Characteristics of carbon isotopic composition of alkane gas in large gas fields in China

Exploration and development of large gas fields is an important way for a country to rapidly develop its natural gas industry.From 1991 to 2020,China discovered 68 new large gas fields,boosting its annual gas output to 1925×108m3in 2020,making it the fourth largest gas-producing country in the world.Based on 1696 molecular components and carbon isotopic composition data of alkane gas in 70 large gas fields in China,the characteristics of carbon isotopic composition of alkane gas in large gas fields in China were obtained.The lightest and average values ofδ^(13)C_(1),δ13C2,δ13C3andδ13C4become heavier with increasing carbon number,while the heaviest values ofδ^(13)C_(1),δ13C2,δ13C3andδ13C4become lighter with increasing carbon number.Theδ^(13)C_(1)values of large gas fields in China range from-71.2‰to-11.4‰(specifically,from-71.2‰to-56.4‰for bacterial gas,from-54.4‰to-21.6‰for oil-related gas,from-49.3‰to-18.9‰for coal-derived gas,and from-35.6‰to-11.4‰for abiogenic gas).Based on these data,theδ^(13)C_(1)chart of large gas fields in China was plotted.Moreover,theδ^(13)C_(1)values of natural gas in China range from-107.1‰to-8.9‰,specifically,from-1071%o to-55.1‰for bacterial gas,from-54.4‰to-21.6‰for oil-related gas,from-49.3‰to-13.3‰for coal-derived gas,and from-36.2‰to-8.9‰for abiogenic gas.Based on these data,theδ^(13)C_(1)chart of natural gas in China was plotted.

Differential interformational velocity analysis as an effective direct hydrocarbon indicator under velocity reversal conditions,an example from the anomalously high temperature and over-pressured DF1-1 Gas Field in the Yinggehai Basin,South China Sea

In the DF1-1 Gas Field in the Yinggehai Basin, South China Sea, the velocity-depth plot and velocity spectra show significant variations from a linear trend, exhibiting a distinct reversal phenomenon. Velocity parameters derived from velocity spectral analysis of the seismic data and sonic logs indicate that the interval velocity reverses below 2,100 m (2.2 s two-way time (TWT)) in the DF1-1 Gas Field. Some direct hydrocarbon indicators (DHIs) models developed for the shallow strata cannot be simply applied to the moderately to deeply buried strata for direct exploration target recognition because the velocity reversal has caused the middle-deep gas reservoirs to exhibit a moderate or weak seismic amplitude. The hydrocarbon indicator method of “Differential Interformational Velocity Analysis (DIVA)” with the aid of other hydrocarbon indicating techniques was employed to effectively identify DHIs in the middle-deep strata under velocity inversion. The result has shown that the DIVA technique can be effectively used as a DHI in both the shallow and the middle-deep strata in the study area with the shallow strata characterized by Type I DIVA anomaly and the middle-deep strata characterized by the Type II DIVA anomaly.

Start of Production at the Ya 13-1 Gas Field in South China Sea

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North slope transition zone of Songnan-Baodao sag in Qiongdongnan Basin and its control on medium and large gas fields,South China Sea

Based on analysis of newly collected 3D seismic and drilled well data,the geological structure and fault system of Baodao sag have been systematically examined to figure out characteristics of the transition fault terrace belt and its control on the formation of natural gas reservoirs.The research results show that the Baodao sag has the northern fault terrace belt,central depression belt and southern slope belt developed,among them,the northern fault terrace belt consists of multiple transition fault terrace belts such as Baodao B,A and C from west to east which control the source rocks,traps,reservoirs,oil and gas migration and hydrocarbon enrichment in the Baodao sag.The activity of the main fault of the transition belt in the sedimentary period of Yacheng Formation in the Early Oligocene controlled the hydrocarbon generation kitchen and hydrocarbon generation potential.From west to east,getting closer to the provenance,the transition belt increased in activity strength,thickness of source rock and scale of delta,and had multiple hydrocarbon generation depressions developed.The main fault had local compression under the background of tension and torsion,giving rise to composite traps under the background of large nose structure,and the Baodao A and Baodao C traps to the east are larger than Baodao B trap.Multiple fault terraces controlled the material source input from the uplift area to form large delta sand bodies,and the synthetic transition belt of the west and middle sections and the gentle slope of the east section of the F12 fault in the Baodao A transition belt controlled the input of two major material sources,giving rise to a number of delta lobes in the west and east branches.The large structural ridge formed under the control of the main fault close to the hydrocarbon generation center allows efficient migration and accumulation of oil and gas.The combination mode and active time of the main faults matched well with the natural gas charging period,resulting in the hydrocarbon gas enrichment.Baodao A transition belt is adjacent to Baodao 27,25 and 21 lows,where large braided river delta deposits supplied by Shenhu uplift provenance develop,and it is characterized by large structural ridges allowing high efficient hydrocarbon accumulation,parallel combination of main faults and early cessation of faulting activity,so it is a favorable area for hydrocarbon gas accumulation.Thick high-quality gas reservoirs have been revealed through drilling,leading to the discovery of the first large-scale gas field in Baodo 21-1 of Baodao sag.This discovery also confirms that the north transition zone of Songnan-Baodao sag has good reservoir forming conditions,and the transition fault terrace belt has great exploration potential eastward.

Research on 1:2 subharmonic resonance and bifurcation of nonlinear rotor-seal system

The 1：2 subharmonic resonance of the labyrinth seals-rotor system is inves- tigated, where the low-frequency vibration of steam turbines can be caused by the gas exciting force. The empirical parameters of gas exciting force of the Muszynska model are obtained by using the results of computational fluid dynamics （CFD）. Based on the multiple scale method, the 1：2 subharmonic resonance response of the dynamic system is gained by truncating the system with three orders. The transition sets and the local bifurcations diagrams of the dynamics system are presented by employing the singular theory analysis. Meanwhile, the existence conditions of subharmonic resonance non-zero solutions of the dynamic system are obtained, which provides a new theoretical basis in recognizing and protecting the rotor from the subharmonic resonant failure in the turbine machinery.

Optimization Design of Fracturing Parameters for Coalbed Methane Wells in Dafosi Mine Field

Being controlled by the gas content of the reservoir, the productivity of CBM well is also mainly determined by its engineering quality and the physical property of the reservoir. In Dafosi mine field, 22 vertical wells from the “26 + 1” CBM wells project that was implemented since 2014 showed great difference in productivity after fracturing construction. With the consistent geological condition of coal reservoirs, fracturing construction parameters are suggested as the priority factor that controlling the well productivity. In this study, being combined with historical production and geological data, an optimized designation of fracturing construction parameters was established based on systematically comparative analysis of construction parameters such as discharge, the total volume of fracturing fluid, the quality of sand, sand ratio, which provides engineering basis for further fracturing construction of CBM wells.

CO2 geological storage into a lateral aquifer of an offshore gas field in the South China Sea： storage safety and project design

The DFI-1 gas South China Sea, contains thus there is great concern field, located in the western a high concentration of CO2, about the need to reduce the CO2 emissions. Many options have been considered in recent years to dispose of the CO2 separated from the natural gas stream on the Hainan Island. In this study, the feasibility of CO2 storage in the lateral saline aquifer of the DF 1-1 gas field is assessed, including aquifer selection and geological assessment, CO2 migration and storage safety, project design, and economic analysis. Six offshore aquifers have been investigated for CO2 geological storage. The lateral aquifer of the DFI-1 gas field has been selected as the best target for CO2 injection and storage because of its proven sealing ability, and the large storage capacity of the combined aquifer and hydrocarbon reservoir geological structure. The separated CO2 will be dehydrated on the Hainan Island and transported by a long- distance subsea pipeline in supercritical or liquid state to the central platform of the DFI-1 gas field for pressure adjustment. The CO2 will then be injected into the lateral aquifer via a subsea well-head through a horizontal well. Reservoir simulations suggest that the injected CO2 will migrate slowly upwards in the aquifer without disturbing the natural gas production. The scoping economic analysis shows that the unit storage cost of the project is approximately US$26-31/ton CO2 with the subsea pipe- line as the main contributor to capital expenditure （CAPEX）, and the dehydration system as the main factor of operating expenditure （OPEX）.

An Installation System of Deepwater Risers by An S-Lay Vessel

Along with the consumption increase of the petroleum products, more countries have transferred their attentions to the offshore fields, especially the deepwater oil and gas reserves. For deepwater exploitation, the risers must be installed to act as the conduits connecting surface platforms to subsea facilities. In this paper, the typical risers sorted by different classes are introduced, and the correspondent installation methods are presented. By investigating the recent projects performed in the deepwater hot spots, and combining the challenges of HYSY20 l for riser installation, a lifting device developed for assisting riser installation is proposed and detailed to satisfy the installation of deepwater risers in the LW3-1 Gas Field of 1500 m in the South China Sea. Tests on both the functions and performances of such a new system exhibit the satisfaction of meeting all challenging requirements of HYSY201 for application to riser installation in waters up to a depth of 3000 m in the South China Sea.