Controlling factors of marine shale gas differential enrichment in southern China

Based on the exploration and development practice of marine shale gas in Fuling, Weiyuan, Changning, Luzhou and Southeast Chongqing in southern China, combined with experiments and analysis, six factors controlling differential enrichment of marine shale gas are summarized as follows:(1) The more appropriate thermal evolution and the higher the abundance of organic matter, the higher the adsorption and total gas content of shale will be.(2) Kerogen pyrolysis and liquid hydrocarbon cracking provide most of the marine shale gas.(3) The specific surface area and pore volume of organic matter rich shale increased first and then decreased with the increase of thermal evolution degree of organic shale. At Ro between 2.23% and 3.33%, the shale reservoirs are mainly oil-wet, which is conducive to the enrichment of shale gas.(4) The thicker the roof and floor, the higher the shale gas content. The longer the last tectonic uplift time and the greater the uplift amplitude, the greater the loss of shale gas will be.(5) The buried depth and dip angle of the stratum have different controlling and coupling effects on shale gas in different tectonic positions, resulting in two differential enrichment models of shale gas.(6) The effective and comprehensive matching of source, reservoir and preservation conditions determines the quality of shale gas accumulation. Good match of effective gas generating amount and time, moderate pore evolution and good preservation conditions in space and time is essential for the enrichment of shale gas.

Numerical simulation of fracture propagation in Russia carbonate reservoirs during refracturing

Refracturing treatment is often performed on Russian carbonate reservoirs because of the quick production decline of reservoirs.The traditional refracturing model assumes that a refracture initiates in the normal direction relative to the initial hydro-fracture.This assumption is inconsistent with oilfield measurements of refracture propagation trajectories.Indeed,the existing model is not based on an indepth understanding of initiation and propagation mechanisms of the second hydraulic fractures during refracturing.In this study,we use the extended finite element method to investigate refracture propagation paths at different initiation angles.Both the enriched function approach and phantom mode technique are incorporated into the refracturing model,thereby ensuring that the refracture can freely extend on the structured mesh without any refinement near the crack tips.Key factors including production time,stress anisotropy and initiation angle,and the propped mechanical effect are analyzed in detail.This study provides new insight into the mechanism of refracture propagation in unconventional reservoirs.

In-situ Fluid Phase Variation along the Thermal Maturation Gradient in Shale Petroleum Systems and Its Impact on Well Production Performance

In-situ fluid phase behavior is important in determining hydrocarbon contents and the multiphase flow through shale reservoirs.The gas-to-oil ratio(GOR) has been recognized as a critical indicator of fluid types.However,little is known about the impact of fluid phase variation across the thermal maturity on shale oil/gas production(e.g.,estimated ultimate recovery,EUR).According to the specific gravity ratio of oil/gas,the producing GOR was converted and normalized into a mass fraction of gas in total hydrocarbons(MGOR) to compare North American shale oil/gas plays with Chinese shale oil and hybrid gas-condensate plays.A correlation between MGOR,the fluid phases,and production data was established to identify five phase stages of flow.MGORvaries systematically with the different production zones,which shows promise in rapidly indicating the well production performance and high production stages of shale oil/gas plays.The hybrid shale gas condensate index,Tmax,and total gas contents were integrated to present the fluid types and maturity of shale gas-condensates,which indicates fluid phase and production variation across thermal evolution.The results offer a unique perspective on the shale oil reservoir producibility based on the impact of GOR on fluid phases and EUR from the dominant global oil/gas plays.

A New Method for Converting T_(2) Spectrum into Pore Radius

In this paper,a new method for converting the T_(2)(relaxation time) of NMR(nuclear magnetic resonance) into the pore radius is proposed.Combined with NMR and centrifugation experiments,the relationship between pore radius and T_(2) of the sample was established.The results show that the new method is more reasonable than the traditional method.When the sample was denser and the mercury saturation was lower,the pore distribution curve was obtained by traditional method had a worse agreement with mercury injection experiment,while pore distribution curve of the new method had a better agreement with the mercury injection curve,which reflected the greater advantage of the new method as the reservoir becomes denser.The new method can obtain all the pore information in the sample.The results show that the pores in tight sandstone are mainly consisted with mesopore and macropore,and the connectivity of macropore is better than that of mesopore.The new method can effectively characterize the full pore distribution and the seepage characteristics in different pores interval of tight reservoirs,which had a great significance to evaluate the recoverable resources of tight reservoir.