Located in offshore South China Sea, Ledong high-pressure/high-temperature (HPHT) gas field has entered the appraisal phase after the first discovery was announced in 2015. The pressure gradient of the main target zone is close to 2.2 g/cm3 whereas the top formation is approximately 1.8 g/cm3; the sand packages are separated by a variable shale layer thickness. To avoid kicks, mud losses, or other drilling problems, mud weight must be adjusted accordingly to preserve the well integrity. Hence, the main objective for this hole section is to stop drilling above the main target sand and set casing to isolate the formation with different pressure gradient. An innovative look-ahead technology based on deep electromagnetic measurements was used to predict the formation change ahead of bit in real time to reduce the drilling risk. After review of the technical and geological challenges encountered in this field, this paper will discuss the successful approach taken to detect formation changes using the new technique. After the shale layer above the target sand has been identified in real-time, drilling will stop above the high-pressured sand to set casing. In addition, the authors will also describe the bottom hole assembly (BHA) configuration and measurement selection in the planning phase to ensure the success of this well. The real-time interpretation of look-ahead measurements enables boundary detection ahead of the bit at distances ranging from 3 to 20 m in this example. The depth of detection depends on the resistivity contrast between formation, layer thickness, presence of laminations, and the transmitter-to-receiver distance. The application of the innovative look-ahead technology has helped to Reduce the drilling risk by detecting formation change ahead of the bit Accurately identify casing shoe position to ensure well integrity Eliminate extra casing string, which will directly increase the well construction cost Avoid unnecessary operation adjustments and improve drilling efficiency Clear prediction of the resistivity profile ahead of the bit enables proactive decision making while drilling. This additional information has removed the need to consider the possibilities for different scenarios and the extra circulating time taken to make decisions among stake holders. The successful implementation of the look-ahead technology and the application in the HPHT well has led to reduction in overall nonproductive time by reducing drilling risk and improving drilling efficiency. This innovative technique changes the real-time decision-making process while delivering a new way to manage drilling risk.
This paper presents a case study of hydraulic flow unit (HFU) based permeability characterization and fast production prediction for an offshore field, South China Sea. The aim is to integrate cores, petrophysics logs, formation tester and drill string well test (DST) data together to build a generally applicable workflow for rapid production prediction immediately after logging, in order to facilitate the decision making of the drill stem testing and completion. Overall productivity of a well and productivity of each zone is critical information for well evaluation. Based on a quick yet reliable answer, operators can decide whether the well should be tested by DST /completed. If the prediction is within the acceptable accuracy range, the number of zones to be tested could be reduced. Therefore well cost can be greatly saved. The major challenges are facing in this field are: highly laminated sand/shale sequences, strong heterogeneities, complex porpermeability relationships, and different scales of different measurements. Based on the field geological settings, total four sub areas, two sub zones were delineated. Different HFUs were identified from core data for each area and zone. The porositypermeability models were built in each HFU. The permeability output was calibrated by formation tester interpreted effective permeability to get calibration scale factor. Based on the scale factor, a workflow was then set up to predict the production for new wells, from either core data or petrophysical data, depending on the data availability. Three DST results were used to validate the results from this workflow, giving great confidence of the applicability in the study result (the prediction accuracy is within 20% range). This case study brings following values: a quick production prediction workflow has been set up for this field; and the methodology applied is not limited to this specific field but applicable to other fields with similar formation types.
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