PETSOC-97-111-P

NATURAL AND GAS-LIFT IN SAGD PRODUCTION WELLS

R.M.BUTLER S.BHARATHA C.T.YEE

this article begins on the next page FF THE PETROLEUM SOCIETY PAPER 97-111 Natural and Gas-lift in SAGD Production Wells R.M. Butler, S. Bharatha, C.T Yee GravDrain Inc.This paper is to be presented at the 48th Annual Technical Meeting of The Petroleum Society in Calgary, Alberta, Canada, June 8 - 11, 1997. Discussion of this paper is invited and may be presented at the meeting if filed in writing with the technical program chairman prior to the conclusion of the meeting. This paper and any discussion filed will be considered for publication in CIM journals. Publication rights are reserved. This is a pre-print and subject to correction. ABSTRACToperation of the SAGD process. In many SAGD projects, the early performance of the process has been severely Adequate lifting of produced fluids is an important issue limited by inadequate lifting. When this occurs the produced for SAGD producers. It is often necessary to evaluate the oil tends to be replaced by water rather than gas and steam, natural lift capabilityof the SAGD process for given and a steam chamber either does not form or is confined to .ft producer well design and modify the design if self-flowing only the upper part of the reservoir. capability of the well is a requirement. If the steam chamber pressure is sufficiently high in This paper describes the methodology for calculating relation to the depth of the reservoir, it may be possible topressure, temperature and fractional vaporization of water achieve natural lifting of produced fluids in a SAGD project pro files along SAGD producers, leading to the development without the use of a pump. While this mode of operation is of a computer program RISEWELL to perform such very convenient, it is clearly not feasible for deep reservoirs calculations. Sample calculations using the program the highoperating required to produce natural indicate unstable flow regimes for low fluid flow rates lift would then result in poor oil-steam ratio making SAGD implications of this instability for lift gas injection and steam not viable for such reservoirs. Even for shallow reservoirs, production into the well to enhance lifting capability are the producer has to be designed appropriately to render itdiscussed in relation to the example used for the sample self-flowing and it is necessary to perform pressure drop calculations. calculations to aid well design. INTRODUCTION This paper describes the development of a computer The analysis of fluid flow in producing wells of the program RISEWELL for the flow analysesof SAGD Steam-Assisted Gravity Drainage (SAGD) process is producers. In this program, pressures, temperatures and fractional water vaporization profiles along the well are necessary for evaluating the natural lift capability of the process and for well design. The lifting of produced fluids at calculated using momentum and energy balance principlesan adequate rate is of prime importance for the successful combined with heat transfer equations and experimental

THE PETROLEUM SOCIETY

Natural and Gas-lift in SAGDProduction Wells

A.M. Butler, S. Bharatha, C.T. YeeGravDrain Inc.

PAPER 97-111

This paper is to be presented at the 48th Annual Technical Meeting of The Petroleum Society in Calgary, Alberta, Canada, June 8 - 11,1997. Discussion of this paper is invited and may be presented at the meeting if filed in writing with the technical program chairman priorto the conclusion of the meeting. This paper and any discussion filed will be considered for pUblication in CIM journals. Publication rightsare reserved. This is a pre-print and is subject to correction.

ABSTRACTAdequate lifting ofproducedfluids is an important issue

for SAGD prodlfcers. It is often necessary to evaluate thenatural lift capability of the SAGD process for givenproducer well design and modify the design if self-flowingcapability ofthe well is a requirement.

This paper describes the methodology for calculatingpressure, temperature and fractional vaporization of waterprofiles along SAGD producers, leading to the developmentof a computer program RISEWELL to perform suchcalculations. Sample calculations using the programindicate unstable flow regimes for low fluid flow rates. Theimplications ofthis instabilityfor lift gas injection and steamproduction into the well to enhance lifting capability arediscussed in relation to the example used for the samplecalculations.

INTRODUCTIONThe analysis of fluid flow in producing wells of the

Steam-Assisted Gravity Drainage (SAGD) process isnecessary for evaluating the natural lift capability of theprocess and for well design. The lifting ofproduced fluids atan adequate rate is of prime importance for the successful

operation of the SAGD process. In many SAGD projects,the early performance of the process has been severelylimited by inadequate lifting. When this occurs the producedoil tends to be replaced by water rather than gas and steam,and a steam chamber either does not form or is confined toonly the upper part ofthe reservoir.

If the steam chamber pressure is sufficiently high inrelation to the depth of the reservoir, it may be possible toachieve natural lifting of produced fluids in a SAGD projectwithout the use of a pump. While this mode of operation isvery convenient, it is clearly not feasible for deep reservoirs- the high operating pressure required to produce naturallift would then result in poor oil-steam ratio making SAGDnot viable for such reservoirs. Even for shallow reservoirs,the producer has to be designed appropriately to render itself-flowing and it is necessary to perform pressure dropcalculations to aid well design.

This paper describes the development of a computerprogram RISEWELL for the flow analyses of SAGDproducers. In this program, pressures, temperatures andfractional water vaporization profiles along the well arecalculated using momentum and energy balance principlescombined with heat transfer equations and experimental

(3)

(1)

(9)

(6)

(5)

(10)

( ) _ ( 1415 )(999)Po IS-C - 1315+' API .

(V...P" +V.'" Po)P -..:......:.,,...-"--~;.:.1- (v.nr + v.,,)

The water and oil densities in kglm3 are calculated from theequations given on pp. 487 - 488 ofButler:

The gas phase density Pg appearing in (5) is calculatedfrom

P.. =1001.7 -O.l6I6T-0.00262T\ (7)

Po =(PO)IS-C +9.6-0.64T (8)

The calculation of the mixture density depends uponempirical correlations obtained by several authors for liquidholdup in two-phase flow of liquid and gas. Thesecorrelations take account of the slip between the liquid andgas phases and will be discussed later on.

Since liquid here stands for water and oil phases, someassumption on the slip between the water and oil phases isnecessary before the correlations for liquid holdup can beused here. It will be assumed that there is no slip betweenthe water and oil phases i.e., the phase velocities of waterand oil are equal. Let Vsw and VSIJ denote the superficialvelocities (volumetric flow rate divided by total cross-sectional flow area) of water and oil, respectively. Becauseof the assumption of no slip, the liquid density PI may bewritten in the fonn

where T is the temperature in C and (Po)IS'C is the density ofoil at 15C in kglm3, calculated from the API gravity of theoil by the equation

M(p-psl)PI' =Psi + RT

where T is the absolute temperature and P.fl is the saturationpressure ofwater at the temperature T. The relation betweenPSI and Twill be given in a later section - see (63) and (64).The dry gas density in (10) is based on the ideal gas equation

f, the specific enthalpies h.., hlJ and hug> latent heat L,saturation relations for water and the heat loss rate per unitlength q appearing in (1) to (4) are required; these areprovided below.

MIXTURE DENSITYIn tenns of the liquid holdup HI (volume fraction of

liquid), the mixture (water, oil and gas) density Pm may beexpressed in the fonn

(2)

where Vm is the mixture velocity (total volumetric flow ratedivided by flow area). It should be noted that manyAmerican authors use the notation I for the quantity 4f interms of the notation here. Using (2) in (1), the momentumbalance may be written in the fonn

~~ =-Pm(gsin9 + 2~m2)

The calculations are based on the fundamental principlesof balance ofmomentum and energy for pipe flow. Considerthe multi-phase flow ofwater, oil and gas through a deviatedproducer well of constant cross-sectional area (notnecessarily circular) as shown in Figure 1. Let s denote thearc length measured along the centre line of the well,increasing along the fluid flow direction. Although the fluidflow in SAGD wells is not strictly steady owing to slowchanges in the temperatures, pressures and flow rates withtime at a well cross-section, the assumption of steady flowwill be made to simplify the equations and is not expected toresult in serious errors. Balance of momentum, ignoringinertia effects, applied to the control volume of length ds (seeFigure 1) leads to the following differential equation (el Eq.(A-I) on p. 616 ofBeggs and Brill l ):

ap =_P gsin9 _ 't Was" AI

correlations for pressure drop in multi-phase pipe flow.Sample calculations are presented to illustrate the use of theprogram for the design of producers in SAGD projects.

BALANCE OF MOMENTUM AND ENERGY

where 't is the mean shear stress at the wall exerted along thedirection opposite to that of flow. The shear stress 'tappearing in (1) is usually expressed in tenns of the Fanningfriction factorlin accordance with the equation

where Dh = 4Af lW is the hydraulic diameter.

An energy balance, ignoring kinetic energy, for thecontrol volume of length ds in Figure 1 leads to thefollowing differential equation:

:s[m"..(h.. +xL) +moho +m..,A{.]=-q-m,gsin9. (4)

The balances (1) and (4) only constitute a framework forflow analyses and are not by themselves adequate tocalculate pressure, temperature and fractional vaporizationprofiles along the well. In order to do these calculations,suitable equations for the mixture density Pm' friction factor

2