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The Leidong depression is one of the secondary tectonic units and the least explored part of the Beibu Gulf Basin, northern South China Sea. Exploration activities in the basin reveal that the Middle Eocene deposition was controlled by NE-SW-striking faults and the second member of the Eocene Liushagang Formation (LⅡ) consists of high-quality source rocks. The nature and origin of this depression has remained enigmatic due to the complex fault structures as compared to those in other basins. It is important to identify whether LⅡ occurs in the Leidong depression in the context of exploration. Here we analyze available geological, seismic, and drilling data of the depression and the controlling factors among faults, deposition, lithology, and hydrocarbon distribution in the basin. Measures such as stress fracture analysis, fault growth index analysis, and sedimentary strata characterization were used to construct an Eocene sedimentary model controlled by NW-SE striking faults. The model with integrated seismic profile characteristics comparison and velocity analysis results was then used to determine whether the Eocene strata in the west of the depression contain the high-quality source rocks that are widely distributed elsewhere in the basin. Finally, calculations were performed to quantify the intensity of hydrocarbon generation and amount of hydrocarbon generated in the depression based on available data of the source rocks, including their distribution and thickness. The results confirmed the assumption that the Middle Eocene strata in the Leidong depression were controlled by NW-SE strike-slip faults and contain high-quality LⅡ source rocks, thus providing an important foundation for future exploration.

The Yinggehai Basin has become the main battlefield for natural gas exploration in recent years in China, but the formation pressure system in this area has the characteristics of poor horizontal distribution regularity and severe vertical distribution changes. This paper proposes a new method for predicting the horizontal and vertical distribution trend of formation pressure for the situation of limited offshore drilling data. The method is based on the topological triangulation algorithm, combined with the data set to fit the multivariate interpolation function, and conveniently realizes the prediction and visualization of the three-dimensional horizontal and vertical distribution trend of regional formation pressure.On this basis, through the analysis of the relevant data of the complex accident points that have been drilled, the characteristics of the single well safety density window and the regional safety density window are clarified. The study found that there is no high pressure in the shallow layer of the block, and the formation pressure spreads smoothly in the lateral direction; the high pressure top surface is at 3500m, and the formation pressure gradually increases with the increase in longitude and decrease in latitude in the lateral direction. The formation pressure rises extremely rapidly with the increase of depth in the longitudinal direction, which has the characteristic of "broken line pressurization". The safety density window of a single well is in the shape of a funnel with the characteristic of turning back, and the turning section is between 3500m and 4000m. The safety density window feature of the block can be divided into three stages according to the depth., and the window appears extremely narrow after 3500m (

Gas kick is generally difficult to discover in time using traditional surface detection methods, which results in a significant wastage of time and money. Owing to the restriction of the low data-transmission speed of measurement-while-drilling system, downhole measured data is usually ignored in gas-kick detection. Furthermore, surface detection methods comprising the use of pressure and flow-rate sensors require professional knowledge and many input parameters, some of which are required to be assumed. In this study, we used downhole dual measurement points for detecting gas kick without the use of other surface input parameters. Firstly, we developed an end-to-end supervised neural network to determine the still and circulation working conditions, which were used for calculating the drilling fluid density and viscosity. Secondly, an unscented Kalman filter was applied to perform a backward gas fraction calculation dynamically. However, this downhole calculation method cannot be used in highly deviated and horizontal wells. Because there is a downhole fluctuating pressure generated during the rock breaking, we proposed an auxiliary gas-kick detection method based on the theory of pressure wave attenuation. This method can be applied to all well types. To evaluate the proposed gas-kick detection method, we used a gas-liquid flow simulation model combined with a pump rate model, screw-drilling-tool pressure-consumption model, rock-breaking model, and formation permeability model to generate transient data with the highest possible accuracy. The advection upstream splitting model was used as the numerical scheme. The accuracy of the simulation model was successfully validated using two field experimental data sets. Finally, we generated a set of vertical and horizontal well data each with the simulation model to test the gas-kick detection method. The experiment results showed that the proposed gas-kick detection model was successful in detecting gas kick and obtaining the accurate gas fraction.

Wavy horizontal sections are typically encountered in horizontal gas wells, which will result in gas accumulation on top of the wavy horizontal sections. This gas accumulation can be a problem and may trigger gas kick or blowout accident while tripping and pulling this gas into the vertical section. In this paper, a numerical model for gas accumulation and gas migration in the wavy horizontal sections of the horizontal gas well is developed; meanwhile, the gas accumulation and gas migration process is numerically investigated. The results show that the gas exhausting time in the wavy horizontal section increases with the increase of the wellbore curvature and the critical drilling fluid flow velocity for gas exhausting increases with the increase of the wellbore curvature. When the drilling fluid flow velocity is higher than the critical drilling fluid flow velocity for gas exhausting, no gas accumulation will occur. With all other parameter values set constant, the number of the wavy horizontal sections has a great effect on the gas-liquid flow pattern while it has little effect on the efficiency of the gas exhausting. This work provides drilling engineers with a practical tool for designing the drilling fluid flow velocity to avoid gas kick or blowout accident in horizontal gas well drilling.

There are rich natural gas resources in the northwestern South China Sea deepwater areas, with poor degree of exploration. Because of the unique tectonic, sedimentary background of the region, velocity model building and time-depth conversion have been an important and difficult problem for a long time. Recent researches in this direction have revealed three major problems for deepwater areas, i.e., the way to determine error correction for drilling velocity, the optimization of velocity modeling, and the understanding and analysis of velocity variations in the slope areas. The present contribution proposes technical solutions to the problems: (1) velocity correction version can be established by analyzing the geology, reservoir, water depths and velocity spectrum characteristics; (2) a unified method can be adopted to analyze the velocity variation patterns in drilled pale structural positions; and (3) across-layer velocity is analyzed to establish the velocity model individually for each of the layers. Such a solution is applicable, as shown in an example from the northwestern South China Sea deepwater areas, in which an improved prediction precision is obtained.

Yinggehai Basin locates in the northern South China Sea. Since the Cainozoic Era, crust has several strong tension: the basin subsides quickly, the deposition is thick, and the crust is thin. In the central basin, formation pressure coefficient is up to 2.1; Yinggehai Basin is a fomous high-temperature overpressure basin.YinggehaiBasin’s in-depth, especially high-temperature overpressure stratum has numerous large-scale exploration goals. As a result of high-temperature overpressure basin’s perplexing geological conditions and geophysical analysis technical limitations, this field of gas exploration can’t be carried out effectively, which affects the process of gas exploration seriously. A pressure prediction model of the high-temperature overpressure basin in different structural positions is summed up by pressure forecast pattern research in recent years, which can be applied to target wells pre-drilling pressure prediction and post drilling pressure analysis of Yinggehai Basin. The model has small erroneous and high rate of accuracy. The Yinggehai Basin A well drilling is successful in 2010, and gas is discovered in high-temperature overpressure stratum, which proved that reservoir can be found in high-temperature overpressure stratum. It is a great theoretical breakthrough of reservoir knowledge.