Field-Proven Well Surveillance Techniques Designed for an Early Well Scaling Diagnosis and Timely Well Performance Reinstatement: Case Study from the Kashagan Field

Some wells in the Kashagan field did not perform as well as expected. Despite producing virtually no water, calcite deposition was found to be the root cause of the problem. A comprehensive well surveillance program, which was proven to be very efficient for an early scaling diagnosis, was developed by the operator, North Caspian Operating Company (hereafter NCOC). As a result, well scaling is currently well managed and prevented from reoccurring. The objective of this paper is to share an early experience with well scaling in the Kashagan field, as well as to describe the developed set of well surveillance techniques. The aim of the various well surveillance techniques discussed in this paper is to improve an Operator's ability to identify the very first signs of scale accumulation. This, in its turn, enables to introduce timely adjustments to the well operating envelope and to schedule a scale remediation / inhibition treatment with the intention to prevent any potential scaling initiation from further development. The approach is quite extensive and incorporates continuous BHP/BHT monitoring, routine well testing, PTA analysis, and fluid/water sampling. Developed approach experienced multiple revisions and modifications. Further optimization continues, however, the described well surveillance techniques represent the latest Operator's vision on the most efficient way for well scaling monitoring and identification. In the Kashagan field, BHP/BHT readings have proved to be the most direct and instantaneous indication of any early signs of potential deterioration in well performance (qualitative analysis) while well testing and PTAs are considered as the most essential techniques in confirming and quantifying scaling severity (quantitative analysis). It is important to mention that BHT increase is explained by Joule-Thomson heating effect being specific for the Kashagan fluid (happening during increased pressure drawdown). This, in turns, enables to predict future well performance, design well operating envelop accordingly and, most importantly, develop a yearly schedule for proactive well treatments with SI. In conclusion, it shall be highlighted that discussed complex of well surveillance techniques has been concluded to be very efficient and reliable tool in identifying any scaling tendencies at its initial stage. Due to successful implementation of this approach in the Kashagan field, scale development is now well-managed and kept under control. To mention, that utilization of well surveillance techniques and methods outlined in this paper may reduce the time required to identify and ultimately mitigate well scale accumulation in any active assets with similar operating environments.

Farmed calcite δ18O, δ13C, and Δ47 at Ascunsă cave, Romania

<p>Ascunsă cave (Romania) is the subject of a monitoring program since 2012. While the cave air temperature was very stable around 7°C for most of the time, it experienced in 2019 a 3°C rise, and remained high until the present.</p><p>We present here δ<sup>18</sup>O, δ<sup>13</sup>C, and clumped isotope results from calcite farmed at two drip points inside the cave (POM X and POM 2). POM X has a slower drip rate than POM 2 and deposits calcite more continuously. Calcite deposition has been shown to depend on cave air CO<sub>2</sub> concentration, which controls the drip water pH and, further, the calcite saturation index.</p><p>In 2019, δ<sup>18</sup>O values at both sites quickly shifted to lower values as a response to the increase in temperature. At POM X, values were situated between approximately -7.2‰ and -7.6‰ before this transition, whereas in 2019 they shifted to -7.8‰ - -8.0‰. At POM 2, where values were generally lower, they shifted from -7.5‰ to -7.8‰ to -8.0‰.</p><p>Clumped isotope temperature estimates mostly agree, within measurement error, with measured cave temperature. This agreement is notable given that strong offsets are commonly observed in mid-latitude caves, reflecting kinetic fractionation effects. However, intervals with deviations from cave temperature are also observed, suggesting variations in isotopic disequilibrium conditions with time.</p><p>Here we will discuss these isotope changes in relation to cave air temperature and CO<sub>2</sub> concentration, drip water isotope values and elemental chemistry, as well as in relation to drip rates, in order to improve our understanding of calcite precipitation and isotope effects in caves.</p>

Novel method for determining 234U-238U ages of Devils Hole 2 cave calcite (Nevada)

<p>Uranium-uranium (<sup>234</sup>U-<sup>238</sup>U) disequilibrium dating can determine the age of secondary carbonates over greater time intervals than the well-established <sup>230</sup>Th-<sup>234</sup>U dating method. Yet it is rarely applied due to unknowns in the initial d<sup>234</sup>U (d<sup>234</sup>U<sub>i</sub>) value, which result in significant age uncertainties. In order to understand the d<sup>234</sup>U<sub>i</sub> in Devils Hole 2 cave, Nevada, we have determined 110 d<sup>234</sup>U<sub>i</sub> values from phreatic calcite using <sup>230</sup>Th-<sup>234</sup>U disequilibrium dating. The sampled calcite was deposited in Devils Hole 2 between 4 and 590 ka, providing a long-term look at d<sup>234</sup>U<sub>i</sub> variability over time. We then performed multi-linear regression among the d<sup>234</sup>U<sub>i</sub> values and correlative d<sup>18</sup>O and d<sup>13</sup>C values. The regression can be used to estimate the d<sup>234</sup>U<sub>i</sub> value of Devils Hole calcite based upon its measured d<sup>18</sup>O and d<sup>13</sup>C values. Using this approach and the measured present-day d<sup>234</sup>U values of Devils Hole 2 calcite, we calculated 110 independent <sup>234</sup>U-<sup>238</sup>U ages. In addition, we used newly measured d<sup>18</sup>O, d<sup>13</sup>C, and present-day d<sup>234</sup>U values to calculate 10 <sup>234</sup>U-<sup>238</sup>U ages that range between 676 and 731 ka, thus allowing us to extend the Devils Hole chronology beyond the <sup>230</sup>Th-<sup>234</sup>U-dated chronology while maintaining an age precision of ~2 %. Our results indicate that calcite deposition at Devils Hole 2 cave began no later than 736 ± 11 kyr ago. The novel method presented here may be applied to future speleothem studies in similar hydrogeological settings, given appropriate calibration studies.</p>

Novel method for determining ²³⁴U–²³⁸U ages of Devils Hole 2 cave calcite (Nevada)

Uranium–uranium (234U–238U) disequilibrium dating can determine the age of secondary carbonates over greater time intervals than the well-established 230Th–234U dating method. Yet it is rarely applied due to unknowns in the initial δ234U (δ234Ui) value, which result in significant age uncertainties. In order to understand the δ234Ui in Devils Hole 2 cave, Nevada, we have determined 110 δ234Ui values from phreatic calcite using 230Th–234U disequilibrium dating. The sampled calcite was deposited in Devils Hole 2 between 4 and 590 ka, providing a long-term look at δ234Ui variability over time. We then performed multi-linear regression among the δ234Ui values and correlative δ18O and δ13C values. The regression can be used to estimate the δ234Ui value of Devils Hole calcite based upon its measured δ18O and δ13C values. Using this approach and the measured present-day δ234U values of Devils Hole 2 calcite, we calculated 110 independent 234U–238U ages. In addition, we used newly measured δ18O, δ13C, and present-day δ234U values to calculate 10 234U–238U ages that range between 676 and 731 ka, thus allowing us to extend the Devils Hole chronology beyond the 230Th–234U-dated chronology while maintaining an age precision of ∼ 2 %. Our results indicate that calcite deposition at Devils Hole 2 cave began no later than 736 ± 11 kyr ago. The novel method presented here may be applied to future speleothem studies in similar hydrogeological settings, given appropriate calibration studies.

Novel method for determining ²³⁴U-²³⁸U ages of Devils Hole 2 cave calcite

Uranium-uranium (234U-238U) dating can determine the age of secondary carbonates over greater time intervals than the well-established 230Th-234U dating method. Yet it is rarely applied due to unknowns surrounding the initial δ234U (δ234Ui) value, which result in significant age uncertainties. In order to understand the δ234Ui in Devils Hole 2 cave, we have precisely determined 110 δ234Ui values from phreatic calcite crusts using a 230Th-234U chronology. The sampled calcite crusts were deposited in Devils Hole 2 between 4 and 590 thousand years, providing a long-term look at δ234Ui variability over time. We then performed multi-linear regressions among the δ234Ui values and correlative δ18O and δ13C values. These regressions allow us to predict the δ234Ui value of Devils Hole calcite based upon its δ18O and δ13C. Using this approach and measured present-day &felta;234U values, we calculate 110 independent 234U-238U ages of Devils Hole 2 cave deposits. In addition, we used newly measured δ18O, δ13C, and present-day δ234U values to calculate 10 234U-238U ages that range between 676 and 731 thousand years, thus allowing us to extend the Devils Hole chronology beyond the 230Th-234U-dated chronology while maintaining an age precision of ~2 %. Our results indicate that calcite deposition at Devils Hole 2 cave began no later than 736 ± 11 thousand years ago. The novel method presented here may be used in future speleothem studies in similar hydrogeological settings, given appropriate calibration studies.

Helium, inorganic and organic carbon isotopes of fluids and gases across the Costa Rica convergent margin

In 2017, fluid and gas samples were collected across the Costa Rican Arc. He and Ne isotopes, C isotopes as well as total organic and inorganic carbon concentrations were measured. The samples (n = 24) from 2017 are accompanied by (n = 17) samples collected in 2008, 2010 and 2012. He-isotopes ranged from arc-like (6.8 RA) to crustal (0.5 RA). Measured dissolved inorganic carbon (DIC) δ13CVPDB values varied from 3.55 to −21.57‰, with dissolved organic carbon (DOC) following the trends of DIC. Gas phase CO2 only occurs within ~20 km of the arc; δ13CVPDB values varied from −0.84 to −5.23‰. Onsite, pH, conductivity, temperature and dissolved oxygen (DO) were measured; pH ranged from 0.9–10.0, conductivity from 200–91,900 μS/cm, temperatures from 23–89 °C and DO from 2–84%. Data were used to develop a model which suggests that ~91 ± 4.0% of carbon released from the slab/mantle beneath the Costa Rican forearc is sequestered within the crust by calcite deposition with an additional 3.3 ± 1.3% incorporated into autotrophic biomass.