A new recombination technique for the gas condensate reservoir fluids

Gas Condensate Reservoir Analysis

Phase BehavoirGas condensate field produce mostly gas, with some liquid dropout, frequently occurring in the separator. The phase diagram shows the retrograde gas field must have a temperature higher than the critical point temperature. The vertical line on the phase diagram shows the phase changes in the reservoir, while the curve line shows these changes as the fluid cools going up the wellbore and into the separator.In both cases, liquids drop out as the pressure drops below dew point pressure.With the retrograde condensate, the %liquid begins to increase to point "A" then decreases with further pressure declines. Thus the name "retrograde" meaning to retreat or go back. So first condensation and then vaporization occurs, and this vaporization can help in further recovery of liquids.

Any hydrocarbon above the dew point line is 100% gas. Any hydrocarbon above the bubble point line is 100% liquid. Hydrocarbons above the bubble point line and close to the critical point are volatile oils. Thecricondenthermis the maximum temperature which two phase flow can exist (maximum temperature on the dew point line of the phase diagram). But a field may have both a oil leg and gas cap, which during depletion produces some condensate from the gas.Fields with active water drive may experience little pressure declines, so condensation occurs only at surface and a constant GLR would be expected.Specific gravity and hydrocarbons in place

Recovery estimatesIn cases where an active water drive will support pressure, so condensation occurs in the surface separators, the conventional volumetric estimates of gas recovery are possible, where Er=(nitial gas volume - final gas volume)/ final gas volume. This is the ultimate or displacement recovery. A sweep factor can be added, since so: Er = (Gi-Gf)/Gi*F.

technique for the gas condensate reservoir fluids

Obtaining representative reservoir fluid samples has become of increasing importance in the

development and exploitation of oil and gas condensate reservoirs. This is especially true of

reservoirs where extensive computer simulations are used to scope out developmental strategies or where enhanced oil recovery options are investigate. Representative fluid samples can usually be obtained from producing reservoirs at surface conditions. Surface samples are removed at either the separator or at the wellhead, with the associated gas and liquid subsequently recombined in proportions to represent the actual reservoir fluid.Unfortunately, in gas condensate reservoir the properties of surface gas are not the same as the properties of the reservoir gas. Liquid condenses from the reservoir gas as it moves from reservoir conditions to surface conditions. Thus the composition of the surface gas is quite different from the composition of the reservoir gas, having considerably fewer of the intermediate and heavy components. Some simulation efforts have been made for gas condensate recombination process based on the matching of initial gas condensate dew point pressure with dew point of the recombined sample. Most of them suggested calculation procedures for the recombination at surface facilities based on dew point pressure matching with initial one. For this purpose, the well head separator gas and condensate samples are mixed at bottom-hole pressure and temperature to represent the initial dew point of reservoir. The equilibrium phases are then mixed with each other to produce a dew point pressure equal to original fluid. In this work, an experimental procedure has been proposed to obtain the best representation of initial reservoir fluid from well head separator samples in a depleted lean gas condensate reservoir that was initially undersaturated. Condensate stabilization technique (CST) is a new recombination method applied to a real lean gas condensate field in Iran namely Sarkhoon to obtain a fluid with the same initinal dew point and gas-condensate ratio (GCR) of the reservoir.Sarkhoon gas field, discovered in 1972 by drilling exploration well SA1, is located 20km away from Bandar-Abbas in south of Iran. The reservoir temperature is 382 K. The available data of the reservoir fluid are initial and dew point pressures of 5200 psia and 4988 psia with a producing CGR of 30 STB/MMSCF, respectively. The pressure and temperature of the test separator in 1976 was 1000 psia and 98 F respectively.

Recombination set up consists of a PVT cell and two high pressure cylindrical stainless steel vessels of type 316 equipped with sealing pistons with the volumes of 300 and 1000 ml, respectively. 1/8 stainless steel tube type 316 is used to connect all parts of the apparatus. A gas chromatography (GC), (Varian model 3800, USA), that is equipped with a Thermal Conductivity Detector (TCD) and Flame Ionization Detector (FID) was used to analysis the samples and recombined fluids. Separator gas and condensate were obtained from South Zagros Oil and Gas Production Company.The average absolute deviation of 4.19% for the GCR and 3.4% for the dew point pressure are achieved for the mixture behaved the phase behavior of initial reservoir fluid with an acceptable accuracy.