Current Technologies and Prospects of Shale Gas Development in China

Shale gas in China has realized the great-leap-forward development because of the significant strides of development technique for marine shale resource, becoming the second largest shale gas producer only to the USA. Benefits from the progress of technique advance, a complete technique series for the development of shale resource buried less than 3500 m has been established by operating technique research and field trail, including five main key techniques of geological evaluation, optimum and fast drilling, multistage hydraulic fracturing, productivity evaluation, and development parameters optimization. At present, shale resource in China has not been fully exploited besides the marine shale resource (3500 m). With the further process of development technique for marine shale, shale gas is expected to the single type of gas reservoir contributing to the highest annual production rate in the near future. Therefore, based on the success in shale industry, this work is organized by reviewing the advances and challenges of current evaluation techniques, and then discussing the possible development trend of shale gas performance evaluation in the future

Mudskipper genomes provide insights into the terrestrial adaptation of amphibious fishes

Mudskippers are amphibious fishes that have developed morphological and physiological adaptations to match their unique lifestyles. Here we perform whole-genome sequencing of four representative mudskippers to elucidate the molecular mechanisms underlying these adaptations. We discover an expansion of innate immune system genes in the mudskippers that may provide defence against terrestrial pathogens. Several genes of the ammonia excretion pathway in the gills have experienced positive selection, suggesting their important roles in mudskippers’ tolerance to environmental ammonia. Some vision-related genes are differentially lost or mutated, illustrating genomic changes associated with aerial vision. Transcriptomic analyses of mudskippers exposed to air highlight regulatory pathways that are up- or down-regulated in response to hypoxia. The present study provides a valuable resource for understanding the molecular mechanisms underlying water-to-land transition of vertebrates

Architecture and quantitative assessment of channeled clastic deposits, Shihezi sandstone (Lower Permian), Ordos Basin, China

Lower Permian Shihezi sandstone in Ordos Basin is the largest gas reservoir in China. Architecture elements of channel, overbank and floodplain facies of braided channel deposits were identified through an outcrops survey, and their proportion of channel facies have been quantitatively estimated from well logging. Characteristics of architecture elements, such as sand thickness, bounding surfaces and lithofacies were investigated through outcrops and core. Petrology of Shihezi sandstone has also been studied in detail. Analysis on sandstone components shows that monocrystalline quartz with approximately 76% bulk volume, and lithic up to 5%–45% bulk volume, are the two main components. Litharenite and lithic quartz sandstone are the main rock types. Compaction is concluded by former researchers as the control factor of low permeability. Examination through thin section reveals that secondary pores developed well in coarse sand. Inter-granular dissolution is included as the positive effect to increasing porosity, and is concluded as the control factor to the generation of net pay. Scale of coarse grained channel fills and channel bar sandstone bodies are quantitatively estimated. Strike-oriented, dip-oriented, and vertical distribution of channel fills and channel bar sandstone bodies have been investigated. The geometry of sand bodies can be depicted as an elongated lens. Subsurface mapping reveals that channel sandstone bodies distribute widely from both lateral and longitudinal cross section profiles, and are poorly connected

Enhanced gas recovery: Theory, technology, and prospects

Research on the enhanced gas recovery (EGR) technology is urgently needed in most of China's major gas fields due to low recovery in their late development stages. To promote progress in ERG theory and technology, this paper establishes a unified and universal model for gas recovery evaluation, analyzes the key factors affecting the gas recovery, and expects the prospect of EGR. The results are obtained as follows. First, the production degree of reserves, pressure drawdown sweep efficiency and pressure depletion efficiency are key factors affecting the gas recovery, which is the product of the coefficients corresponding to these factors. Second, according to the development practice of Anyue Longwangmiao gas reservoir, Kela 2 gas field, Sulige gas field and Southern Sichuan shale gas, it is estimated that the recovery of conventional water-driven gas reservoirs and unconventional gas reservoirs (incl. tight gas and shale gas) can be improved by 6–15 percentage points by increasing the production degree of reserves, the pressure drawdown sweep efficiency and pressure depletion efficiency. Third, it is proposed that clarifying the EGR mechanism, developing new EGR methods, and promoting the field test of EGR technologies are directions for theoretical and technical researches. The study results provide a theoretical foundation for EGR. The EGR methodologies for different types of gas reservoirs provide technical support for improving gas recovery and stable production of existing gas fields, promote healthy and rapid development of natural gas industry in China and provide guidance for guaranteeing national energy security

Well spacing optimization for Sulige tight sand gas field, NW China

Reasonable well spacing has been studied and assessed to increase the reserves and improve the recovery factor of the Sulige gas field, where the reserves controlled by single well are small. An evaluation methodology for optimizing well spacing in a tight sandstone gas field was established from five aspects including geologic model, drainage radius, interference testing, numerical simulation and economic benefits. The well spacing has been optimized using this method. Through geological modeling, the rational inter-well distance is estimated to be less than 600 m and rational well array distance should be less than 1 000 m. Results of drainage areas estimation reveal that the rational inter-well distance should be ranged from 400 m to 600 m. Analysis of interference well test shows that the low limit for inter-well distance is 400 m and the favorable well array distance is 600 m. Numerical simulation and economic evaluation shows that an inter-well distance of 500 m and well array distance of 700 m are favorable. It is concluded from the above five aspects that, in current economic and technological conditions, the well pattern with inter-well distance at 500 m and well array distance at 700 m is rational in the Sulige gas field, and it can improve the recovery factor to 45%. Key words: tight sand gas field, well spacing optimization, sand body scale, drainage area, economic evaluation, recovery facto