| In recent years,in a large number of exploration and development studies on unconventional gas reservoirs such as tight sandstone gas reservoirs,fractures have had a significant impact on the formation,migration and accumulation of unconventional gas reservoirs.Reservoir of the fifth member of Xujiahe Formation in Xinchang Gas Field,West Sichuan Depression is a typical tight sandstone reservoir with low porosity and low permeability,because of the influence of the late tectonic movement,a large number of fractures were developed in the fifth member of Xujiahe Formation,which greatly improved its storage capacity and permeability characteristics.For a long time,the oil and gas exploration of the Xujiahe Formation was mainly aimed at the second and the fourth members.The predecessors used the fifth member as a source rock interval to study and analyze.Therefore,there is a lack of understanding of the crack development law in this area.During the fracturing and reconstruction of the fifth member of Xujiahe Formation reservoir,the volumetric fracturing technology of fracture network is needed to communicate with natural fractures and form a fracture network to maximize the volume of reconstruction to achieve the purpose of increasing the utilization of reserves.Therefore,there is an urgent need to study the characteristics of the natural fractures closely related to the development of the fifth member of Xujiahe Formation in the western Sichuan.In this paper,the macro and micro characteristics of the core fractures in the study area,the development of single well effective fractures,fracture stage and development influencing factors are clarified.The fractures are comprehensively evaluated to determine the effective fracture development area and the fracturing effect.Through literature research,on the basis of theoretical research on fracture mechanics and reservoir geology,etc.,from the aspects of fracture feature identification,influencing factors,distribution and fracturing effects,the following understandings were obtained:(1)By observing and describing the 5 coring wells in the study area,the main development types and basic characteristics of fractures in the study area are clarified: The natural fractures in the study area mainly develop structural fractures,of which tensile fractures are mainly in sandstone and shear fractures are in mudstone.The oblique cross fracture is the main inclination feature of the fifth member of Xujiahe Formation.The upper sub-formation of the fifth member of Xujiahe Formation is mainly unfilled cracks,the proportion of semi-filled cracks in the central sub-formation increases,and the proportion of fully filled cracks in the lower sub-formation increases,the cracks with different inclination angles are mainly half-filled and unfilled.The crack width in the study area is generally around 0 ? 0.5mm.The fractures in the upper sub-formation are the most developed,with an average line density of 1.12 fractures / m;the lower sub-formation is followed by 0.71 fractures / m,and the fracture development in the central sub-formation is relatively low at 0.53 fractures / m.(2)Through core observation,combined with imaging logging data,the characteristics of sand and mudstone fracture logging are summarized,and the effective fractures of single well sections in this area are identified by stepwise discrimination method,BP neural network method and variable scale fractal method.Among them,the BP neural network method has the best recognition effect,and the variable-scale fractal method also has a better recognition effect,and can be comprehensively applied with the stepwise discrimination method to improve the accuracy of crack recognition.The characteristics of fracture development in a single well were clarified,and the comprehensive identification standard for conventional logging of sand and mudstone fractures was established.(3)Based on the study of fracture development characteristics,the formation period and development influencing factors of structural fractures in the study area were determined,on the basis of rock mechanics experiment and rock acoustic emission experiment,the thesis considers that the fracture development of the fifth member of Xujiahe Formation is mainly affected by factors such as brittle mineral content,lithology,rock physical properties,rock thickness and faults.The linear density of core fractures has a significant negative correlation with the thickness of the rock layer,which has a greater impact on the degree of fracture development.(4)Comprehensively considering the influencing factors of fracture development,the fracture development model in the study area was established;focusing on the evaluation index parameters such as lithology,structure and faults,the plane distribution of the comprehensive evaluation coefficient of fractures in the fifth member of Xujiahe Formation reservoir was established,finally,the boundaries of the fracture development area,the more developed area and the non-developed area in the study area were determined.(5)Based on the analysis of fracturing construction data,fracturing monitoring data and typical gas production profiles,the fracturing effect was evaluated: Through repeated fracturing and mixed fracturing,the fracture conductivity is improved,and the generation of secondary fractures is increased.The results of fracturing monitoring show that the trend of fracturing fractures in the study area is highly correlated with the structure in which it is located,and may also be affected by the original fractures in the formation.In the fracturing test section,the fracturing construction parameters have a certain effect on the test production of low-production wells,but have little effect on the extra-low production wells.(6)The fracturing effect of existing wells depends on the geological conditions of the fracturing section(porosity,organic carbon content,natural fracture development,etc.),on the other hand,it has a significant positive correlation with the fracturing scale(total fluid volume and proppant dose)and the fracturing fluid return rate.In addition,the lithologic interface of the fifth member of Xujiahe Formation sedimentary strata is continuous,and fracturing mainly exists in the following two situations,that is,the artificial cracks must either expand on the original cracks or generate new cracks;In the areas where natural fractures are developed and more developed areas,he fifth member of Xujiahe Formation geological conditions are suitable for fracture network volume fracturing,considering the distribution of gas and water,and the longitudinal consideration of fracture height control.Through research,the basic characteristics of cracks in the study area were clarified,the method for identifying effective fractures was established,the fracture development control factors were summarized,the boundaries of the fracture development area were determined,and the comprehensive analysis of the fracturing construction effect was carried out for further exploration and development.Provide a certain reference basis for further exploration and development in the study area. |
PDF Full Text Request