A Statistical Analysis for Comparison Between Lip Pressure and Separator in Production Well Testing at Lahendong and Ulubelu Field

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1 Proceedings World Geotheral Congress 05 Melbourne, Australia, 9-5 April 05 A tatistical Analysis for Coparison Between Lip Pressure and eparator in Production Well Testing at Lahendong and Ulubelu Field Mohaad Husni Mubarok, Yanuaris Dwi Cahyono, alvius Patangke 3 and Eben Ezer iahaan 4 34 Manageent of Operation and Utilization, PT. Pertaina Geotheral Energy, Jakarta, Indonesia husniubarok@pertaina.co, yanuarisdwic@pertaina.co, s.patangke@pertaina.co, eesiahaan@pertaina.co Keywords: well testing, lip pressure, separator, statistical coparison ABTRACT In order to deterine the potential of power generation in a geotheral well that have been copleted; there should be a series of production tests to deterine precisely and accurately the characteristics of the geotheral production well in ters of physical and cheical properties. There are several ethods in geotheral well testing, but there are two ethods coonly used, lip pressure and separator ethod. Lip pressure ethod can be perfored in an upright (vertical discharge) or in a flat configuration (horizontal discharge). Basically, both ethods have the sae purpose; it s to clean the drill cuttings or ud drilling fro the well and to get an estiate of the potential electrical energy that can be generated by these wells. During the progress of a production test, the separator ethod can present the characteristics of the data in a ore coprehensive way than lip pressure. However, separator ethod need longer preparation, larger costs, ore an power and ore coplicated calculations than the lip pressure ethod. Fro that background, a coparative analysis between the two ethods was ade by using the statistical tool of the t-test ethod. The coparisons were done on one well at Lahendong (North ulawesi) and seven wells at Ulubele (Lapung) geotheral fields. The coparison was conducted on the rate of vapor and enthalpy for both test ethods by calculating the error value and the standard deviation at 95% confidence level obtained by the noral distribution curve. Acceptance range for enthalpy is between 596 kj/kg to,850 kj/kg and for stea rate is between 3 tons/hour to 98 tons/hour. In other words, when the lip pressure ethod is used and the results are inside those ranges, then we can say that this ethod is accurate with a confidence level of 95%.. INTRODUCTION Geotheral energy is a renewable energy that currently is urgent because the deand of energy increase every year significantly, particularly in Indonesia. While the ajority of energy sources in use today are based on petroleu. As one type of non-renewable energy, oil reserves in Indonesia and its production continue to decline. Therefore, with the largest geotheral reserves in the world (40% of world reserves), geotheral is an excellent alternative to address the energy crisis in the country and its developent ust be assive and consistent. PT. Pertaina Geotheral Energy (PGE), a subsidiary of PT. Pertaina (Persero) currently has seven new projects and four existing areas. PGE is given a task by the governent to accelerate the progra of geotheral energy utilization in Indonesia. To eet the existing targets, it is needed to drill in the new projects and build ake up wells in the existing areas. When geotheral wells finish its drilling stage, production tests need to be done. Production test are perfored to deterine the characteristics of the wells based on the ass flow rate at a variable wellhead pressure (WHP). In general, production test ethods can be divided into three types, vertical lip, horizontal lip and the separator ethod. For lip ethods the well potential is calculated based on the Russell Jaes equation. While in the separator ethod, two-phase fluid is separated in the separator and then easureents of the ass flow of stea and brine are done. eparator ethod has better accuracy than the lip ethods.. ULUBELU AND LAHENDONG FIELD. Ulubelu Field Ulubelu Geotheral Field is located in Indonesia, uatra, Lapung Province, the District Ulubelu Tanggaus - Lapung, about 00 k west of Bandar Lapung. Geotheral Project Ulubelu can be reached by car via a paved road fro Bandar Lapung to the District tage Island, then proceed to Ulubelu through rocky roads and paved ost of the approxiately 5 k, with an average elevation of 800 eters above sea level. The location of production and reinjection wells is divided into five (5) clusters, which are Cluster A, B, C, D and F. tea supplied fro three (3) clusters (B, C and D) and separated brine will be injected with the condensate fro the plant to the reinjection wells in clusters A and F.. Lahendong Field Lahendong Geotheral Field is located in Indonesia, ulawesi, North ulawesi Province, the District Toohon North ulawesi, about 30 k southwest of Manado. Lahendong Geotheral Field can be reached by car via a paved road fro Manado to the Toohon, with an average elevation of 800 eters above sea level. The location of production wells is divided into five (5) clusters which are Cluster 4, 3, 5, 37 and 4. tea supplied fro these five (5) clusters together with separated brine will be injected with the condensate fro the plant to the reinjection wells in Cluster 7.

2 3. BAIC THEORY Production tests are conducted to deterine the potential, physical and cheical characteristics of a well. Production test ethods are based on the type of fluid and different ethods can be distinguished in Figure. GEOTHERMAL WELL TETING VAPOR DOMINATED LIQUID DOMINATED LIP PREURE ORIFICE LIP PREURE EPARATOR VERTICAL LIP HORIZONTAL LIP VERTICAL LIP HORIZONTAL LIP TEAM FLOW FLOW (TEAM+BRINE) EPARATOR ORIFICE WEIR V-NOTCH TEAM or BRINE FLOW BRINE FLOW Figure : Flowchart of well testing ethods based on fluid characteristics. Figure is divided into three test ethods; separator, vertical lip, and horizontal lip. eparator ethod have the greater accuracy copared to lip ethods. The principle of the separator ethod is the separation of the two phase fluid in the separator; stea and brine coe out separately fro the outlet of the separator. The rate of stea can be deterined by using an orifice-eter, while the rate of brine could be easured using a weir box equipped with a V-Notch. 3. Vertical Lip Test In the ethod of vertical lip, enthalpy values are obtained fro the reservoir teperature. By using the Russel Jaes equation, the total ass flow of the well can be obtained (Figure ). Figure : Vertical lip ethod facilities. According to Figure, is two-phase ass flow rate (t/h), H is specific enthalpy of the discharge fluid (kj/kg), d is lip pressure pipe diaeter (), and P is lip pressure (bar abs). The weakness of this ethod is that the value of the enthalpy of the well is not calculated, but assued fro reservoir teperature obtained fro the downhole easureent (P&T). In its application, this ethod is only used to clean the wells and calculate the potential of the wells. Another significant weakness worth to ention (especially for lower-enthalpy wells) is the discharge of brine all over the well pad and surrounding areas. This is a significant disincentive for the vertical discharge ethod in environentally sensitive areas. 3. Horizontal Lip Test In the horizontal lip ethod, the enthalpy is obtained fro the correlation of lip pressure and water flow (through the weir box). The accuracy of this ethod is better than vertical lip. The horizontal lip test facilities can be seen in Figure 3.

3 Figure 3: Horizontal lip ethod facilities. The calculation of enthalpy and total ass flow is conducted using the Russell Jaes equation (Jaes, 970). The flowing enthalpy and two-phase flow can be calculated using the following equation: Gxh P,0 0, 96 G 0,84 A 0.84 xaxp H.0 0, 96 Where w(at) is brine ass flow rate at weir box (t/h) and A is a lip pipe area (c ). Brine flow rate fro the Atospheric Flash Tank (AFT) is correlated with the total ass flow rate using Flash Correction Factor (FCF) as below: w ( at ) xfcf FCF X w ( at ) h gat xh fgat H ubstitution of the above equation to Russell Jaes equation produces: w ( at ) 3600 xaxp xh 0.96 fgat xh x ( h gat.0 H ) 0,84 For atospheric pressure at bar absolute (hg at = 675 kj/kg and hfg at = 58 kj/kg) next is obtained: w ( at ) AxP ,84 x 3600 x ( xh 0.93 x ( 675 H ).0 H H ) by using the previous equation, W(at) /(A x P 0.96 ) can be calculated for a range of H. If H is plotted vs log[ W(at) /(A x P 0.96 )], then an alost linear relationship is obtained: If we assued the atospheric pressure is bara, the equation is: 3

4 After flowing enthalpy is obtained, the total ass flow rate can be calculated by using the Flash Correction Factor (FCF), the above equation becoes: In horizontal lip test, two-phase fluid flows into an atospheric flash tank (AFT) which will be flashing. Brine flow easureent ethods can use the V-Notch Weir as in Figure 4 below. Figure 4: cheatic picture of weir box (v-notch). Brine flow can be calculated by using this equation (Hirowatari, 986): k h 8.4 p 0.5 h B 0.09 w ( at ) kxh.5 x 60 x f Where h is the water level height through v-notch (), p is the height of weir box excluding v-notch (), B is the width of the weir box (), w(at) is the brine flow at atospheric condition (t/h) and ρ f is the density of brine (kg/3). 3.3 eparator Method In the test separator ethod, the two phase fluid is separated in the separator. The rate of stea is easured using an orifice flow eter and brine rate using the weir box. The scheatic drawing of the separator test facility can be seen in Figure 5 below. Figure 5: eparator ethod facilities. Measureents of the stea rate that coes out fro the outlet of the separator are ade by using orifice eters. The principle of speed easureent by using a disturbance is used, so the next paraeters ay be obtained: the upstrea pressure (PU), downstrea pressure (PD) and differential pressure (ΔP). By using the principle of Bernoulli's law, the rate of vapor can be calculated: M C Z E d P Where M is stea flow (t/h), C is basic coefficient, Z is the correction factor, ϵ is the expansion factor, E is velocity factor, d is inside diaeter orifice (), ΔP is delta pressure of upstrea and downstrea of orifice (kg/c3) and ρ is density of stea (kg/3). 4

5 3.4 t-test as Coparison Method Tool One of the functions of statistic is to copare between two ethods. The coparison of two ethods in general can be ade using the t-test and f-test. If the data used in the analysis is ore than 30 saples, the appropriate ethod is f-test. Vice versa, if the saple size is less than 30, then it is better to use the t-test. In the t-test, the first step is aking a hypothesis; H 0 and H. H 0 is the average values of the two ethods are sae and H otherwise. (Kurniawan, 008). H 0 will be accepted if the t-calculated is saller than t-table, t-calculated can be obtained using the following equation: t test x x n n Where x is the ean, is the standard deviation, and n is the size of population. The value of t-table, which can be seen in the reference, varies and depends on the level of confidence. In this case, the level of confidence that is used is 95%. If the calculation results show that H0 is accepted, the next step is to create a noral distribution curve for both ethods. The area of intersection of two noral distributions is a range of acceptance areas at a certain confidence level. The schee of the distribution curve is shown in Figure 6 below. H 0 Area H Area Confident level 95 % x x x Figure 6: Distribution curve for two tails and 95% acceptance range 3. REULT AND ANALYI Production testing in Lahendong and Ulubelu was done using two different ethods, which are horizontal lip and separator ethod. The test results were analyzed using statistical tools to deterine the extent to which the accuracy of horizontal lip approaches the separator estiate. Moreover, the final product of this statistical analysis is to obtain an accurate value range of horizontal lip if it will be used as a replaceent ethod of the separator technique. eparator ethod has a higher accuracy copared to other ethods; however, it requires long preparation ties and a huge cost. Therefore, in consideration of cost and tie constraints, the horizontal lip ethod is expected to replace the separator ethod as a test ethod of production with relatively low costs and short tie, but also having a good accuracy. In this experient, the test results were copared to enthalpy and stea rate data. The hypothesis H0 is that the values of the enthalpy and stea rate for both ethods are relatively equal and H otherwise. Table below presents the results of the calculation of the t-test. Table : t-test analysis for the two ethods under study. NO WELL ENTHALPY (kj/kg) TEAM (t/h) LIP EPARATOR LIP EPARATOR UBL-A,89.7, UBL-B,480.45, UBL-C,58.5, UBL-D,3.55, UBL-E,94.87, UBL-F,78.63, UBL-G,79.09, LHD-A,078.80, MEAN (x),6.48, TANDARD DEV t-calculated t-table, α=0.05, n= Fro Table it is seen that the value of t-calculated is less than t-table. In other words, the hypothesis H 0 is accepted, the accuracy of enthalpy and stea easureents for both ethods are relatively equal. To obtain an accurate range for the lip ethod, a distribution curve for each variable enthalpy and stea is created (Figure 7). 5

6 Figure 7: Distribution curve for lip and separator ethods for enthalpy and stea flow. The acceptance range is the area under the intersection of the two noral distributions. Fro the graph with the noral distribution overlapping ranges, the obtained acceptance range with the 95% confidence level is as follows: Table : The result of acceptance ranges for the horizontal lip ethod Fro Table above, it can be said that the horizontal lip ethod will be accurate with a 95% confidence level if the enthalpy of the well is in the range between 596 to,850 kj/kg. Whereas, for the stea rate the range is between 3 to 98 t/h. The horizontal lip ethod is quite reasonable to be able to replace the separator ethod on geotheral production testing. Norally, the average value of stea rate and enthalpy of two-phase wells in Indonesia enter into the acceptance range obtained. The advantages of the horizontal lip ethod is the siplicity of facilities, low cost (50% saller), faster preparation and test execution ties than the separator ethod, but with the sae relative accuracy. 4. CONCLUION In conclusion, horizontal lip is an alternative ethod that can be used to replace the separator ethod. The coparative study of the two ethods with the t-test shows that the stea rate and enthalpy using the horizontal lip ethod has good accuracy with a 95% of confidence level. The ajority of wells in Indonesia are two-phase, thus horizontal lip can be used as a standard ethod for obtaining the characteristics and potential of the well. The test results using a horizontal lip can refer to the acceptance range. The acceptance range of stea rate and the enthalpy is kj/kg and 3-98 t/h, respectively. To increase the confidence level of accuracy, well nubers should be added and then copared with tracer flow test results (Broaddus et. al., 00) and two-phase orifice flow eter (Helbig & Zarrouk, 0). REFERENCE Broaddus, M., Katz, J.I.K., Hirtz, P., Kunzan, R.: Advanceents in Tracer Flow Testing: Developent of Real-Tie Technology for Flow and Enthalpy Measureents, Proceedings World Geotheral Congress, Bali, Indonesia, (00). DiPippo, R.: Geotheral Power Plants, Dartouth, (007) Grant, M. A, Donaldson, L. G, Bixley, P. F.:Geotheral Reservoir Engineering, nd Edition, New York, (0) Helbig,.,Zarrouk. J.: Measuring Two-Phase Flow in Geotheral pipelines Using harp Edge Orifice Plates, Geotherics 44, p. 5-64, (0) Hirowatari, K.: Field Training of Geotheral Production Engineering, Kyushu University, Japan, (986) Jaes, R.: Maxiu Discharge of Geotheral Wells, DIR, Wairakei, Taupo, New Zealand, (970). 6

7 Kurniawan, D.: Tabel Distribusi, FORUM TATITIKA, Jakarta, (008). ( NI ,: Fluida uur Panasbui-Prosedur Uji Alir, Jakarta, (005) NI ,: Kriteria Peralatan Uji Alir Fluida uur Panasbui, Jakarta, (005) 7