ABSTRACT producing for over 50 years and are maturing, which meansthey require energy to artificially lift the wells since the reser-voir pressure is not sufficient to lift fluids to the surface natu-rally; in these cases, the reduction in costs by energy harvestingwill be tangible. Energy harvesting and conservation has be-come a topic of great importance in the industry, and this arti-cle will mainly discuss the amount of energy loss across achoke valve in a condensate gas well. The wellhead pressure(upstream) gets reduced by the choke valve, which causes anincrease in the gas velocity and a reduction in the temperature,as well as a loss in the energy. This article will discuss theamount of energy loss in a selected gas condensate well andpresent alternative methods for capturing this energy instead ofwasting it.

ENERGY ANALYSIS FOR CHOKE FLOW

We first investigate gas production controlled with a surfacechoke from an energy perspective. A control volume is selected,enclosing the choke valve between the cross section at point 1,some distance upstream of the choke, and the cross section atpoint 2, some distance downstream from the choke, Fig. 1.

Applying the first law of thermodynamics to this controlvolume1 under steady-state production, we have:

(1)

Chokes are control valves built into the production systems sothat wells can be produced at desired rates, while at the sametime reservoir depletion and sweep can be optimized, and for-mation and well completion integrity can be protected. The useof surface chokes also allows surface flow lines and facilities tobe designed more economically due to reduced pressure rat-ings. Substantial pressure drops can occur through well surfacechokes, especially at early stages of production when the reser-voir pressure is still high and lower choke settings are applied.This article investigates energy loss through wellhead chokesfor gas wells, with attention to the laws of thermodynamics.Calculation methods and applicable equations are presented.Example calculations show that energy loss can be significantas a result of choke control applied to wells. This is especiallytrue when one considers the number of wells and the totaldaily production that a company has. The energy waste can bepotentially avoided by extracting the otherwise lost fluid flowenergy to produce electricity. In remote areas, such as subsealocations or on offshore platforms, the generated electricitycan be used to power equipment present at the well sites, suchas sensors, communication devices, heaters or pumps. In lessremote sites, the electricity can be fed into existing powergrids. The potential benefit includes reduction in field energyconsumption and operating costs, energy conservation and environmental protection.

INTRODUCTION

The oil and gas industry faces many challenges in terms of en-ergy consumption and costs. In some wells — those with ex-tremely low rates of oil or gas daily production and highpercentages of water cut — the high consumption of electricityand other energy sources can serve as a determining factor indeciding whether the well is economical to produce or not. Un-doubtedly, energy cost might not be of significant importancein Saudi Arabia, where most of the existing oil and gas wellsare producing at very high rates, but the fact that there arethousands of gas and oil wells all around the Eastern and Cen-tral regions of Saudi Arabia makes the total energy consump-tion cost a major factor. In addition, many wells have been

Well Site Energy Harvesting from High-Pressure Gas Production

Authors: Dr. Jinjiang X. Xiao, Wessam A. Busfar, Rafael A. Lastra and Muhammad Adnan

Fig. 1. Control volume encompassing a choke valve for thermodynamic analysis.

WINTER 2014 SAUDI ARAMCO JOURNAL OF TECHNOLOGY

where u is the internal energy (associated with the kinetic en-ergy of the molecules and energy due to inter-molecule interac-tion);

p—� is the flow energy or transport energy (p is pressure

and � is density); a v2

—2 is the bulk kinetic energy (a is the veloc-ity correction factor to account for the nonuniform velocityprofiles and a value of 1.05 can be taken for turbulent flow); vis the fluid velocity; gz is the bulk potential energy (g is thegravitational acceleration and z is the elevation); q is the heattransferred into the fluid from the surrounding area outside thecontrol volume (this is negative if it is lost to the surroundingarea); and ws is the work done by the fluid on a shaft protrud-ing out of the control volume and delivered to the surroundingarea outside the control volume, all on a per unit mass basis.The flow energy term represents work needed to push fluidinto and out of the control volume. The terms inside the brack-ets represent total energy associated with the fluids cominginto the control volume at the cross section of point 1 and outof the control volume at the cross section of point 2. Theseitems can also be viewed as energy density with a unit of kJ/kg.When a liquid condensate phase is also present, the internalenergy and flow energy will be the average internal energy andflow energy of the two phases based on mass fraction. Thevelocity will be the mixture velocity, assuming no slippage between gas and liquid.

Equation 1 is the law of conservation of energy understeady-state production applied to the selected control volume.Due to the small size of the control volume and high fluid ve-locity, heat transfer can be neglected, so the process can beconsidered adiabatic, i.e., q = 0. Because there is no compressoror turbine inside the control volume, there is no shaft work doneby the fluids, i.e., ws = 0. It is worth pointing out that there isfriction at the control volume surface (pipe and choke interiorwall) and energy is needed to overcome the friction. Since thepipes and the choke are stationary, there is no movement ordisplacement of the control volume surface, and there is noslippage between the fluids and pipe wall — a non-slip condi-tion. Therefore, there is no work done by the frictional forceor viscous stress. Under these considerations, Eqn. 1 becomes:

(2)

This equation states that the total energy associated with thefluids is conserved across the choke. There can be significantpressure and temperature changes before and after the choke,but the total energy of the fluids is conserved. This seems intu-itively to be a contradiction, but energy conservation is dic-tated by the governing law.

Using the definition of specific enthalpy, h = u + p

—� , we canrewrite Eqn. 2 as:

(3)

The kinetic energy terms are small relative to enthalpy.

There is no elevation change before and after the choke, andthe bulk potential energy terms cancel each other. Equation 3is further reduced to:

h1 = h2 (4)

Equation 4 shows that enthalpy before and after the chokeremains constant, i.e., choking is an isenthalpic process, just asmany others have reported in the literature for flow-throughchokes or throttling devices2.

How can one reconcile the notion that there is significantpressure and temperature reduction through the choke, and yetthere is no loss in the total energy of the fluids as they flowthrough the chokes? The answer lies in the fact that energy ischanged from one form to another going through the choke.The mechanical energy is defined asEquation 2 can be rewritten as

(5)

Or in terms of mechanical energy change, we have

(6)

The reduction in pressure and temperature leads to a reduc-tion in mechanical energy. Equation 6 means that the loss inmechanical energy is equal to the gain in internal energy whilethe total energy is preserved. Mechanical energy is “useful” energy since it can be converted into mechanical work via devices,such as a turbine, whereas internal energy is nonrecoverable.

% mol

N2 4.83

CO2 8.30

H2S 2.06

Methane 82.47

Ethane 1.62

Propane 0.33

I-Butane 0.06

N-Butane 0.12

I-Pentane 0.04

N-Pentane 0.04

Hexanes 0.02

Heptanes 0.00

Octanes 0.04

Nonanes 0.06

Decanes 0.01

Total 100.0

Table 1. Fluid composition for an offshore gas well

SAUDI ARAMCO JOURNAL OF TECHNOLOGY WINTER 2014

Therefore, there is mechanical or “useful” energy loss throughthe choke.

Consider an example calculation. An offshore well producesat a rate of 114 million standard cubic ft per day (MMscfd)(31.43 kg/s). The measured choke upstream pressure and tem-perature are 6,589 psia and 222 °F, and the downstream pres-sure and temperature are 1,478 psia and 158 °F. The pipe sizeis 8”. The fluid composition, Table 1, is reported by Al-Qah-tani and Garland (2013)3.

Performing pressure-temperature (P-T) flash calculationswith a pressure-volume-temperature (PVT) package, we calcu-late density, enthalpy and other parameters. The results aregiven in Table 2.

As shown in Table 2, the kinetic energy terms are confirmedto be small and negligible relative to enthalpy, even thoughthere is a tenfold increase going through the choke. The en-thalpy terms before and after the choke are nearly identical.The 2% to 3% difference is within the accuracy of the fielddata measurement and modeling with the equation of state.This confirms that choking is an isenthalpic process and the to-tal energy is preserved; but there is a significant reduction inflow energy. This represents the loss in mechanical or “useful”energy. For the rate of 114 MMscfd (31.43 kg/s), the powerloss is calculated to be as high as 48.2 × 31.43 = 1.5 MW. Thismissing mechanical energy has been transformed to fluid inter-nal energy, i.e., there is an equal amount of internal energy increase with the fluids.

A question one may ask is, how can there be an increase ofinternal energy when the temperature has dropped from 222°F to 158 °F. This behavior can be explained by the fact thatthe fluid mixture is not an ideal gas where the internal energyis only a function of temperature. For real gas, internal energyis a function of both temperature and pressure. For example,the internal energy of methane (CH4) increases with decreasingpressure4. As the production mixture flows through the choke,the temperature drop will cause a reduction in its internal en-ergy, whereas the pressure drop will cause an increase in its internal energy. The combined effect is a net increase in its internal energy.

ENERGY HARVESTING AT THE WELL SITE

Instead of a choke, a turbo-expander or flow turbine can beused to take advantage of the pressure reduction and to extractenergy. The high-pressure gas is expanded through the deviceto produce work, as indicated in Fig. 25, 6.

Again, considering that the process is adiabatic, i.e., q = 0,kinetic energy terms are negligibly small, and there is nochange in vertical elevation and so no change in bulk potentialenergy, from the first law of thermodynamics (Eqn. 1), wehave:

ws = h1_ h2

(7)

The extracted shaft work is equal to the reduction in fluidenthalpy. If frictional loss is neglected, the adiabatic expansionis approximated by an isentropic process, i.e., a constant en-tropy process, based on the second law of thermodynamics. AP-T flash calculation at the upstream condition followed by apressure-entropy (P-S) flash at the downstream pressure andthe upstream entropy — keeping the entropy constant — willprovide values of enthalpy for conditions upstream and down-stream of the expander, which will allow the shaft work to becalculated from Eqn. 7. The same P-S flash will also provide

Upstream Downstream Difference

Pressure (psia) 6,589 1,478 -5,111

Temperature (°F) 222 158 -64

Density (kg/m3) 253.1 77.6 -175.5

Velocity (m/s) 3.83 12.49 8.66

Enthalpy (kJ/kg) 56.0 57.5 1.5

Kinetic Energy (kJ/kg) 0.008 0.082 0.074

Flow Energy ( ) (kJ/kg) 179.5 131.3 -48.2

p–ρ

Table 2. Calculated energy change across wellhead choke

Fig. 2. Control volume for energy generation with a turbo-expander.

WINTER 2014 SAUDI ARAMCO JOURNAL OF TECHNOLOGY

the value of the downstream temperature. This isentropic shaftwork, wsi, represents an expander working at 100% efficiency.Friction will occur with the expander, which reduces its effi-ciency. The frictional loss will be converted to thermal energy,resulting in a higher fluid downstream temperature. As an ex-ample, assuming the expander efficiency is 70%, i.e., the actualshaft work, wsa, is 70% of the isentropic shaft work deter-mined previously, the expander downstream enthalpy will be:

h2 = h1 _ 0.7 wsi (8)

A pressure-enthalpy (P-H) flash at the downstream pressureand the enthalpy from Eqn. 8 will provide the value of down-stream temperature at the 70% expander efficiency. Details onP-S and P-H flash calculations can be found in other literature7.

Table 3 provides values for power generation by installingan expander at the same offshore well producing at 114 MMscfd (31.43 kg/s).

We include a high loss case, with an efficiency of only 26%,to illustrate what the downstream condition will be if we onlyrecover the mechanical loss calculated in the previous sectionwhen the production is controlled with a choke.

As shown in Table 3, using a moderate efficiency of 70%,we can harvest 4.1 MW of power from just one well. The fieldhas 14 wells producing a total of 1,500 MMscfd of gas. There-fore, it has the potential to generate over 50 MW of power.This is just one field of the company’s operation. When all existing and future operations are considered, the potentialpower capacity can be significant.

The expander is not meant to be a replacement of the chokevalve. In actual field implementation, the choke and expandercan be arranged either in parallel or in series to enhance operability.

From the values in Table 3, we can see that harvesting powerwith an expander will lead to lower downstream temperature.Because hydrate inhibition is required to prevent flow assurance

issues in the pipeline, the existing inhibitor injection infrastruc-ture and dosage rates may need some adjustments to cope withthe lower temperature. There is also a possibility of higher liquid dropout because of the lower temperature, which couldimpact the slug catcher and pigging operations. The implicationsfor the processes of the gas receiving plant will also need to beinvestigated.

CONCLUSIONS

Well surface chokes are an integral part of a production system.They are used to control well production and protect the assets.For high-pressure reservoirs, severe choking is typically appliedfor many years of operation. Significant mechanical energyloss occurs with choking. The energy can be extracted by useof turbo-expanders. For one field alone, the potential powerthat can be generated is over 50 MW. Harvesting this energywill bring benefits, including reduction in field energy con-sumption and operating costs, energy conservation and environ-mental protection.

ACKNOWLEDGMENTS

The authors would like to thank the management of SaudiAramco for their support and permission to publish this article.

REFERENCES

1. Sonntag, R.E. and Van Wylen, G.J.: Introduction toThermodynamics, Classical and Statistical, 3rd ed., JohnWiley & Sons, 1991, 800 p.

2. Li, C., Jia, W. and Wu, X.: “Temperature Prediction forHigh-Pressure High Temperature Condensate Gas Flowthrough Chokes,” Energies, Vol. 5, No. 3, 2012, pp. 670-682.

3. Al-Qahtani, M.H. and Garland, P.: “Khursaniyah Gas

100% Effi ciency 70% Effi ciency 26% Effi ciency

Upstream Pressure (psia) 6,589 6,589 6,589

Downstream Pressure (psia) 1,478 1,478 1,478

Upstream Temperature (°F) 222 222 222

Downstream Temperature (°F) 29 64 121

Upstream Enthalpy (kJ/kg) 56.0 56.0 56.0

Downstream Enthalpy (kJ/kg) -129.0 -73.5 7.9

Upstream Entropy (kJ/kg-K) -1.991 -1.991 -1.991

Downstream Entropy (kJ/kg-K) -1.991 -1.794 -1.528

Shaft Work (kJ/kg) 185 130 48

Power (MW) 5.8 4.1 1.5

Table 3. Power generation with expander.

SAUDI ARAMCO JOURNAL OF TECHNOLOGY WINTER 2014

Wessam A. Busfar joined SaudiAramco in 2010. Since joining, he hasworked in several areas, including a15-month experience in fieldoperations with the Northern AreaProduction Engineering and WellServices Department. Wessam also

worked for a year in the Artificial Lift and Intelligent FieldUnit under the Production and Facilities DevelopmentDepartment. Since January 2013, he has been working as aPetroleum Engineer in the artificial lift focus area of theProduction Technology team at the Exploration andPetroleum Engineering Center – Advanced Research Center(EXPEC ARC).

Wessam received his B.S. degree in PetroleumEngineering from the Colorado School of Mines, Golden,CO.

Rafael A. Lastra works as an ArtificialLift Engineering Consultant for SaudiAramco’s Production TechnologyDivision of the Exploration andPetroleum Engineering Center –Advanced Research Center (EXPECARC). He provides internal

consultation related to artificial lift technologies, especiallyin the area of electrical submersible pumps. Rafael hasmore than 20 years of extensive and versatile oil industryexperience obtained through a variety of internationalengineering and management positions, including researchand development, management of artificial lift operations,project control and business development.

He received his B.S. degree in Electrical Engineeringfrom Universidad del Valle, Cali, Colombia, and a M.S.degree in Management of Information Systems from TexasA&M University, College Station, TX.

Muhammad Adnan joined SaudiAramco in 2008 and is working as anElectrical Engineer in the SouthernArea Gas Producing Department,which is responsible for hundreds ofgas wells. His previous experienceincludes working with several different

companies in Pakistan and North America prior to joiningSaudi Aramco.

Muhammad received his B.S. degree in ElectricalEngineering from the University of Engineering andTechnology, Lahore, Pakistan.

BIOGRAPHIES

Dr. Jinjiang X. Xiao is a PetroleumEngineering Specialist working withthe Production Technology team ofSaudi Aramco’s Exploration andPetroleum Engineering Center –Advanced Research Center (EXPECARC). His interests are well

productivity improvement and water management. Prior to joining Saudi Aramco in 2003, Jinjiang spent

10 years with Amoco and later BP-Amoco, working onmultiphase flow, flow assurance and deepwater productionengineering.

He received both his M.S. and Ph.D. degrees inPetroleum Engineering from the University of Tulsa, Tulsa,OK.

Plant Experience with Foaming during Startup of KaranNonassociated Gas,” IPTC paper 17175, presented at theInternational Petroleum Technology Conference, Beijing,China, March 26-28, 2013.

4. Younglove, B.A. and Ely, J.F.: “Thermophysical Propertiesof Fluids II: Methane, Ethane, Propane, Isobutane andNormal Butane,” Journal of Physical and ChemicalReference Data, Vol. 16, No. 4, October 1987.

5. Ardali, E.K. and Heybatian, E.: “Energy Regeneration inNatural Gas Pressure Reduction Stations by Use of GasTurbo Expander; Evaluation of Available Potential inIran,” Proceedings of the 24th World Gas Conference,Buenos Aires, Argentina, October 2009.

6. Eber, S. and Cavanagh, C.: “Energy Recovery from GasDistribution Operations,” presentation given at theAdvanced Energy Research and Technology Center(AERTC) Conference, Hauppauge, New York, November19-20, 2008.

7. Michelsen, M.L.: “State Function Based FlashSpecification,” Fluid Phase Equilibria, Vol. 158-160, June1999, pp. 617-626.

WINTER 2014 SAUDI ARAMCO JOURNAL OF TECHNOLOGY