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Nowadays, the stakes of operating hydrocarbon producing wells in mature field are getting higher. Advanced technologies are needed, and the industry must improve the cost efficiency of maturing assets to compensate for declining production and high fixed costs. One of the methods developed for efficient operation in producing hydrocarbon in mature field is one-phase-well (OPW), which is a well architecture without 9-5/8" surface casing. For safety reason, the conventional pressure temperature fluid analysis (PT-FA) logging can no longer be performed in OPW; alternative approaches are required to determine the potential of reservoirs. Pulsed neutron log (PNL) is proposed as a one-stop-shop solution to determine main reservoir characteristics and fluid status in anticipation of future OPW implementation. Latest generation PNL technology utilizes high-counting neutron generator coupled to high-resolution nuclear detectors to measure accurate oil and gas saturation by means of carbon/oxygen ratio (COR), in-situ total organic carbon, sigma, neutron porosity and novel fast neutron cross section (FNXS) measurements, while simultaneously providing accurate lithology volumes and porosity by means of advanced elemental spectroscopy combined with cased-hole porosity (TPHI). To evaluate the robustness and applicability of the method, the latest generation PNL was run in three recently drilled wells which have complete open-hole (OH) logs dataset along with fluid analysis (FA) and mudlog information. The PNL data were processed and interpreted independently, without utilizing any input from the OH log data and without the support of fluid analysis and mudlog information. A criterion based on comparison and correlation between fluid volumes and saturation defined by the PNL and the fluid analysis from FA was defined. For all the wells, the PNL interpretation results matched the hydrocarbon information from FA with success ratio higher than 90 percent over the logged interval, confirming its ability to perform accurate standalone evaluation and its value as alternative technology for effective fluid analysis. Because of the complexity of the system, conditions and potential risks must be properly analyzed case-by-case to encourage more massive application in the future.
Nowadays, the stakes of operating hydrocarbon producing wells in mature field are getting higher. Advanced technologies are needed, and the industry must improve the cost efficiency of maturing assets to compensate for declining production and high fixed costs. One of the methods developed for efficient operation in producing hydrocarbon in mature field is one-phase-well (OPW), which is a well architecture without 9-5/8" surface casing. For safety reason, the conventional pressure temperature fluid analysis (PT-FA) logging can no longer be performed in OPW; alternative approaches are required to determine the potential of reservoirs. Pulsed neutron log (PNL) is proposed as a one-stop-shop solution to determine main reservoir characteristics and fluid status in anticipation of future OPW implementation. Latest generation PNL technology utilizes high-counting neutron generator coupled to high-resolution nuclear detectors to measure accurate oil and gas saturation by means of carbon/oxygen ratio (COR), in-situ total organic carbon, sigma, neutron porosity and novel fast neutron cross section (FNXS) measurements, while simultaneously providing accurate lithology volumes and porosity by means of advanced elemental spectroscopy combined with cased-hole porosity (TPHI). To evaluate the robustness and applicability of the method, the latest generation PNL was run in three recently drilled wells which have complete open-hole (OH) logs dataset along with fluid analysis (FA) and mudlog information. The PNL data were processed and interpreted independently, without utilizing any input from the OH log data and without the support of fluid analysis and mudlog information. A criterion based on comparison and correlation between fluid volumes and saturation defined by the PNL and the fluid analysis from FA was defined. For all the wells, the PNL interpretation results matched the hydrocarbon information from FA with success ratio higher than 90 percent over the logged interval, confirming its ability to perform accurate standalone evaluation and its value as alternative technology for effective fluid analysis. Because of the complexity of the system, conditions and potential risks must be properly analyzed case-by-case to encourage more massive application in the future.
Pulsed neutron log (PNL) is well known for its main application to determine hydrocarbon saturation in cased-hole wells by means of neutron inelastic reaction with carbon and oxygen elements. However, the computed hydrocarbon saturation might be biased by the presence of hydrocarbon in the annulus between casing and formation, in which the cement is not evenly distributed. Cement quality analysis is important to complement the cased-hole analysis, not only to ensure the validity of the computed hydrocarbon saturation but also to provide valuable insights in term of drilling and workover to take necessary action in ensuring zonal isolation between reservoirs. Despite the available technology for cement evaluation, elemental spectroscopy reading from PNL can also be used to give an insight about the cement quality in the annulus since the emitted neutrons from PNL also interact with other elements in the formation and near borehole environment such as silicon, calcium, iron, magnesium, etc. which can be detected and quantified in term of count rate. The quantification of inelastic response between emitted neutron and calcium nuclei from PNL can serve as an indicator to define cement quality in annulus. This paper explains the proposed technique to determine the cement presence by utilizing the elemental spectroscopy reading of calcium since typical elemental composition of cement is dominated by calcium. First, the zones with good cement condition and engaged wellbore need to be defined from CBL and caliper data as reference zones. Then, elemental spectroscopy reading of calcium over these reference zones is used to create the estimated standard calcium spectroscopy response for the whole interval with the same completion feature. Poor cement quality or lack of cement volume is indicated whenever the actual calcium spectroscopy reading is significantly less than the estimated standard calcium spectroscopy response. The result is then validated using the available conventional cement evaluation log such as cement bond log (CBL) and variable density log (VDL). This proposed technique shows satisfactory result in most of the observed intervals. However, it is suggested to perform this technique using multi-arm caliper in order to better simulate the annulus volume for cement identification using down-hole spectroscopy data. It is also advised to utilize the latest generation triple-detector pulsed neutron log to obtain more precise elemental spectroscopy measurement.
ABC-1 exploration well was drilled through a carbonate build-up structure of Ngimbang Formation in offshore East Java, Indonesia. Standard triple-combo open hole logs were acquired by means of logging while drilling, while more advance wireline loggings were planned subsequently. Unfortunately, there were total losses during drilling which had to be managed by pressurized mud cap drilling (PMCD) which prevent from cuttings recovery for the rest of the interval. Multiple trips were also required to drill the well safely resulting in rugose and enlarged borehole. These conditions did not allow open-hole wireline log to access the target located at the lower interval of the well. It was inevitable to complete the well despite not enough data had been acquired to perform a comprehensive formation evaluation. In order to obtain remaining required data, it was decided to complement the compromised open-hole data with an advance pulsed neutron log (PNL) device, which offered several unique measurements to tackle the harsh conditions. Some of these key measurements are: (1) a self-compensation algorithm which provided robust sigma (SIGM) and cased-hole porosity measurement (TPHI), which was used to further validate neutron from LWD. (2) A combination of both capture and inelastic high definition elemental spectra measurement were utilized to obtain accurate mineralogy fraction. (3) carbon-oxygen ratio (COR) high precision measurement to calculate oil saturation. Lastly (4), fast neutron capture cross-section (FNXS) measurement was also acquired to give insight on possible gas occurrence even in tight zones. The advance PNL, acquired over 3 passes, showed consistent reading of sigma, TPHI, FNXS and elemental spectroscopy measurement. However, there were some discrepancies in between COR passes, which eventually has shed some light on what happened in this well. The first pass did not really show any potential oil along the carbonate body. Then, the second pass started to reveal potential oil around the top part of the carbonate, where resistivity is low with no distinctive neutron-density crossover. The third pass revealed an even more oil volume along the top carbonate. There is a possibility that the increase of oil reading might be due to the changing environment during logging, allowing some invasion to dissipate along the carbonate tops. This implies that there might be yet another oil zone below the revealed oil interval, should the invasion fluid start to dissipate. Subsequent well test showed significant oil production over the interval identified from the PNL interpretation, which put ABC-1 as one of the most successful Indonesian exploration well in 2021. This case study shows the success of utilizing advance pulsed neutron log to perform comprehensive formation evaluation under challenging condition, which can be used as reference for tackling similar drilling challenges in the future.
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