DIRECTIONAL WELLS AT THE PAILAS GEOTHERMAL FIELD, SUPPORTING GEOLOGICAL AND STRUCTURAL DATA FOR THE GEOTHERMAL MODEL

Transcription

1 PROCEEDNGS, Thirty-Sixth Workshop on Geothermal Reservoir Engineering Stanford University, Stanford, California, January 31 - February 2, 211 SGP-TR-191 DRECTONAL WELLS AT THE PALAS GEOTHERMAL FELD, SUPPORTNG GEOLOGCAL AND STRUCTURAL DATA FOR THE GEOTHERMAL MODEL Hakanson, Edward Charles Gálvez Orellana, María de los Angeles Mora Protti, Oscar Rojas Barrantes, Martín nstituto Costarricense de Electricidad Centro de Servicio Recursos Geotérmicos Guayabo de Bagaces Guanacaste, Costa Rica Echarles@ice.go.cr; mgalvez@ice.go.cr; omorap@ice.go.cr; mbarrantesr@ice.go.cr ABSTRACT The Pailas Geothermal Field is located on the southern flank of the Rincón de la Vieja volcanic complex in northwestern Costa Rica. Since 9, the Costa Rican Electricity Company (CE) has drilled 7 deep directional boreholes (in addition to the 9 existing vertical boreholes). The purpose of directional drilling has been to intersect faults and permeable fractured rock layers suggested by the geothermal model proposed by the Geothermal Resources Service Center (CSRG) Geology Department (GD), and by the consulting company West Japan Engineering Consultants (WestJEC) for the Pailas Geothermal Field and thereby locate zones for production and reinjection of geothermal brine. These deep directional boreholes have been drilled from existing well pads where vertical boreholes were drilled during the Feasibility Study in which the existence of an economically exploitable geothermal reservoir was confirmed. The results of directional drilling have been positive since they have made it possible to find permeable zones interpreted as open high-angle faults that otherwise would not have been possible to find with vertical boreholes, thereby giving insight to structural evidence that supports the geological model, and incrementing the production and injection zones in the field. Lithostratigraphic correlations between directional boreholes and vertical boreholes support structural field data and permit the estimation of fault dip. Also, with directional drilling we have found a possible new deep aquifer to the west of the field that had not been observed in the other boreholes in the field and may be used as a new reinjection zone. These and other findings associated with directional drilling at Pailas Geothermal Field have allowed us to supplement the needed vapor and reinjection capacity so that in 211 Pailas 1, the first 35 MWe Binary geothermal plant in this field, can be a reality. NTRODUCTON Pailas Geothermal Field is the first borehole field in Costa Rica to employ directional wells. The Costa Rican Electricity Company (CE) is developing this field and concurrently installing the first 35 MWe binary geothermal power plant unit (Pailas ) which is scheduled to start operation in 211. Up until 9 only vertical deep boreholes had been drilled at the Pailas Field but due to limited land accessibility and the need to find more vapor and injection capacity to support Pailas, CE decided to initiate a directional drilling campaign for which West JEC was contracted as a consulting company and Baker HUGHES for directional drilling technical assistance. Since June of 9, seven directional deep boreholes have been drilled at this field, all of which have been successful in that they have allowed exploration of new areas at this geothermal field, thus increasing the production and reinjection capacity. Furthermore, a completely new injection area may have been discovered to the west of the field. This paper focuses on the information obtained from the directional wells and includes general data regarding

2 the field and borehole location, directional drilling data, geologic units, clay zonification, lost circulation zones, permeable zones, injectivity indices and isotherms. LOCATON The Pailas Geothermal Field is located on the southern face of the Quaternary age Rincón de la Vieja volcanic complex in northwestern Costa Rica, between the elevations of 55 and 75 meters above sea level. The developed part of the field is about eight kilometers squared (Figure 1). n Chavarría et al (21) a geologic model for this field is presented based on the information from these deep vertical boreholes. DRECTONAL BOREHOLES Starting in June 9 a directional drilling campaign has been carried out at the Pailas Geothermal Field and to date a total of seven wells have been completed (Table 2, Figures 1-11) from pre-existing wellpads. Table 2: Directional boreholes drilled at the Pailas Geothermal Field up to December, 21. Pad Well TVD MD Az 1 PGP E 1 PGP NNE 3 PGP-17A NE 4 PGP NE 4 PGP ESE 5 PGP N 6 PGP NNW Figure 1: Pailas Geothermal Borehole Field (Geologic base map from the Centro de Servicio Recursos Geotérmicos with the structural model proposed by WestJEC- CE 21, unpublished internal report). NF1-7 are geologic structures interpreted by WestJEC. BOREHOLE FELD AND GEOLOGC MODEL n all there are a total of 9 wellpads at the Pailas Geothermal Field where vertical wells have been drilled (Table 1 and Figure 1). Table 1: Vertical boreholes drilled at Pailas Geothermal Field (elevation and depth in meters). Pad Well East North Elev. Depth 1 PGP PGP PGP PGP PGP PGP PGP PGP PGP GEOLOGC UNTS The generalized downhole lithostratigraphic sequence present at the Pailas Geothermal Field encompasses the five geologic units reported Chavarría, et al (21). These directional wells have found the same units although in some cases a thickening of the deeper units is observed with respect to the vertical wells drilled at a given wellpad. The following section addresses the variations observed in the downhole lithostratigraphic sequence between vertically and directional wells as observed from each of the wellpads (Figures 2-6, Table 3). Elev. Depth PGP-12 DU NNW WELLPAD 1 (659 m.a.s.l.) PLAN VEW PGP PGP-11 Figure 2. Lithostratigraphic correlation at wellpad E njectivity ndex DU >

3 Elev. Depth PGP NE 3 WELLPAD 3 (757 m.a.s.l.) PGP-17A Figure 3. Lithostratigraphic correlation at wellpad 3. Elev. (m a.s.l.) Depth (TVD) PGP () > NE Figure 4. Lithostratigraphic correlation at wellpad 4. Elev. Prof Figure 5. Lithostratigraphic correlation at wellpad 5. PLAN VEW WELLPAD 4 (63 m a.s.l.) PGP-4 GB SE njectivity ndex PGP-5 ND 14 4 PLAN VEW N WELL PAD 5 (554 m a.s.l.) PGP-27 ND njectivity ndex PGP njectivity ndex PLAN VEW Elev. (m a.s.l.) Prof. (TVD) PGP-6 GB < N WELL PAD 6 (589 m a.s.l.) PGP-23 Figure 6. Lithostratigraphic correlation at wellpad 6, Rincón de la Vieja Volcanic Products () This unit is composed of a lahar apron containing andesitic blocks in a yellow-brown silty to sandy matrix and recent andesitic lava flows from the Rincon de la Vieja volcanic complex (restricted to the northeast part of the field). A uniform thickness of this unit is maintained at all of the wellpads between the vertical wells and the directional wells. t was found thickest at wellpad 3 (due to the presence of recent andesitic lava flows) and thinnest at wellpad 6. Pital Formation () Pumiceous and crystalline lithic pyroclastic flows with few intercalated lacustrine deposits. This unit has been found in all of the deep wells and is associated with Cañas Dulces Caldera fill material. The Pital Formation basically has uniform thickness at each of the wellpads with small lateral variations. t is important however at wellpad 5 this unit thins slightly. Domes Unit (DU) The Domes Unit is composed of intra-caldera dacitic and rhyo-dacitic domes to the west of the Pailas Geothermal Field and within the Cañas Dulces Caldera. This unit has only been found at depth in the directional wells from wellpad 1 as can been seen by the outcropping dome unit to the west of the field. Biotite Flow () This unit is associated with the Liberia Formation, which is an extensive rhyolitic biotite and quartz bearing pumiceous pyroclastic flow sequence locally containing andesitic lithic clasts and rarely occurring granitoid clasts. t has been used as a marker bed in this field and is thickest in the central part of the field GB njectivity ndex PLAN VEW 23 6

4 at wellpad 1, thinning out slightly towards wellpad 3 to the north and notablemente to the east and south (wellpads 4, 6 and 5). Bagaces Group () The Bagaces Group is a sequence of pyroclastic events and intercalated lava flows. This group thickens from the northeast towards the southwest although at wellpad 3 a slight thickening is observed to the northeast. Locally, at wellpad 1 the Bagaces Group thickens to the south; at wellpad 6 it thickens to the north and at wellpad 5 the thickness of this unit is uniform towards the north. At wellpad 4 the total thickness of the Bagaces Group could not be determined due to total circulation loss in the three wells drilled at that pad, however bottomhole cores at wells PGP-4 and PGP- suggest that it may be nearly meters thick. Alcántaro Formation () The Alcántaro Formation () is a sequence of glassy dacitic lavas and ignimbrites which forms part of the Bagaces Group and is considered to mark the Cañas Dulces caldera border (Chavarría, et al. 21). This analysis shows that there is a uniform thickness the Alcántaro Formation at wellpads 4 and 6; at wellpad 5 there are thin intercalations of this unit within the Bagaces Group. The Alcántaro Formation was not observed at the wellpads in the northern part of the field (wellpads 1 and 3). Aguacate Group () This is the oldest rock unit drilled at the Pailas Geothermal Field and does not crop out in the area. t is comprised primarily of andesitic lavas with a few intercalated pyroclastic flows. Table 3. Unit thickness in vertical and directional wells (thickness in meters). PGP DU GA A ; ; Borehole stratigraphic correlation shows that the top of this unit deepens towards the west. However, lost circulation within the Bagaces Group limits the precision of the top of the Aguacate Group in some wells. CLAY ZONES n house x-ray diffraction analysis of clay contained in rock cuttings is used to estimate the thermal condition of the lithology and the geothermal reservoir at the Pailas Geothermal Borehole Field while drilling, based on the methodology presented in Sanchez et al,. Five clay zones have been observed in the boreholes and may be correlated between wellpads. The following sections compare the correlation of these clay zones as well as measured borehole temperatures at and between wellpads (Figures 7-11, Table 4). aolinite +/- ectite Zone ( +/- ) This clay zone is common to all of the wellpads at the Pailas Geothermal Field and represents an estimated temperature of <165 C. t is thickens from the northeast to the southwest which indicates lower temperature conditions in this sector of the field. Locally, at wellpad 1 this clay zone thickens in the directional wells towards the north; at wellpad 4 it thins in the directional wells which suggests better conditions to the northeast and east; and at wellpads 5 and 6 it thins towards the north. t is notable that to the north of wellpad 5 this clay zone is only 866 m thick as opposed to 134 m in well PGP-5, suggesting hotter conditions to the north of wellpad 5, although the bottom hole temperature is similar. Elev. Depth NNW PGP-12 PGP-1 PGP Tmax (S): 1 C C 165 C 22 C C Tmax (S): C Figure 7. ay zone correlation at wellpad PLAN VEW 12 1 WELLPAD 1 (659 m.a.s.l.) / C C E C 22 C C 22 C sotherm llite/ectite Curve llite Curve idote Curve C C C / / / / + / 935 / / / / + / 1592 Tmax (S): 225 C

5 Elev. Depth PGP Tmax (S): 8 C PGP-17A 1431 Tmax (S): 237 C Figure 8. ay zone correlation at wellpad 3. Elev. (m a.s.l.) Depth (TVD) Figure 9. ay zone correlation at wellpad 4. Elev. Prof Figure 1. ay zone correlation at wellpad 5. NE C C 22 C 3 C PLAN VEW WELLPAD 3 (757 m.a.s.l.) / C sotherm llite/ectite Curve llite Curve idote Curve NE SE PGP- PGP-4 PGP-25 Tmax (S): 5 C PGP Tmax (S): 155 C / C C 489 WELLPAD 4 (63 m a.s.l.) C C 4 C 1418 Tmax (S): 233 C C 1 23 C PLAN VEW N C WELL PAD 5 (554 m a.s.l.) PGP-27,, y / y / 1595 Tmax (S): sotherm llite Curve C 22 C llite/ectite Curve idote Curve C / + + / + / Tmax (S): 23 C sotherm llite/ectite Curve PLAN VEW Elev. (m a.s.l.) Prof. (TVD) PGP-6 / / Tmax (S): 139 C C N 22 C PGP Tmax (S): 138 C Figure 11. ay zone correlation at wellpad 6. C WELL PAD 6 (589 m a.s.l.) ectite +/- Chlorite Zone ( +/- ) The smectite +/- chlorite zone appears as a transition between the kaolinite +/- smectite zones and the illite/smectite +/- chlorite zone. This zone seems to indicate higher temperatures than zones where only smectite has been found however a definitive temperature range has not been determined. n all of the wells it is of reduced thickness, commonly intercalated between other clay zones and possible temperature inversions. This clay zone is more common in the western sector (wellpads 5 and 6) than elsewhere in the field. Chlorite Zone () This zone was only seen at wellpad 1 at well PGP-1, intercalated in the illite/smectite +/- chlorite zone however a definitive temperature range has not been determined. llite/ectite +/- Chlorite Zone ( +/- ) This zone is has been found below the clay zone containing smectite +/- kaolinite +/- chorite and continues to total depth at the Pailas Geothermal Field representing a temperature range from C. n some wells this zone is well defined from its first appearance and in others it is affected by temperature inversions represented by local intercalations of chlorite +/- smectite, illite +/- chlorite, and illite due to local variations in temperature. Also, this zone occurs locally within the smectite +/- kaolinite +/- chlorite zones in short intervals. Discontinuities in the illite/smectite +/- chlorite zone can be related to some lost circulation zones. t is within this clay zone that the majority of lost circulation zones have been found at this field. Also, local temperature inversions with depth correlate well with small intercalations of smectite / + / C sotherm llite/ectite Curve idote Curve PLAN VEW 23 6

6 clays within the illite/smectite clay zone. This zone has been observed most stable at wells PGP-12, PGP-3, PGP-17A, PGP-4, PGP- and PGP-25. llite +/- Chlorite Zone ( +/- ) This zone was found intercalated in the illite/smectite +/- chlorite zone at wellpads 3, 4, and 1 representing temperatures equal to or greater than 22 C. At wellpads 1 and 4 this clay zone occurs in two intervals within the illite/smectite +/- chlorite zone and may be correlated between these two wellpads. The presence of this clay zone in directional wells and not in vertical wells drilled at the same wellpad suggests hotter conditions along the azimuth of the directional wells and may possibly indicate a heat source or greater heat flow related to faulted rock layers nearby the wellpads. The illite zone is very shallow at some wells which indicates very localized high temperature zones that may be related to small open fractures. However it is difficult to determine true thickening or thinning of this clay zone since the base of its lower limit is unknown. Table 4. Total clay zone thickness (in meters) in vertical and directional wells (intercalations in parentheses) and maximum measured borehole temperatures. PGP +/- +/- +/- +/- T max ( C) (3) (5) (5) A (3) LOST CRCULATON, PERMEABLE ZONES, NJECTVTY NDCES AND STRUCTURAL NFERENCES The lost circulation zones are most common in the Bagaces Group and are distributed from the top to the base of this unit; however the wellpads at lower elevations towards the southwest (further from the Rincón de la Vieja volcanic complex) found lost circulation zones restricted to the basal section of the this group. n the following paragraphs only the lost circulation zones in the Bagaces and Aguacate groups are considered and their variations at each of the wellpads due to their affiliation with the geothermal reservoir. Permeable zones are mostly restricted to the Aguacate Group. Once the first total loss circulation zone is crossed, the wells are drilled using water and geodrill, occasionally aerated, as drilling fluids. Permeable zones are identified from the downhole temperature and pressure tests and quantified with injectivity tests (Figures 2-11, Table 5). n most cases the directional wells show a higher injectivity index than in the vertical wells which is interpreted as interception of wide influence fracture zones or relatively high angle dipping faults and fracture zones that were found at only a few hundred meters displacement from the wellpad. Table 5. Permeable zones and injectivity indicies (L/s/bar) at vertical and directional wells. PGP Permeable zones ii (L/s/bar) A > ND 27 2 ND 6 1 < Wellpad 1 Four permeable zones were found in wells PGP-12 and PGP-1 while only two permeable zones were found at PGP-11. The high injectivity index found at well PGP11 suggests that this well intercepted structure NF3 (Figure 1), an open high-angle fault to the east of the wellpad that based on borehole evidence is dipping 86 towards the southwest. Although well PGP-12 was drilled to the north looking for a NE-SW fracture taken from an older structural map prior to the confection of the structural framework suggested by WestJEC, the permeability

7 found in this borehole may possibly be related to the NF3 structure. The thickness of the permeable zones seems to be greater in the directional wells possibly associated with a wide influence fracture zone and/or proximity to a higher heat flow area which may move along this fracture zone. Wellpad 3 Two permeable zones were found in PGP-17A and six permeable zones were found in PGP-3. The high injectivity index found at well PGP-17A suggests that this well intercepted structure NF5 (Figure 1), an open fault to the east of the wellpad that based on borehole evidence is nearly vertical or dipping very slightly to the west. Wellpad 4 Two permeable zones were found in well PGP- and well PGP-25 while six permeable zones were found in well PGP-4. Although PGP- was drilled to the northeast looking for higher permeability and higher temperature suggested by borehole evidence from other vertical wells, prior to the confection of the structural framework suggested by WestJEC, the high injectivity index found at this well suggests that it may have intercepted NF3 (Figure 1), an open high angle fault to the northeast of the wellpad that may be dipping towards the southwest. Although there are only two permeable zones in well PGP-25, it is believed that this borehole intercepts the same fault as well PGP-4, (NF2B) only at a higher angle which permits a greater span along the face of the fault and thus better permeability. Based on borehole evidence the structure NF2B may be dipping 85 to the northwest. Wellpad 5 The only permeable zones determined at wellpad 5 are in well PGP-27 although it was not possible to quantify them. Well PGP-27 was drilled towards the north in search of better permeability possibly associated with the structure NF2A (Figure 1) and although some thermal zones were found the permeability is very low and the well had to be abandoned due to problems of formation stability. Further tests and data analysis need to be done at this well to determine if this borehole crossed the structure NF2A. Wellpad 6 Two permeable zones were found in well PGP-23 while only one permeable zone was found in well PGP-6 possibly due to its shallow depth. Well PGP-23 was drilled towards the north in search of better permeability possibly associated with the structure NF2A. nitially, the permeability at well PGP-23 was not sufficient for commercial brine injection however after repeated injectivity tests the well acceptance capacity increased enough to be used for commercial injection. n order to reach this objective a one kilometer separation to the north was necessary. SOTHERMS n this paper the C, 165 C, C, 22 C, 23 C and C isotherms are considered at each of the wellpads where directionally oriented wells have been drilled for a correlation between wellpads. From this analysis it can be seen that the isotherms increase in depth from the northeast towards the southwest. At wellpad 3 the C isotherm is found close to the surface (Figure 8) and at wellpad 5 the C isotherm is found at more than 7 meters depth. At wellpad 3 the depth of the isotherms decreases towards the northeast. A horizontal tendency is seen with the C and 22 C isotherms although the C isotherm deepens to the east. At wellpad 1 there is a slight deepening of the isotherms towards the north and a steep deepening of the isotherms towards the east. At wellpad 4 the depth of the isotherms decreases towards the directional wells (PGP- and PGP-25) as can be seen at the 23 C is found at a shallower depth to the east. At wellpad 6 a determinative comparison could not be made since well PGP-6 is very shallow and although the C appears to decrease towards the south, PGP-23 is in a period of temperature recuperation following repeated water injections. At wellpad 5 the isotherms decrease slightly in depth towards the north (Figures 7-11). CONCLUSONS Horizontal displacement in the seven directional boreholes at Pailas Geothermal Field has helped to determine fault/fracture-related zones to confirm the geologic model presented in Chavarría et al (21) and the structural model proposed by WestJEC since better permeability (possibly fracture-related) was found at a short distance from the wellpad, thereby permitting and inference of fault dip angle and direction based on correlation between directional wells and their vertical counterparts at each wellpad. Directional boreholes at wellpads 3, 1 and 4 have more favorable heat and permeability conditions than those found with vertically drilled wells at these wellpads and may be structurally controlled by faults. n the central and northern parts of the field (wellpads 1, 4 and 3), at a maximum distance of m to the north and east of the vertical wells there are much better temperature and permeability conditions. These directional wells were oriented in directions

8 where other pressure-temperature tests in other vertical wells indicated higher temperature and permeability conditions. There may be a new deep high permeability zone to the west of the current borehole field and possibly associated with the NF2A structure. n the western central part of the field well PGP-23 found better permeability conditions than well PGP-6 although it was necessary to drill up to one kilometer to the north to find these conditions. Unfortunately permeability could not be determined at PGP-27 due to formation instability. Better temperature and permeability conditions have been found towards the north and northeast of PGP-1 within a 2 km wide NE-SW oriented belt extending from borehole PGP-17A to boreholes PGP-1 and PGP- in the central part of the field and possibly extending towards wellpad 2 to the east (Figure 12). (-1) (-75) (-) (-23) (-35) Figure 12. Higher temperature and permeability zone (discontinuous blue line) and 22 C isotherms (black lines) placed over the geological and structural basemap (numbers in parentheses are meters above sea level of the 22 C isotherm). () Based on the current information, the highest permeability seems to be concentrated in the central part of the field, however this condition does not appear to be strictly related to higher temperatures since some high temperature-low permeability zones have also been found that may be structurally related although auto sealed by medium to high-temperature alteration minerals. FNAL REMARS Directional drilling has allowed CE to find better permeability conditions inaccessible with vertical boreholes, and thereby supply the necessary vapor and reinjection capacity so that Pailas 1 can start operation in April of 211 with a 35 MWe Binary Plant. Pailas Geothermal Field is the first borehole field in Costa Rica to employ directional drilling. To the west of the currently developed borehole field downhole evidence suggests a new permeable zone for the injection of geothermal brine that may be explored in the future. REFERENCAS Chavarría L., Mora O., Hakanson E., Gálvez M., Rojas M., Molina F. & Murillo A. (21) Geologic Model of the Pailas Geothermal Field, Guanacaste, Costa Rica. Proceedings World Geothermal Congress 21. Bali, ndonesia, April 21 Sánchez E., & Vallejos O. () Practical Uses of ay Minerals at the Miravalles Geothermal Field, Costa Rica. Proceedings World Geothermal Congress. yushu-tohoku, Japan, May 28 June 1,. WestJEC (21) nternal Report. Unpublished. The Alcántaro Formation is restricted to wellpads 4, 6 and 5. This suggests that these wellpads are placed on or close to the caldera rim. ay mineralogy analysis and the isotherms from measured borehole temperatures show that to the north-northeast of wellpad 4 there are higher temperature conditions. Lost circulation zones found in the directional wells indicate that there may be better permeability conditions in the northern and eastern parts of the field than in the southern part of the field.