Sandy Hook #1: Produced Water Volume Inversion

Transcription

1 Brine evaporates Gas condenses Sandy Hook #1: Produced Water Volume Inversion Overview This chapter discusses the common client application of producing gas at a low water cut. In these cases, the production water undergoes evaporation and condensation along the production line. Some of the formation water evaporates as pressure drops under constant temperature. The water then re-condenses with possibly additional condensed water near the surface, as temperature drops quickly, relative to pressure, as shown in the diagram below. A conventional gas well comes on line producing a small amount of water. This reservoir is a high pressure, high temperature tight sand. You will model the initial reservoir fluid is set to 100% (mass basis). There is a significant pressure drawdown from the reservoir to the well, which creates an evaporative environment. Up to 50% of the brine evaporates into the gas. As the fluid flows up the well, T & P decrease, and some of the H 2O returns to the liquid. Downstream in a cold pipe (i.e., a seabed flow line), there is a sharp temperature drop relative to pressure. Additional H 2O condenses from the vapor, and now starts to dilute the original brine salinity. The brine composition and properties change with location, and the laboratory data will vary with sampling location. The true (reservoir) water composition will need to be computed from whatever sample was done and at the conditions it was sampled. Sandy Hook #1: Produced Water Volume Inversion 18-1

2 Input Information The following tables contain the brine and gas analysis data. The tables also contain information about sampling and production conditions. The brine analysis below contains several elements, that are ambiguous, since they do not exist, they are not stable, in water. Task #1 Create the Production Brine The instructions for this case will be terse, since it is assumed that the reader is familiar with the software interface. Create a new and save it as SSC Sandy Hook or another filename The hypothetical case is the Sandy Hook field off the coast of New Jersey (wishful thinking, perhaps). We can write this in the description tab for the Brine we just created. Add a new Brine Analysis and label it Sandy Hook #126 Enter the Cation and Anion data from the table below Cation mg/l Anion mg/l Elements mg/l Properties (measured data) Na Cl Si 11 ph 7.0 K SO IC 46 Alkalinity 255 Ca B 10.6 Density (g/ml) Mg S 47 Elec Cond (mho/m) 14.9 Sr TDS (mg/l) Ba Fe+2 26 Table 1 Brine sample taken at the Test Separator. There is the issue of the four elements, Si, IC, B, and S that cannot be entered directly. These elements can be entered but their concentrations need to be adjusted. The conversions are straightforward. Si is silica and the multiplier i, IC is bicarbonate, B is boric acid, and S is ambiguous. Here are the multiplication factors. Element Species Multiplier Si SiO B B(OH) IC HCO S SO4-2 or HS- 3.0 for SO for HS-1 Enter the Si and B elements in the Netutrals section. They will convert directly to SiO2 and B(OH)3. Use the multipliers and enter the concentrations Enter into the HCO3-1 row the IC x 5.08 concentration Sandy Hook #1: Produced Water Volume Inversion 18-2

3 Ignore the S, since you don t know if its S(VI) or S(-2) or a combination of those and other S oxidation states. Select the Show Non-zero Only box Click on the Reconcile tab Enter the Properties data from Table 1 above (7.0 ph, 255mg HCO 3/L alkalinity, g/ml density, 14.9 mho/m conductivity, and mg/l total dissolved solids) Calculate Compare the measured and computed values. The computed ph is the same as the reported value, and the computed alkalinity is lower than measured. Task #2 Create the Production Gas Add a Gas Analysis and name it Sandy Hook #126 Gas Select the Design tab then enter the composition from the table below Component Mole% Component Mole% N2 1.0 i-c4 1.6 CO2 2.0 n-c4 0.7 H2S 0.01 i-c`5 1.1 CH n-c5 0.8 C2 3.2 C6 0.4 C3 2.7 Keep Makeup as the Normalize Option and the box next to CH4 checked Sandy Hook #1: Produced Water Volume Inversion 18-3

4 The software will adjust the CH 4 value automatically to create a 100% composition. Note that there is no reported H 2O. You will next estimate the H 2O content using the Saturate With calculation in the Reconcile tab. The H 2O results will be overestimated slightly because this calculation assumes pure water with ideal activity. We will improve on the calculation in the next task. Click on the Reconcile tab Change the Saturate With button from None to H2O Enter the Test Separator conditions of 40C and 30 atm Calculate The calculated H 2O saturation is 0.27 mole%. The software automatically adds this to the gas composition, and reduces the CH 4 content accordingly. Task #3 Selecting process point to Set Mass and Energy Balance The water and gas are sampled at the test separator, a process point where the phases are at or near equilibrium. Equilibrium is a critical simulation state, because it enables simulation software to set mass and energy balance within a process. Thus, finding a point in the process where we can equilibrate the gas with the water is critical from a modeling point of view. The simple flow diagram below illustrates this point. Mass Gained (e.g., corrosion, additives) Production Gas Inlet to Process (Deep Reservoir) Production Line - Static Reservoir to Separator Production Oil Mass Lost (e.g., scale, leakage) Production Brine Thermodynamic models are used to compute the System State of the multi-phase fluid at each process point, i.e., from the reservoir to the surface outlets. Computing a system state requires that we know the total, mass, composition, temperature, and pressure. Mass is computed from volumetric flow rates at a known temperature and pressure. Compositions are obtained from sample analysis, and temperature and pressure are obtained from measurements or from fluid flow models. Sandy Hook #1: Produced Water Volume Inversion 18-4

5 Accounting for every mole of material improves thermodynamic accuracy. That is why getting the correct gas-phase water content is important. In this case, the test separator is an ideal Mass and Energy Balance set point, because at that location, the fluids flow rates were measured, the phases were sampled, and the T, P conditions are known. Task #4 Saturating the Gas and Brine at the Test Separator sampling point - Equilibrium State #1 The Test Separator is the chosen point along the production path where the gas and brine are set to equilibrium. Add a Saturator and rename it Test Separator Stop here we need to change the units to metric Side Task Change Units to Metric Creating the Scale-Metric Units Set We have to modify the flow units before proceeding. Open the Units manager shown below and confirm the Inlet units are set to the values Returning to the Task Make sure you are in the Design tab of the Test Separator object Add the Sandy Hook #126 brine to the first row at 5.5 m3/day Add the Sandy Hook #126 Gas gas to the second row at 800 std E3m3/day Enter the Test Separator conditions of; 40C, 30 atm Calculate (<F9>) Sandy Hook #1: Produced Water Volume Inversion 18-5

6 Select the Report tab and scroll to Gas Composition at the bottom of the report The computed H 2O content in the gas is mole%, lower than the mole% computed Task 2. This difference resulted from using brine instead of pure water to equilibrate with the gas. The brine has a lower water activity and draws H 2O from the vapor 1. Confirm this by viewing the water volume in the Saturation Data table (top of screen) and then the total Cl - concentration in the Brine Totals table. Review the Standard Conditions volume at the bottom of the Phase Properties table The lower row is the standard volume. The liquid increased by m3. This is a small value, and the difference is due to water condensation. The volume at 40 C and 30 bar is the upper row. It is a m3 increase and is due to water condensation and thermal expansion. Scroll down to the Brine Composition section Compare the computed Cl(-1) to the amount 74,100 mg/l entered in Task 1 The chloride concentration is lower mg/l, and reflects the dilution effects of condensed H 2O. Therefore, the H2O content in the gas is NOT precisely the value needed. We need to remove some H 2O from the gas analysis, so that it does not condense into the brine phase. This will then make the equilibrium chlorides at or close to 74,100 mg/l. This will give us a more representative condition. Modifying the Gas Analysis with the Computed Water Content We will assume that the above H 2O value in the vapor of 0.257% is more accurate than that estimated in the Gas analysis object. We will make the change to the Sandy Hook #126 Gas. Click on the Sandy Hook #126 Gas object in the Navigator Pane Select the Design tab and the Inflows grid, then enter the value for H2O 1 We care about brine salinity because the salinity affects the H 2O activity coefficient. This in turn, affects the H 2O vapor pressure, which is what determines what the gas-phase H2O (v) will be. The following correlation, derived from chemical potential equations, illustrates this: γ H2 O (activity of water) = f(salinity & ion interactions) = p H2O = real H 2O partial pressure X H 2O p H 2O pure H 2 O partial pressure If the activity coefficient were equal to one (i.e., ideal) then the real H 2O partial pressure would equal to the ideal partial pressure. As the water activity deviates from one, the real and ideal vapor pressures diverge. Thus, the H 2O vapor pressure produced by a dilute water (γ 1) will differ from that produced by a highly saline water (γ 1). Sandy Hook #1: Produced Water Volume Inversion 18-6

7 Select the Reconcile tab Change the Saturate With button to None (THIS IS IMPORTANT) If we do not set the Saturate with to none, then the software will override the entered H 2O value. Calculate Retesting the Test Separator calculation Click back on the Test Separator object and Calculate Review the Standard flows According to these results there is virtually no mass transferred. There is partitioning of gas components to the water and visa versa, but this is small relative to H 2O transfer. Notice that the 40 C/ 30 atm is m3, or about 0.9% greater than the standard volume. The standard volume differs by 0.12% and is the result of other gas components dissolving in the water. Review the gas composition and brine volume tables, and the chloride concentration The calculated values should be near / almost identical to the value entered in the gas. This also confirms negligible partitioning. Lastly, review the chloride concentration The chlorides are now mg/l, slighly lower than the input value, but this is because the liquid volume increased slightly because of thermal expansion. The brine composition table is based on the measured volume, and not the Volume at standard conditions. Sandy Hook #1: Produced Water Volume Inversion 18-7

8 The purpose of this step is to reduce the error between the daily mass rate of material exiting the separator in the field compared to this calculation. If this calculation is matches the field mass flow, then there is greater confidence in taking the next steps. Task #4 Equilibrium State #2, Static Reservoir conditions The test separator fluid (brine and gas) was used as the mass balance and equilibrium unit. The assumption is that any mass exiting the reservoir flows into this separator. Therefore, the mass rate at the separator equals the mass rate at the perforations. This assumption is the basis for the next calculation. Add a new Saturator and name it Static Reservoir Select the Whole Fluid, Test Separator and keep the flow <Automatic> This confirms 100% mass balance between the test separator to the static reservoir. Enter the Reservoir temperature and pressure of 130C and 455 atm Calculate Review the Report and compare it to the Test separator properties Static Reservoir Report at 130C and 455 atm Sandy Hook #1: Produced Water Volume Inversion 18-8

9 Test Separator Report at 40C and 30 atm The Reservoir ph - is lower than the laboratory- and test-separator value. This is because of the high PC O2. The Reservoir brine mass flow (Mass kg/day) is about 37% of the test-separator. Water (H 2O) mass partitioned to the vapor phase because of the high reservoir temperatures The total mass (Aqueous + Vapor) is kg/day are identical for both calculation. This confirms that the software conserved mass and between the two calculations. The Ionic Strength (salinity) at reservoir conditions is about three-times higher than separator, which is caused by water evaporated. The electrical conductivity at Reservoir is nearly five times than at Test Separator. This is because of the higher salinity and the higher temperature. The Osmotic pressure (used in membrane applications) also differs by a factor of five. Osmotic pressure is a function of water activity and temperature. The following equation shows this. Π water = RT V ln (a water) The Gas flow rate is 2.9 E 3 m 3 /day at Reservoir compared with 27 E 3 m 3 /day at Test separator. This is the effect of compression. The standard gas flow is sm3/d compared to 800 std E 3 m 3 /day in the Test separator. This small difference is the result of water evaporation. Review the Gas Composition table The H2O content in the vapor is about 3.5 times greater in the gas phase. This difference results in the salinity difference between the reservoir and test separator water. Static Reservoir Scaling Tendencies Review the Scale Tendency table Test Separator Scaling Tendencies Sandy Hook #1: Produced Water Volume Inversion 18-9

10 There are four minerals computed to be near- or super-saturated at reservoir conditions. This compares to one mineral pyrrhotite at separator conditions. Static Reservoir Scaling Tendencies Review the Brine Composition table Test Separator Scaling Tendencies The reservoir chlorides (and other ion) are three-times the separator concentration. This is expected, based on the water mass computed to evaporate per the bullet points above. Static Reservoir Brine Composition Test Separator Brine Composition Sandy Hook #1: Produced Water Volume Inversion 18-10

11 Task #5 Computing fluid properties as it flows from Static Reservoir to the Surface The case created above has two distinct sets of fluid properties; static reservoir conditions and test separator (sampling point). This is by design since the objective of this example case is to show how the chemistry of a fluid can change with condition. Our interest now is to see how these conditions change as the fluid flows through the production line. The table below contains the temperature/pressure conditions. We will enter them in a Scale Scenario and compute the fluid properties, including scale tendencies. Add a Scale Scenario and name it Sandy Hook #126 Production Select Whole Fluid, Static Reservoir, automatic flow Click on the blue, Conditions tab and enter the conditions from the table below Location T, C P, atm Static Reservoir Reservoir Drawdown Bottomhole Tubing Safety valve Wellhead Choke Flowline 1 km Flowline 3 km Flowline 5 km Riser base 5 80 Topside Separator Select the Solid tab then select Standard box Calculate then select the Plot tab and view the default pre-scaling tendencies plot Sandy Hook #1: Produced Water Volume Inversion 18-11

12 The initial view shows a maximum y-axis scale value of ~20 for FeS, which is compressing the other values. If we remove the FeS pre-scale tendency or change the y-axis scale to log, we can view the remaining lines. Right-mouse click on one of the y-axis numbers Select Logarithmic scale It may still be difficult for some to interpret the plot in detail, largely because the FeS scale tendency range (0.01 at drawdown and ~40 at Choke) compress the graph. As a practice step, we will edit it once more. Right-mouse click on the FeS-pre scale tendency curve then select Hide Right mouse-click on the y-axis and unselect Logarithmic scale Sandy Hook #1: Produced Water Volume Inversion 18-12

13 The plot above is the final format. At Bottomhole conditions NaCl and SrSO 4 are slightly supersaturated. If we mouse over the NaCl symbol at drawdown conditions, we will see the value is about ST=1.1. This is sufficient to cause NaCl to precipitate. CaCO 3, and SrSO 4 are also supersaturated but their values are probably too low to result in rapid precipitation. Downstream of the Bottomhole, the scale tendencies are below 1. Thus, the greatest scale risk is the drawdown from the static reservoir to the Bottomhole. Studio ScaleChem quantifies the scale mass precipitating at each point and we will view this next. Click the Curves button in the upper right of the plot Select the Y1-axis header then press the << button to remove the variables Expand the Solid section on the left side and select Dominant Solid Make sure your y-axis is set to the Linear scale The software computes that several solid phases form. We will view the mass in a table format. Sandy Hook #1: Produced Water Volume Inversion 18-13

14 Select the View Data button The software computes that a large amount of NaCl, and lesser amounts of CaSO 4, and CaCO 3 from at drawdown conditions. Downstream from that point FeS is the dominant scale. Comparing the computed Test Separator chlorides to the measured values Click on the Report tab Scroll down to the Separator or press <Ctrl>+<F> and search for separator Look for the section called Location Name: Separator Scroll down to the Separator s Brine Composition table The chloride concentration is 73,512 mg/l, this compares with 73,511 mg/l at the end of Task #3. We should expect nearly identical results because mass was conserved at each location. Summary The case is mostly complete, in that we created a mass-balance from the surface to the reservoir. We also computed a scale risk of FeS, and CaCO 3 scale. What remains as possible simulation options is to saturate the fluid at either Static Reservoir or Drawdown reservoir conditions. Another simulation option is to deposit precipitated solids at the point where they may form. The above case assumes that the solids move with the liquid. We can consider these modeling options in the next case. Production Gas Inlet to Process (Static Reservoir) Sandy Hook #126 production- static reservoir to test separator Scale buildup in drawdown/bottomhole 25,000 mg/l NaCl 90 mg/l CaSO 4 40 mg/l CaCO 3 Scale buildup in in well ~60 to 70 mg/l FeS Scale buildup DS of choke 0-30 mg/l FeS Production Brine Sandy Hook #1: Produced Water Volume Inversion 18-14