Pre-Drilling Prediction Techniques on the High-Temperature High-Pressure Hydrocarbon Reservoirs Offshore Hainan Island,China

Decreasing the risks and geohazards associated with drilling engineering in high-temperature high-pressure(HTHP) geologic settings begins with the implementation of pre-drilling prediction techniques(PPTs). To improve the accuracy of geopressure prediction in HTHP hydrocarbon reservoirs offshore Hainan Island, we made a comprehensive summary of current PPTs to identify existing problems and challenges by analyzing the global distribution of HTHP hydrocarbon reservoirs, the research status of PPTs, and the geologic setting and its HTHP formation mechanism. Our research results indicate that the HTHP formation mechanism in the study area is caused by multiple factors, including rapid loading, diapir intrusions, hydrocarbon generation, and the thermal expansion of pore fluids. Due to this multi-factor interaction, a cloud of HTHP hydrocarbon reservoirs has developed in the Ying-Qiong Basin, but only traditional PPTs have been implemented, based on the assumption of conditions that do not conform to the actual geologic environment, e.g., Bellotti's law and Eaton's law. In this paper, we focus on these issues, identify some challenges and solutions, and call for further PPT research to address the drawbacks of previous works and meet the challenges associated with the deepwater technology gap. In this way, we hope to contribute to the improved accuracy of geopressure prediction prior to drilling and provide support for future HTHP drilling offshore Hainan Island.

Simulation of Shallow Gas Invasion Process During Deepwater Drilling and Its Control Measures

Shallow gas is considered one of the most serious geological hazards in deepwater drilling because it has the characteristics of suddenness and is difficult to deal with.To perform a quantitative evaluation of shallow gas risk during deepwater drilling,a numerical model for calculating gas invasion volume is established based on gas-water two-phase flow theory.The model considers the effect of the dynamic drilling process,and the influencing factors which affect the gas invasion volume are analyzed.Results indicate that the gas invasion rate and accumulated gas invasion volume increase with increasing bottom-hole pressure difference.A linear relationship exists between gas invasion volume and bottom-hole pressure difference.The duration of gas invasion increases as the shallow gas zone thickness increases,and the accumulated gas invasion volume grows as shallow gas zone thickness increases.The increase in formation permeability,water depth,and rate of penetration will enhance the gas invasion rate.However,these three factors can hardly affect the accumulated gas invasion volume.The gas flow rate increases significantly with increasing burial depth of shallow gas.On the basis of influencing factor analysis,a series of methods that consider different risk levels is proposed to control shallow gas,which can provide a reference for the prevention of shallow gas disasters during deepwater drilling.

Development of the Miocene Guangle Carbonate Platform in the South China Sea:Architecture and Controlling Factors

This study investigates the evolution of the Miocene Guangle carbonate platform(or Triton Horst)of the northwestern South China Sea margin.The platform is located at a junction area surrounded by Yinggehai basin,Qiongdongnan basin and Zhongjiannan basin.Well and regional geophysical data allow the identification of the morphologic and stratigraphic patterns.The Guangle carbonate platform was initiated on a tectonic uplift during the Early Miocene.The early platform was limited at Mesozoic granitic basement,pre-Paleogene sediments localized tectonic uplift and was small extension at the beginning stage.While during the Middle Miocene,the carbonate buildup flourished,and grow a thrived and thick carbonate succession overlining the whole Guangle Uplift.The isolated platforms then united afterward and covered an extensive area of several tens of thousands of square kilometers.However,it terminated in the Late Miocene.What are the control factors on the initiation,growth and demise of the Guangle carbonate platform?The onset of widespread carbonate deposits largely reflected the Early Miocene transgression linked with early post-rift subsidence and the opening of the South China Sea.Stressed carbonate growth conditions on the Guangle carbonate platform probably resulted from increased inorganic nutrient input derived from the adjacent uplifted mainland,possibly enhanced by deteriorated climatic conditions promoting platform drowning.Therefore,tectonics and terrigenous input could be two main controlling factors on the development of the Guangle carbonate platforms and main evolution stages.

Seismic Imaging and 3D Architecture of Yongle Atoll of the Xisha Archipelago, South China Sea

Yongle atoll in the Xisha(Paracel) Archipelago is an isolated carbonate platform developed on Precambrian metamorphic and Mesozoic volcanic rocks since the early Miocene. To identify the 3D stratigraphic architecture and evolution of this platform, 13 high-resolution seismic profiles and shallow-to-deep water multi-beam data were processed and analyzed to reveal seismic facies, sequence boundary reflectors, seismic units, and platform architecture. Nine types of seismic facies were recognized based on their geometry, which included seismic amplitude, continuity, and termination patterns;additionally, six reflections, i.e., Tg, T60, T50, T40, T30, and T20, were identified in the Cenozoic strata. Five seismic units, SQ1(lower Miocene), SQ2(middle Miocene), SQ3(upper Miocene), SQ4(Pliocene), and SQ5(Quaternary), were identified from bottom to top across the platform. The platform grew rapidly in the middle Miocene and backstepped in the late Miocene–Pliocene. Here, we discuss the developmental characteristics and evolution of the Yongle Atoll, in combination with drilling wells, which can be divided into four stages: the initiation stage in the early Miocene, the flourishing stage in the middle Miocene, the partial-drowning stage in the late Miocene–Pliocene, and modern atoll in the Quaternary.

Saturation evaluation for fine-grained sediments

Accurate quantification of the gas hydrate content in the deep sea is useful for assessing the resource potential and understanding the role of gas hydrates in the global carbon cycle.Resistivity logging data combined with Archie’s equation are often used to calculate gas hydrate saturation,but the reliability is dependent on the rationality of the empirical parameter cementation factor and saturation index.At present,an increasing number of fine-grained hydrate-rich sediment regions have been discovered worldwide through drilling efforts,and the reservoir types and hydrate distribution are diverse,which differs greatly from that of coarse-grained reservoirs of hydrate-bearing sediment.This results in vertical variations in m and n through stratigraphy.At present,the saturation evaluation effect of these reservoirs cannot be improved.In this work,a theory for the determination of the cementation factor and saturation index was first proposed to obtain reliable and variable values of the empirical parameters.Then,a hydrate saturation evaluation technique with variables m and n was formed based on the well logging data.This technique was used to evaluate complex fine-grained hydrate-bearing reservoirs in several regions worldwide.It was found that the highest n could be 16,and the log calculation results were more consistent with the core hydrate saturation.Additionally,the cause of the excessively high n values was explained from physical principles,and the result was verified with actually well log data.In future evaluations of the amount of hydrate resources in fine-grained sediment reservoirs worldwide,new saturation estimation methods should be taken into account to advance hydrate research.