Abstract Groundwater at depths exceeding 500 m can be an important source of freshwater. However, the characteristics of deep groundwater in many regions of the world, including Indonesia’s sedimentary basins, remain vaguely defined. This study investigates the pressure regimes, hydraulic head distributions, salinity, and hydrochemical facies of deep groundwater using available evidence from oil and gas exploration wells in the Lower Kutai Basin of Indonesia. Pressure measurements and wireline log data reveal three pressure regimes within the basin: hydrostatic, overpressure, and underpressure. The top of the overpressure varies, from close to the surface onshore to depths of ~4.5 and ~3.8 km in the Mahakam Delta and offshore, respectively. Computed hydraulic heads at the top of the overpressure range from ~191 m above sea level onshore to ~71 m below sea level offshore, and are indicative of regional groundwater flow. The observed salinity of deep groundwater within the basin indicates predominantly brackish or saline conditions. Fresh (total dissolved solids < 1 g/L) groundwater to a depth of ~2 km is found at a small minority of wells onshore and in the delta; no fresh groundwater is found offshore. Four hydrochemical facies are observed: Na + /Cl – , Ca 2+ /Cl – , Na + /HCO 3 – , and Na + –Ca 2+ /HCO 3 – . This study indicates that deep fresh groundwater in the Lower Kutai Basin is of localized occurrence. Recharge from meteoric water may replenish deep fresh groundwater within the hydrostatic zone and sustain water supplies, whether brackish or fresh. Water produced from clay diagenesis is also cited as a possible process of freshening deep groundwater.
Summary Deep (>500 m below ground level) groundwater in Indonesia's sedimentary basins, including the Lower Kutai Basin, has yet to be well characterized. Here, we present an approach to characterise groundwater salinity in the Lower Kutai Basin using resistivity logs, which are tested against available measurements of groundwater salinity. Synthesised groundwater salinity data suggest that fresh (total dissolved solids ≤ 1000 mg/L) groundwater can occur to depths of up to ∼1.3 km. The data indicate that the most deep groundwater is either brackish or saline. By applying Archie's equation and adjusting its empirical constant, we generate a formation resistivity cut-off to identify fresh groundwater in sandstone. We note, however, that the presence of hydrocarbons can increase the formation resistivity. We highlight that knowledge of hydrocarbon occurrence is essential in the characterisation of groundwater salinity from resistivity logs in deep sedimentary basins.
ABSTRACT The use of tritium isotope is one of the methods for determining groundwater age. It can be used to determine the age of groundwater classified as young. However, if it is the only method used, the results may not be valid. In this study, tritium application in determining groundwater age was evaluated based on deuterium and oxygen-18 isotopes and hydrochemical data from seven mud volcanoes and one hydrocarbon production well in East Java Basin. The tritium analysis shows that the age of groundwater samples is young, 1.75 to 9 years. However, deuterium, oxygen-18, and hydrochemical analysis indicate that the groundwater age is relatively old. It shows that the results of groundwater age analysis using tritium are not valid in this study. It is because tritium is not only from the atmosphere but also from tritium enrichment below the surface through water and rock interaction. The shifting of oxygen-18 isotope, which becomes heavier, indicates that isotope enrichment occurred in the subsurface. Based on the composition of the major cations and anions, the groundwater samples in this study have Na-Cl type with high TDS values as saline water. The shifting of oxygen-18 isotope, the water type, and the high TDS value also indicate that water and rock interaction occurs beneath the surface and can increase the tritium content in groundwater.Keywords: deuterium, groundwater age, hydrochemical data, mud volcano, oxygen-18, tritiumABSTRAK Salah satu metode untuk menentukan umur airtanah adalah dengan menggunakan isotop tritium. Tritium dapat digunakan untuk menentukan umur airtanah yang tergolong muda. Namun, jika hanya menggunakan metode ini, maka hasil yang diperoleh mungkin tidak valid. Dalam studi ini, penggunaan tritium untuk menentukan umur airtanah dievaluasi berdasarkan data isotop deuterium dan oksigen-18, serta data hidrokimia dari tujuh gunung lumpur dan satu sumur produksi hidrokarbon di Cekungan Jawa Timur. Analisis tritium menunjukkan umur sampel airtanah yang tergolong muda, yaitu 1,75 hingga 9 tahun. Namun, analisis deuterium, oksigen-18, dan analisis hidrokimia menunjukkan bahwa umur airtanah tergolong tua. Hal tersebut memperlihatkan bahwa hasil analisis umur airtanah berdasarkan tritium pada studi ini tidak valid. Hal ini disebabkan karena tritium tidak hanya berasal dari atmosfer, tetapi dapat juga berasal dari pengayaan tritium di bawah permukaan melalui reaksi air dan batuan. Pergeseran nilai oksigen-18 yang menjadi semakin berat mengindikasikan bahwa terjadi pengayaan isotop tersebut di bawah permukaan. Berdasarkan komposisi kation dan anion utama, sampel airtanah dalam studi ini memiliki tipe Na-Cl dengan nilai TDS yang tinggi dan air yang tergolong asin. Pergeseran oksigen-18, tipe air dan nilai TDS yang tinggi tersebut juga menunjukkan bahwa terjadi interaksi air dan batuan di bawah permukaan yang dapat meningkatkan nilai tritium pada airtanah.Kata kunci: deuterium, umur airtanah, data hidrokimia, mud volcano, oksigen-18, tritium
Quantifying the change of minimum horizontal stress (Shmin) in response to pore pressure is vital, particularly at the development stage of a hydrocarbon field. However, estimates of the coupling ratio between these parameters, either at basin or reservoir scales, are not well defined in Indonesia’s sedimentary basin, such as Lower Kutai Basin. This study investigates the coupling ratio between Shmin and pore pressure in one of the gas fields within the basin, Tunu Field, using available evidence from an extensive data set of direct pressure measurement, leak-off test, extended LOT, Mini-Frac, and lost circulation. Previous studies have reported overpressured and depleted reservoirs. Nevertheless, the evolution of Shmin due to the change in pore pressure has yet to be well established. This study reveals a Shmin and vertical stress ratio of 0.87 which may relate to the stiffening of the shales via clay diagenesis/cementation. Coupling ratios between Shmin and pore pressure are 0.3 for the overpressured zone at the basin scale and 0.42 for the depletion at the reservoir scale. The approach used has been shown to produce similar coupling ratio (or stress path) results to other basins globally, thus providing a useful estimate of the Shmin and pore pressure relationship for the Tunu Field, and potentially other fields in the Lower Kutai Basin. For the application to infill drillings within the field, this study indicates Shmin decreases in response to pore pressure depletion due to production, ranging from 1.7 to 2.3 ppg within the hydrostatic zone and from 2.5 to 3.7 within the overpressured zone, equivalent to a Pore Pressure (PP)-Shmin window decrease of approximately 28% to 37% and approximately 40% to 88%, respectively. In addition, strike-slip faulting is suggested as the likely in situ stress regime in the Tunu Field.
Understanding pore pressure characteristics in carbonate reservoirs is essential, because it concerns all aspects of subsurface, drilling, and occupational safety, as well as the environment in oil and gas exploration and exploitation. In this study, the pore pressure regime and connectivity of the carbonate reservoir (compartmentalization) of Kujung and Ngimbang Formations in the northern Madura Platform of East Java Basin are characterized by utilizing direct pressure data and wireline logs from five wells. The result shows that both hydrostatic and slight overpressure conditions are found in these formations, further indicating the presence of carbonate compartmentalization. The magnitudes of overpressure, however, are mostly very low, with an average of ~37 psi. Four compartmentalized carbonates are identified based on the presence of different pressure gradients in the water leg. The slight overpressure in the carbonates is likely owing to pressure transference from overpressured shale at depth. In addition, by using acoustic impedance from seismic, the depositional environments of the carbonates are interpreted as lagoon and tidal flat. These environments support the occurrence of patch reef carbonate buildups in the Kujung and Ngimbang Formations, which later become a good environment for overpressure generation and carbonate reservoir compartmentalization. Keywords: carbonates, compartmentalization, depositional environment, slight overpressure
Abstract The 3D distribution of hydraulic conductivity in the unconfined aquifer is needed to understand the interaction between groundwater and Citarum River, which may relate to water contamination and flooding. In this study, it was estimated using the geostatistical method. The data used are the hydraulic conductivity parameter from thirteen large-diameter dug wells determined using the slug test method. Further analysis was conducted using groundwater and Citarum River physical parameters, i.e. total dissolved solids, pH, and temperature. The results show that the hydraulic conductivity of the unconfined aquifer in the Upper Citarum River ranges from 8.34 x 10 −7 to 2.19 x 10 −5 m/s. The area with relatively high hydraulic conductivity, i.e. Ciparay and Solokan Jeruk Sub-districts, will have a greater possibility of groundwater or river water contamination depending on their interaction. Meanwhile, the area with relatively flat topography and low hydraulic conductivity, i.e. Majalaya and Baleendah Sub-districts, will be more susceptible to flood. Therefore, the mitigation of water contamination and flooding in the Upper Citarum River should consider the 3D hydraulic conductivity distribution, e.g. by monitoring the groundwater quality in some dug wells located in high hydraulic conductivity area and installing recharge or injection well to eliminate the artificial runoff.
ABSTRAKUji pemompaan merupakan metode yang dianggap paling baik dalam menentukan nilai parameter akuifer. Namun, ada kemungkinan bahwa nilai parameter akuifer yang dihitung menggunakan data uji pemompaan tersebut tidak merepresentasikan nilai parameter akuifer yang sebenarnya. Sebagai sebuah studi kasus, data uji pemompaan akuifer tidak tertekan dari dua sumur observasi (1 dan 2) pada salah satu daerah di SE Asia dianalisis menggunakan metode pencocokan kurva dengan dua skenario. Pada skenario 1, nilai parameter akuifer dihitung dengan mengasumsikan sumur pemompaan dan observasi menembus seluruh ketebalan akuifer (fully penetrating well), sedangkan pada skenario 2 nilai parameter akuifer dihitung berdasarkan kondisi sumur pemompaan dan observasi yang sebenarnya, yaitu menembus hanya sebagian ketebalan akuifer (partially penetrating well). Selain itu, nilai konduktivitas hidraulik akuifer juga diestimasi berdasarkan analisis ukuran butir. Hasilnya menunjukkan bahwa asumsi pada skenario 1 akan menyebabkan nilai konduktivitas hidraulik dalam arah vertikal menjadi lebih besar dari yang seharusnya. Nilai specific yield yang mendekati nilai referensi, berdasarkan litologi pasir kerikilan yang dominan pada daerah studi, hanya diperoleh menggunakan data uji pemompaan dari sumur observasi 1 pada kedua skenario. Adapun hasil estimasi nilai konduktivitas hidraulik berdasarkan analisis ukuran butir memperlihatkan nilai konduktivitas hidraulik yang lebih besar dibandingkan nilai konduktivitas hidraulik yang dihitung berdasarkan data uji pemompaan.Kata kunci: akuifer tidak tertekan, analisis ukuran butir, parameter akuifer, uji pemompaanABSTRACT The pumping test is considered as the best method in determining the values of the aquifer parameter. However, there is a possibility that the values calculated using the pumping test data do not represent the actual aquifer parameter values. As a case study, pumping test data of unconfined aquifer from two observation wells (1 and 2) in an area of SE Asia were analyzed using the Neuman’curve matching method with two scenarios. In scenario 1, the aquifer parameter values were calculated by assuming the pumping and observation wells penetrate the entire thickness of the aquifer (fully penetrating well), while in scenario 2 the values were calculated based on the actual conditions of the pumping and observation wells, i.e., penetrating the aquifer thickness partially (partially penetrating well). In addition, the hydraulic conductivity of the aquifer was also estimated based on grain size analysis. The results show that the assumptions in scenario 1 will cause the vertical hydraulic conductivity value to be greater than it should be. The specific yield value close to the reference value, based on the dominant lithology of gravelly sand in the study area, was only obtained using the pumping test data from the observation well 1 in both scenarios. The results of the hydraulic conductivity value estimation based on grain size analysis show that the hydraulic conductivity value is greater than the hydraulic conductivity value calculated based on the pumping test data. Keywords: unconfined aquifer, grain size analysis, aquifer parameter, pumping test
Abstract The Indonesian Buoy for Tsunami Early Warning System (INATEWS - Buoy) consists of a tsunami buoy, an ocean bottom unit (OBU), a satellite and a ground station. The changes of the water pressure due to the seismic motion prior to the tsunami event will be acquired by OBU and sent the reading to the tsunami buoy by acoustic signal. One of the conditions that affect the success of sending data from the OBU to the buoy is the distance between the two (offset). Several parameters that are considered to have an effect on the occurrence of offsets on the buoy are the magnitude of the environmental load around the buoy and the value of the scope mooring line used. Environmental loads that consist of current, wave and wind loads are analyzed. Analysis of each environmental load is carried out separately to determine the level of load sensitivity to the offset buoy. In addition, a buoy offset study was also carried out on several scope values from the mooring line to determine the change in the buoy offset. Offset prediction on buoys is calculated by numerical simulation. From this numerical simulation, it found that changes in environmental load, especially changes in current speed, significantly affect the offset value of the buoy that occurs. Likewise with the determination of the mooring line scope, where the smaller the scope value used in the mooring line system, the smaller the offset value that occurs. Finally, the designed mooring systems while applied at the selected operational buoy location, will have Safety Factor 3.61 and 4.57 for wire and nylon rope mooring line components, respectively.
High overpressure is a critical drilling issue in the Lower Kutai Basin. Typical pore pressure prediction approaches involve an empirical relationship, such as Eaton’s method using sonic log data. In areas with high geothermal gradients, such as the Lower Kutai Basin, there is evidence for additional overpressure from gas generation such that sediment unloading must be considered to interpret pore pressure correctly. In this paper a repeatable deterministic model is presented for pore pressure from sonic data and, using selected wells from the Lower Kutai Basin, also the use of the resistivity log in a similar model. In the Lower Kutai Basin, sonic logs are often absent from the logging suite or otherwise running over limited intervals, making an alternative log-based prediction method particularly valuable. As a caveat, shallow freshwater encroachment is reported in the Lower Kutai Basin, means the shallow resistivity data can be problematic to use to define both top of overpressure and a normal compaction trend. Care must therefore be taken if resistivity is to be used for the interpretation of unloaded pore pressure, and chiefly applied and this likely to be more successful where encroachment is less pronounced, such as pro-delta shales. Assuming the additional care needed in using resistivity data, this paper suggests that resistivity can be a useful tool for pore pressure prediction in unloaded shale at elevated temperatures within the Lower Kutai Basin. At present the technique has been applied to only a limited dataset due to data availability limitations, but it is hoped with further refinement it will form a helpful additional approach in the pore pressure prediction toolkit. Keywords: pore pressure, resistivity, unloading, Lower Kutai Basin
Abstract Knowledge of a well-constrained geomechanical model provides valuable information for designing development wells more efficiently for drilling and completion in a mature gas field. This paper presents results of a recent geomechanical analysis designed to evaluate reservoir and overburden rock behavior in order to drill horizontal wells in Badak field, VICO Indonesia’s largest gas fields. During development of the geomechanical model, an enormous effort focused on maximizing the use of available information from all the Badak wells in the field. Data included open hole logs, core data, pore pressure data, minifrac data, high-resolution image logs, previous drilling experiences and regional studies. The synthesis of this data resulted in a well-constrained geomechanical model that could explain the previously unsuccessful and problematic horizontal and vertical wells; but more importantly, it provided the needed information to predict well performances for future designs that minimize the risks associated with future horizontal wells. VICO has implemented this geomechanical approach to designing and drilling wells as part of VICO’s low permeability development program. A deep horizontal well has successfully drilled to the main target using geomechanical modeling information.
'%3e%3cpath fill='%23012169' d='M0 0v30h60V0z'/%3e%3cpath stroke='%23fff' stroke-width='6' d='m0 0 60 30m0-30L0 30'/%3e%3cpath stroke='%23C8102E' stroke-width='4' d='m0 0 60 30m0-30L0 30' clip-path='url(%23b)'/%3e%3cpath stroke='%23fff' stroke-width='10' d='M30 0v30M0 15h60'/%3e%3cpath stroke='%23C8102E' stroke-width='6' d='M30 0v30M0 15h60'/%3e%3c/g%3e%3c/svg%3e)
