Fractional flow in radial flow systems: a study for peripheral waterflood

Transcription

1 J Petrol Expl Prod Technol (2016) 6: DOI /s ORIGINAL PAPER - PRODUCTION ENGINEERING Fractional flow in radial flow systems: a study f peripheral waterflood Kegang Ling 1 Received: 28 December 2014 / Accepted: 2 August 2015 / Published online: 16 November 2015 The Auth(s) This article is published with open access at Springerlink.com Abstract The Buckley Leverett displacement mechanism has been used to predict the perfmance of waterflood. With Buckley Leverett method, oil recovery from waterflood is calculated and required water injection volume to achieve that oil recovery is estimated. This method does provide a very useful tool in waterflood design. Our experience in oil industry and a though literature review indicates that Buckley Leverett method was used to analyze waterflood project directly without any adjustment based on the real reservoir and production situations. By doing so, errs are introduced into the analysis. Buckley Leverett method assumed that displacement occurs in a linear system. This is true f some waterflood scenarios while f hers it is n. F some waterflood scenarios, a radial system is me appropriate than a linear system. In this study, we investigated the fractional flow in a radial system and derived the solutions to predict the perfmance of water displacing oil in radial system. With this radial displacement model, design and prediction of waterflood can be achieved by Buckley Leverett method and our model, whichever fits the waterflood pattern. Considering the fact that many waterflood scenarios follow radial displacement, our model is an imptant supplement to Buckley Leverett method. Keywds Waterflooding Radial flow system Fractional flow List of symbols A Flow area & Kegang Ling kegangling@yahoo.com 1 University of Nth Daka, Grand Fks, ND, USA c w f w h k k ro k rw P c P d p p o p w p wf q o q t q w r r D r e r f r Sw r w Dr S w S wi T Dt t V x f a q w l o l w k Water compressibility Water fraction Reservoir thickness Reservoir permeability Relative permeability to oil Relative permeability to water Capillary pressure Threshold pressure in capillary pressure curve Pressure Oil pressure Water pressure Flowing btomhole pressure Oil rate Tal liquid rate Water rate Radius from center of wellbe Dimensionless radius Reservoir outer boundary radius Displacement front position in radial system Position of any water saturation in radial system Wellbe radius Radius incremental Water saturation Irreducible water saturation Temperature Time period Time Volume Displacement front position in linear system Posity Water density Oil viscosity Water viscosity Rock property parameter related to the distribution of pe sizes

2 442 J Petrol Expl Prod Technol (2016) 6: Introduction When reservoir engineers analyze the waterflood perfmance, they rest to the conventional frontal advance they of Buckley and Leverett (1942). With Buckley Leverett method, oil recovery from waterflood is calculated and required water injection volume to achieve that oil recovery is estimated. This method does provide a very useful tool in waterflood design. Welge (1952) proposed a tangent construction method to estimate the water saturation, water fraction at the water front and oil recovery fact. Several her investigats studied the multilayer reservoir waterflood perfmance. Stiles (1949) investigated the multilayer reservoir displacement by assuming the displacement velocity in a layer to be proptional to its absolute permeability. Dykstra and Parsons (1950) developed their famous multi-permeability model f noncommunicating layers without crossflow. Hearn (1971) derived expressions f communicating stratified reservoirs using the pseudo-relative permeability functions. El Khatib (1999) advanced the closed fm analytical solution f communicating stratified systems with log-nmal permeability distributions. To the best of our knowledge, none of the study considered water displacing oil in a radial reservoir system. It should be ned that Buckley Leverett method and all the afementioned studies assumed that displacement occurs in a linear system. This is true f some waterflood scenarios, while f hers it is n. Our experience in oil industry and a though literature review indicates that petroleum engineers used Buckley Leverett method to analyze the waterflood project directly without any adjustment based on the real reservoir and production situations such as production-injection patterns. By doing so, a l of errs are introduced into the analysis. F some waterflood scenarios, a radial system is me appropriate than a linear system. Therefe, a radial displacement model is a necessary supplement to Buckley Leverett linear displacement model. With bh displacement models, design and prediction of waterflood can be achieved by selecting the appropriate model that fits the waterflood pattern. Considering the fact that many waterflood scenarios follow radial displacement, our model is very useful in field application. Derivation of fractional flow in a radial reservoir system Figure 1 shows a circular reservoir with a well located in the center. F oil reservoir with strong peripheral water drive surrounded by peripheral injects, Fig. 1 can represent the displace procedure well. The fractional flow can be viewed as water displacing oil into the central well. Figure 2 illustrates the flow line and pressure distribution in the reservoir. To make the analysis simple, the following assumptions are made: 1. A circular reservoir with constant height 2. Reservoir is homogenous in all rock properties 3. The dip angle of the fmation is zero 4. Oil and water two-phase flow in reservoir, no gas presents in the reservoir 5. Compressibilities of oil and water are negligible 6. The variation in oil and water densities can be neglected 7. Constant reservoir temperature is applied 8. All rock properties do n change as pressure changes 9. Constant oil and water viscosities during the displacement Starting from Darcy s equation, we have oil and water flow rates to be calculated as: q o ¼ kk ro oðap o Þ ð1þ l o q w ¼ kk rw oðap w Þ ð2þ l w where A is the flow area, k is the reservoir permeability, k ro is the relative permeability to oil, k rw is the relative permeability to water, p o is the oil pressure, p w is the water pressure, q o is the oil rate, q w is the water rate, r is the radius from wellbe, l o is the oil viscosity, and l w is the water viscosity. Recalling the concept of capillary pressure we have P c ¼ p o p w ð3þ where p c is the capillary pressure. p p wf r w Fig. 1 A circular reservoir with a well located in the center q r e k h

3 J Petrol Expl Prod Technol (2016) 6: rw p e r e p wf r w pe pwf h k r re (a) (b) Fig. 2 Radial flow reservoir system: a plan view, b lateral view Replacing water pressure by oil and capillary pressure Eq. (2) becomes q w ¼ kk rw o½aðp o p c Þ : ð4þ l w Expressing in pressure gradient, Eqs. (1) and (4) are changed to oðap o Þ ¼ l o kk ro q o oðap o Þ oðap cþ ¼ l w q w : kk rw Subtracting Eq. (5) from (6), we obtain oðap cþ oðap c Þ ¼ 1 k ¼ l w kk rw q w l o kk ro q o ð5þ ð6þ l o q o l w q w : ð7þ k ro k rw Fig. 3 A control volume in a circular reservoir with a well located in the center At this stage, we can introduce the concepts of tal liquid rate and fractional flow, which are defined as: q t ¼ q o þ q w ð8þ f w ¼ q w ð9þ q t where q t is the tal liquid rate and f w is the water fraction. Substituting Eqs. (8) and (9) into (7) yields f w ¼ 1 oðap cþ kk ro q t l o 1 þ k : ð10þ rol w k rw l o Flow area is defined as: A ¼ 2prh ð11þ where h is the reservoir thickness. Substituting Eq. (11) into (10), we have f w ¼ 1 2phkk ro q t l ð rop c o þ P c Þ 1 þ k : ð12þ rol w k rw l o dr h qt=qo+qw r+dr qt=qo+qw r re rw r

4 444 J Petrol Expl Prod Technol (2016) 6: Fig. 4 The pl of water saturation versus dimensionless radius based on Eq. (31) It should be ned that capillary pressure decreases as op radius increases in this case. Therefe, c is negative. Comparing Eq. (12) with the fractional flow of linear displacement, we found that they are different. In linear displacement, the water saturation is calculated by (Buckley and Leverett 1942) f w ¼ 1 Akk ro op c q t l o ox 1 þ k : ð12aþ rol w k rw l o Therefe, linear displacement fractional flow equation cann be used f radial displacement fractional flow. Equation (12) is the crect equation we should use in the radial system. If capillary pressure is negligible, Eq. (12) collapses into 1 f w ¼ 1 þ k : rol w k rw l o ð13þ Now, we derive the continuity equation of radial displacement. Considering the water displacing oil situation, material balance equation provides that the mass change in a control volume f a time period can be shown as Fig. 3. Since the flow direction is from reservoir outer boundary to the wellbe, f convenience, we define the reservoir boundary as the start point, where r = 0, and the wellbe as the end point where r = r e. Therefe, in the dimensionless analysis, dimensionless radius can be defined as, r D = r/r e, the start point at the reservoir outer boundary will have r D = 0, and the end point at the wellbe will have r D = 1. Material balance gives us ½ðq w q w Þ r ðq w q w Þ rþdr Dt ¼ phfðr e rþ 2 ð14þ ½r e ðr þ DrÞ 2 g/½ðs w q w Þ tþdt ðs w q w Þ t where S w is the water saturation, q w is the water density, Dt is the time period, t is the time, Dr is the radius incremental, r is the radius from reservoir outer boundary to wellbe, r e is the distance between reservoir outer boundary to wellbe, and / is the posity. Simplifying Eq. (14), we have ½ðq w q w Þ r ðq w q w Þ rþdr Dt ¼ ph½2r e Dr 2rDr ðdrþ 2 /½ðS w q w Þ tþdt ðs w q w Þ t ð15þ : As Dr? 0 and Dt? 0, we have 2r e Dr 2rDr ðdrþ 2 2r e Dr 2rDr: Equation (15) becomes partial differential equation oðq wq w Þ ð16þ ¼ð2r e 2rÞph/ oðs wq w Þ : ð17þ Assuming constant density ( Appendix A shows the derivation of governing equation including the change of fluid density), we have oq w ¼ð2r e 2rÞph/ os w : Substituting Eq. (9) into (18) gives oðf wq t Þ ð18þ ¼ð2r e 2rÞph/ os w : ð19þ If the water encroachment rate is constant, we have a constant tal liquid rate. Equation (19) can be simplified to

5 J Petrol Expl Prod Technol (2016) 6: Fig. 5 The crect pl of water saturation versus dimensionless radius of w ¼ ð2r e 2rÞph/ os w : ð20þ q t Since water fraction is function of water saturation, f w ðs w Þ applying chain rule to partial differential equation results in df w ds w os w q t ¼ ð2r e 2rÞph/ os w : ð21þ At the first look Eq. (21) is similar to the Buckley Leverett equation f linear displacement, we should nice that the term on the right-hand side befe partial derivative is n constant. Observing that water saturation is function of time, t, and position, r, we can express ds w ¼ os w dt þ os w dr: ð22þ The fact that at the displacement front the water saturation is constant provides us a boundary condition. Table 1 The input data f water displacing oil in a radial system Injection rate (BWPD) 50,000 Radius (ft) 5000 Reservoir thickness (ft) 50 Initial oil saturation (fraction) 0.8 Irreducible oil saturation (fraction) 0.2 Oil viscosity (cp) 0.93 Water viscosity (cp) 0.32 Posity (fraction) 0.2 Table 2 The relative permeabilities and calculated parameters versus water saturation S w (%) S o (%) K ro K rw f w (%) df/ds w

6 446 J Petrol Expl Prod Technol (2016) 6: Table 2 continued S w (%) S o (%) K ro K rw f w (%) df/ds w ds w ¼ os w dt þ os w dr ¼ 0 Table 3 Comparisons of locations of different water saturations at a waterflooding time f linear and radial systems Water saturation os w ¼oS w dt dr : Substituting Eq. (23) into (21) yields df w ð os w dt ds w dr Þ¼ð2r e 2rÞph/ os w Location of water saturation in linear system (ft) q t Location of water saturation in radial system (ft) ð23þ

7 J Petrol Expl Prod Technol (2016) 6: Fig. 6 Comparisons of locations of different water saturations at a waterflooding time f linear and radial systems df w dt ¼ ð2r e 2rÞph/ dr: ð24þ ds w q t Integrating Eq. (24) yields an equation f displacement front position, r f. ph/ ð2r e r f rf 2 q Þ¼ df w t ds w t f rf 2 2r er f þ tq ph/ ds w f ¼ 0 ð25þ where r f is the displacement front position in radial system. There are two solutions to Eq. (25), which are sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi r f ¼ r e re 2 tq : ð26þ ph/ ds w f Obviously only one solution is crect to match with the physical phenomenon. Considering at the beginning of the displacement as t? 0, we have r f? 0; therefe, we can eliminate the solution sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi r f ¼ r e þ re 2 tq : ð27þ ph/ ds w f Therefe, the crect solution is sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi r f ¼ r e re 2 tq : ð28þ ph/ ds w The distance between the wellbe and water front will be calculated by f Table 4 Comparisons of water front location versus waterflooding time f linear and radial systems Waterflooding time (days) Location of water front in linear system (ft) Location of water front in radial system (ft) sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi r welltowaterfront ¼ r e r f ¼ re 2 tq : ð29þ ph/ ds w Again, if we compare Eq. (28) with the linear displacement, we found that it is distinct from the f

8 448 J Petrol Expl Prod Technol (2016) 6: Fig. 7 Comparisons of water front location versus waterflooding time f linear and radial systems displacement front of linear displacement system, which is x f ¼ tq ð30þ A/ ds w f where x f is the displacement front position in the linear system. Therefe, linear displacement fractional flow equation cann be used to locate the position of displacement front in a radial displacement system. Equation (28) is the crect equation we should use in the radial system. F any water saturation, S w, the position can be calculated by sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi r Sw ¼ r e re 2 tq ð31þ ph/ ds w where r Sw is the position of any water saturation in radial system. The pl of water saturation versus dimensionless radius based on Eq. (31) indicates that the pl needs to be modified to match with the physical model. Here, dimensionless radius is defined as: r D ¼ r r e : S w ð32þ Figure 4 shows the pl of water saturation versus dimensionless radius based on Eq. (31). Figure 4 gives two saturation values f the same position. Physically, it is impossible. The modification to Fig. 4 to get the crect water saturation distribution can be accomplished by determining displacement front position. To determine the displacement front location, one can define a saturation discontinuity ( displacement front) at r f and balancing of the areas ahead of the front (Area 1) and below (Area 2) the saturation curve shown in Fig. 4. Then, the water saturation ahead of the displacement front should be the initial water saturation. The crect water saturation distribution is shown in Fig. 5. Case study A case study was conducted to illustrate the analysis of radial water displacing oil procedure. The input data are shown in Table 1. Table 2 shows the relative permeabilities and calculated parameters versus water saturation. The pl of water saturation versus dimensionless radius is shown in Fig. 5. If linear displacement equation is used, the position of the displacement front will be quite different. The processes of water displacing oil in linear and radial systems are compared to illustrate the difference. All inputs are the same in the comparison. The comparisons of locations of different water saturations at a waterflooding time f linear and radial systems are shown in Table 3 and Fig. 6. The comparisons of water front location at different waterflooding times f linear and radial systems are shown in Table 4 and Fig. 7. The differences in these tables and figures indicate that Buckley Leverett linear displacement is n appropriate f peripheral waterflood reservoirs. Conclusions The following conclusions can be drawn upon this study.

9 J Petrol Expl Prod Technol (2016) 6: The perfmance of radial water displacing oil system is different from that of linear water displacing oil system. If consider the effect of capillary pressure, equations used to calculate water fraction are different between linear and radial displacement systems. Linear displacement system equation cann be used f the radial displacement system. Equation (12) should be used to estimate the water fraction. Equations used to calculate the position of any water saturation are different between linear and radial displacement systems. Linear displacement system equation cann be used f the radial displacement system. Equation (31) should be used to estimate the position of any water saturation. Acknowledgments This paper was firstly presented at the SPE Latin American and Caribbean Petroleum Engineering Conference held in Mexico City, Mexico, April The auths thank Society of Petroleum Engineers (SPE) in allowing us to publish this paper with Journal of Petroleum Explation and Production Technology. Open Access This article is distributed under the terms of the Creative Commons Attribution 4.0 International License ( creativecommons.g/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the iginal auth(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. Appendix A Expanding Eq. (17) we have oq w q w q oq w w ¼ð2r os w oq e 2rÞph/ q w þ S w w : ð33þ Applying chain rule we obtain oq w q w q oq w op w w op w ¼ð2r e 2rÞ ð34þ os w oq ph/ q w þ S w op w w : op w Now introducing the concept of water compressibility, which is defined as c w ¼ 1 ov ¼ 1 oq w : ð35þ V op T q w op Substituting Eq. (35) into (34) gives oq w q w q op w wc w q w ¼ð2r e 2rÞ os w op w ph/ q w þ S w c w q w : ð36þ Substituting Eq. (9) into (36) yields oðf w q t Þ op w q w f w q t c w q w ¼ð2r e 2rÞ os w op w ph/ q w þ S w c w q w : ð37þ If the water encroachment rate is constant, we have a constant tal liquid rate. Equation (37) can be simplified to of w þ f op w wc w ¼ ð2r e 2rÞph/ q t os w op w þ S w c w : ð38þ Expressing the pressure in terms of radius we have p w ¼ f wq t l w 2pkk rw h ln r þ p wf : ð39þ r w Taking the derivative of bh sides of Eq. (39) with respect to r gives op w ¼ f wq t l w 1 2pkk rw h r : ð40þ F the same location ( radius in this case), water pressure change with respect to time can be approximated by capillary pressure change with respect to time, op w ¼ op c : ð41þ Accding to Brooks and Cey (1964) capillary pressure model capillary pressure can be expressed as 1 S w S k wi P c ¼ P d ð42þ 1 S wi where, S wi = the irreducible water saturation, P d = the threshold pressure, k = rock property parameter related to the distribution of pe sizes. Brooks and Cey related the parameter k to the distribution of pe sizes. F narrow distributions, k is [ 2; f wide distributions, k is \ 2. Substituting Eq. (42) into (43) we have 1 op w P d S w S 1 k wi os w ¼ : ð43þ kð1 S wi Þ 1 S wi Substituting Eqs. (40) and (43) into (38) yields of w þ f f w q t l w 1 wc w ¼ ð2r 2r eþph/ " 2pkk rw h r q t # 1 P d S w S k 1 ð44þ wi os w 1 þ S w c w : kð1 S wi Þ 1 S wi Equation (44) can be solved numerically to obtain the location of any water saturation at any waterflooding time.

10 450 J Petrol Expl Prod Technol (2016) 6: If the capillary pressure is small and can be igned, Eq. (44) becomes of w þ f f w q t l w 1 wc w ¼ ð2r 2r eþph/ os w 2pkk rw h r q t : ð45þ References Brooks RH, Cey AT (1964) Hydraulic properties of pous media. Hydrology Paper No. 3, Colado State University, Ft Collins, Colado, pp Buckley SE, Leverett MC (1942) Mechanism of fluid displacement in sands. Trans AIME 146:107 Dykstra H, Parsons RL (1950) The prediction of oil recovery by waterflooding. In: American Petroleum Institution (ed) Secondary recovery of oil in the United States, 2nd edn. API, pp El Khatib NA (1999) Waterflooding perfmance of communicating stratified reservoirs with log-nmal permeability distribution. SPEREE 2: Hearn CL (1971) Simulation of stratified waterflooding by pseudo relative permeability curves. J Pet Technol 23(7): Stiles WE (1949) Use of permeability distribution in water-flood calculations. Trans AIME 186:9 13 Welge HJ (1952) A simplified method f computing oil recovery by gas water drive. Trans AIME 195:91 98