RESERVOIR PRESSURE ESTIMATION USING PTS DATA. Rodolfo Belen Jr., Zim Aunzo, Cal Strobel, and Phil Mogen

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1 PROCEEDINGS, Tweny-Fourh orkshop on Geoherma Reservoir Engineering Sanford Universiy, Sanford, Caifornia, January 25-27, 1999 SGP-TR-162 RESERVOIR PRESSURE ESTIMATION USING PTS DATA Rodofo Been Jr., Zim Aunzo, Ca Srobe, and Phi Mogen Phiippine Geoherma Inc. 12/F Ciibank Tower 8741 Paseo de Roxas Makai Ciy, Phiippines, 1226 ABSTRACT Big-hoe producion we Buao 1 is crossfowing whie shu-in based on shu-in pressure, emperaure, and spinner (PTS) survey resus. The apparen crossfow is probaby caused by a verica upfow from he we s deep, ess permeabe and pressure suppored feedzones ino is shaow, more permeabe and pressure depeed enry zone. Consequeny, shu-in webore pressures a he enry zones do no represen rue saic pressures. Dynamic boomhoe pressures underesimae pressures a he deep producing zones and overesimae pressures a he shaow pressure depeed zones. Crossfowing in Buao 1 woud expain he arge pressure decine raes observed in he fied based on saic pressure gradien surveys despie he fied s uniformy ow producion decine raes. Reservoir pressures a he permeabe zones of Buao 1 were hen esimaed using pressure, emperaure and fow daa obained from shu-in and injecing PTS surveys. Linear fow equaions ha define fow a he we s enry zones a crossfowing and injecing condiions were soved simuaneousy o esimae pressures. refec rue saic pressures. The dynamic fowing pressures underesimae pressures a he producing zones and overesimae pressures a he pressure-depeed zones. Therefore, verifying he crossfow hypohesis hrough a shu-in PTS survey woud expain he arge pressure decine raes observed in he fied and woud provide a means of esimaing pressures more accuraey. PTS surveys were hen performed in Buao 1 whie he we was shu-in o verify he crossfow hypohesis. Buao 1 is a big-hoe wo-phase seam producer siuaed in he cener of he Buao fied (Figure 1). The we was compeed in Augus 1996 o a oa dried deph of 7328 MD (649 bs). The we produces abou 2 kph seam wih an enhapy of 89 Bu/b a a fowing wehead pressure of abou 2 g. BACKGROUND Hisorica Buao pressures based on saic pressure gradien surveys indicae arge pressure decine raes greaer han 5 /year ha are difficu o reconcie concepuay and mahemaicay wih he fied s uniformy ow producion decine raes ha are generay ess han 4%/year. To reconcie pressure and fied producion daa, Srobe (1995) hypohesized ha wes exhibiing arge pressure decine raes are crossfowing whie shu-in. This woud impy ha measured webore pressures during shu-in do no Figure 1. Buao Geoherma Fied, Phiippines. The permeabe zones of Buao 1 were deermined based on resus of injecing PTS surveys performed afer we compeion in Sepember 1996 (Figures 2 and 3). Injecion rae was varied a 9, 14 and 2 bpm. Three major permeabe zones were idenified a

2 25-3 MD ( bs), 45 MD ( bs) and 7-73 MD ( bs) (Aunzo 1996). The emperaure profie during injecion (Figure 2) indicaes an infow of wo-phase seam a he shaowes enry zone beow he surface casing shoe. Injeced waer hen fows ou ino he formaion a he oher wo enry zones. Furhermore, he injecing spinner profie (Figure 3) suggess anoher possibe permeabe zone a MD ( bs). Shu-in PTS survey resus (Figures 4 and 5) indicae an apparen upward crossfow of wo-phase seam aong he webore whie he we is shu-in. The deep permeabe zone a abou 6 bs and he inermediae zone a abou bpm 14 bpm 2 bpm 9 bpm 14 bpm 2 bpm Temperaure, F Injecing 9 bpm Injecing 14 bpm Injecing 2 bpm Injecing 14 bpm Injecing 9 bpm Injecing 2 bpm Spinner Veociy, rpsa Figure 3. Buao 1 Spinner Veociy Profies During aer Injecion Pressure, a Figure 2. Buao 1 Pressure and Temperaure Profies during aer Injecion. RESULTS AND DISCUSSION The shu-in PTS surveys in Buao 1 were conduced by Scienific Driing Inernaiona in Apri Spinner ogs a a ine speed of 8 fpm and saionary runs a seeced dephs were performed. Two spinner oos wih 2 pich hreebaded impeers were used during he survey. The firs spinner oo was used during he down and up ogs and was pued ou and repaced when he maximum operaing emperaure imi of he oo was reached. The saionary runs beow 465 MD (3559 bs) were compeed using he second oo.

3 1 Temperaure, F Down 8 fpm Up 8 fpm Saionary Log Shu-in P Sa'n T Shu-in T new oo Spinner Veociy, rpsa Pressure, a Figure 4. Buao 1 Shu-in Spinner Profies. 4 bs coninue o produce wo-phase seam even whie he we is shu-in. Seam fows up he webore and fows ou ino he formaion hrough he shaow feedzones beow he surface casing shoe a abou 15 bs. A saic wophase seam coumn exends above he shaow permeabe zone up o he wehead. The apparen upward crossfow is probaby caused by a verica upfow from he deep, ess permeabe and pressure suppored feedzone ino he shaow, more permeabe and pressure depeed feedzone. Reservoir pressures were hen esimaed by soving simuaneousy equaions invoving he produciviy and injeciviy of he we s permeabe zones using boomhoe pressures, emperaures and fow raes obained from he shu-in and injecing PTS surveys. Figure 5. Buao 1 Shu-in Pressure and Temperaure Profies. A he shaow permeabe zone, here is an infow of wo-phase seam during injecion and an oufow of wo-phase seam coming from he deeper zones during shu-in. The equaions of produciviy and injeciviy defining his feedzone are given by Equaions 1 o 4. PI i PI (1) 66, µ i (2) (3) (4) s 66, w i µ s w s where is he fow in kph, PI is he produciviy index in kph/, is he injeciviy index in kph/, P is he pressure in a, P w is he webore pressure in a, s is he specific graviy, and µ is he viscosiy in cp. The subscrips i and s denoe injecing and shu-in condiions. A he deep and inermediae permeabe zones, here is an oufow of injeced waer during injecion and infow of wo-phase seam during shu-in. The

4 equaions of produciviy and injeciviy defining hese zones are given by Equaions 5 o 8. (5) (6) i s PI 66, µ i w i PI (7) 66, µ s where he variabes and subscrips are defined above. w s (8) Shu-in P Simuaed Fowing P Injecing P Iniia Reservoir P 1998 P 1996 P The raes of infow and oufow a he permeabe zones were esimaed using he shu-in and injecing spinner veociy profies. The infow of wo-phase seam a he shaow zone during injecion was esimaed by performing an energy baance around he feedzone. Two-phase fuid properies were based on assumed pressures and fuid enhapies and were esimaed using Equaions 9 o µ 1 H v v H H v + H v H (9) µ v µ X + ( 1 X ) (1) H H X (11) H v where H is he enhapy, is he densiy, µ is he viscosiy and X is he seam quaiy. The subscrips, and v denoe wo-phase, iquid and vapor fuids. The pressures and he produciviy and injeciviy indices were hen cacuaed by soving Equaions 1 o 8 simuaneousy. The permeabiiy hickness was assumed o remain consan wih ime. Furhermore, pressures were assumed o decine by abou 2 per year a he shaow seam zone and by abou 3 per year a he inermediae and deep feedzones. The esimaed pressures and he pressure profies a shu-in, injecing and producing condiions are poed in Figure 6. The pressure profie during producion was simuaed using esim and Buao 1 producion daa Figure 6. Pressure, a Buao 1 Pressure Profies. A coheren picure of Buao 1 during shu-in, injecing and fowing condiions is iusraed by he pressure profies. The ess permeabe deep enry zones produce during shu-in and fowing condiions bu accep waer during injecion. On he oher hand, he more permeabe shaow zone produces during injecion and producion bu acceps upfow coming from he deep zone during shu-in (Figure 7). The esimaed curren pressures are abou 35 g a 2 bs, 85 g a 4 bs and 151 g a 6 bs. The shaow feedzone has an esimaed permeabiiy hickness of abou 5, md-f and he deep zone has ony abou 2, md-f. Based on he cacuaion resus, pressures in he seam zone have decined by abou 4 a an average pressure decine rae of ony abou 2 /year since he sar of fied producion. In he iquid zone, pressure drawdown is abou 75-9 a an average decine rae of ony abou 3-35 /year. Tabe 1 summarizes Buao 1 webore and pressures as we as fow raes during shu-in and fowing condiions.

5 28 kph 27 kph REFERENCES Subsea Dephs shaow kph 16 kph 11 kph 4 kph Lee, John (1982). e Tesing. New York. Sociey of Peroeum Engineers of AIME. Srobe, C. J. (1983), Iniia Sae of he Buao Reservoir. midde kph 1 kph 6 kph Srobe, C. J (1995). PGI Inerna Memo o Phi Mogen and Zim Aunzo. Buao 1 Injecing PTS Survey (Sepember 1996), Scienific Driing Inernaiona. deep Injecing 8 kph Shu-in 1 kph Producing 17 kph Buao 1 Shu-in PTS Survey (Apri 1998), Scienific Driing Inernaiona. Figure 7. Buao 1 During Injecion Shu-in and Producion. Tabe 1. Buao 1 Pressures and Fow Raes. zone Subsea Deph fee Iniia P reservoi Curren P reservoi Pressure Decine Decine Rae /yr r r shaow midde deep P Shu-in Condiions zone fow PI a P P kph kph/ wf a shaow midde deep P Fowing Condiions zone fow PI a P P kph kph/ wf a shaow midde deep CONCLUSIONS The apparen crossfowing in Buao 1 whie i is shu-in woud expain he arge pressure decine raes observed in he Buao fied despie he fied s uniformy ow producion decine raes. Daa obained from PTS surveys performed during wo differen we condiions can be used o esimae pressures by soving fow equaions simuaneousy.