connate water = 30% Porosity = 25% Initial cell vol. = cu cm MW of C7+ in initial fluid = 114 lb/lb-mole Sp.gr. Of C7+ in initial fluid = Assume: Liquid recovery is 25% butane, 50% pentane, 75% hexane and 100% heptanes and heavier.

14 Use of Material Balance with Retrograde Condensate Reservoirs The single phase gas material balance may be applied to gas condensate reservoirs under a partial or total water drive. From basic principles: Psc Gp / Tsc = Pi Vi / Zi T P ( Vi We + wp Bw ) / ZT Z is a two phase factor G (Bg Bgi ) + We = Gp + Bw Wp Plot P/Z Vs Gp as in the case of gas

15 Field Application East Texas Field Depth = 7600 ft Area = 3100 acres Permeability = md Porosity = 10% Temperature = 220ºF Initial Pressure = 3691 psia Very heterogeneous reservoir Poor communication between wells Pressure depleted reservoir 600 psia = 48.6 %

16 Well Tests and Sampling To obtain representative samples for laboratory analysis. To make field determinations on gas and liquid properties

17 Well Conditioning Lowest rate, minimum drawdown, gas liquid ratio should remain constant for several days. Recombined separator samples better than subsurface. Exclude produced water. Keep well rate constant during analysis.

18 Sample Collection and Evaluation Samples of gas and liquid are taken from first stage. Obtain bp and separator pr. Determine hydrocarbon composition of separator samples by GLC or fractional distillation. Recombine samples in the ratio produced. PVT Simulated depletion

19 Numerical Simulation of Condensate Reservoirs Compositional simulator should be used for gas condensate reservoir simulation. Use thee dimensional simulation.

20 Water Drive G-C Reservoirs Reservoir pressure declines very little below the initial pressure. Little or no retrograde condensation. Recovery same as in nonretrograde gas reservoirs under the same conditions and will depend upon: 1-initial connate water 2-residual gas sat. in water invaded zone 3-fraction of the initial reservoir volume invaded by water

21 Recovery From Water Drive G-C Reservoirs Recovery = ( 1-Swi Sgr ) F ( 1 - Swi ) Where F is the fraction of the initial reservoir volume invaded.