Geophysical Modeling Of Shale Gas

Due to the successful exploration and development of shale gas in North American, Chinese scholars begun to pay close attention to shale gas, a kind of unconventional oil and gas resources, which gradually has become a hot topic of oil and gas fields in recent years. At present, the research of shale gas in China is still in the initial exploratory stage, because it is not easy to achieve large-scale successful development and production of shale gas, because of the complex geological conditions and tectonic background and the large differences between the shale gas characteristics of China and that of North America.Compared with conventional reservoirs, shale gas has many features which include two or more occurrence modes, complex reservoirs rock composition, different types and high abundance of organic matter, strong heterogeneity and variable pore spaces.These features determine that shale gas interpretation and evaluation has large difference from conventional reservoirs, which lead to higher request for evaluation ideas and methods. So far, domestic research mainly focuses on accumulation pattern and conditions, reservoir geology and resource potential evaluation, but seldom involves geophysical modeling and prediction of shale gas. Furthermore, there is limited research on geophysical modeling and prediction of shale gas, which gives the strong need for further research.Using Longmaxi Formation in Jiaoshiba area as a case study, a research is conducted on geophysical modeling method and its application on shale gas prediction. The research area is located in the Jiaoshiba slope in Sichuan basin covering an area of 56.1 km2, in which there are 3 vertical wells and several horizontal wells. The target layer is the Lower Longmaxi gas shale formation (including Wufeng formation) in Silurian.The main objective of this dissertation is gas-bearing properties evaluation of shale gas. Through the analysis of geological characteristics of shale gas, a set of geophysical modeling and prediction methods and processes is established, to evaluate the gas-bearing properties for the target layer in the research area. Studies start with the investigation of shale gas geological characteristics and main controlling factor.Based on the geological characteristics of Longmaxi Formation in Jiaoshiba area, the logging modeling method is applied for the single well modeling and evaluation of main geological controlling parameters using JY1 well. Based on the analysis result of logging curve characteristics and petrophysical parameters, the shale gas rock volume model of this area is established, and single well petrophysical modeling and prestack AVO forward modeling is processed to analyse the variation relationship between the main geological parameters with petrophysical parameters and AVO response. Finally seismic inversion and attribute analysis method is used for the lateral prediction of main geological parameters (enrichment parameters) and then overlapping display analysis of enrichment parameters which used for the prediction of shale gas gas-bearing distribution.According to the research and analysis, the main achievement and conclusion were as follows:(1) Evaluation results of main geological parameters of JYl well using single well modeling were as follows: ① Percentage Weight of QFM (mixture of Quartz, Feldspar, Mica) content was 4.8%-61.2% with an average of 52.04%,carbonte content was 0-11.4% and averaged 3.8%, clay content was 20.8%-68.5% with an average of 42.8%, pyrite content was 0-4.5% with an average of 1.34%. ②Total organic carbon(TOC) content was 1%-4.5% and averaged 3.1%. ③Porosity was 3%-7% and averaged 4.8%. ④Vitrinite reflectance (Ro) was 1.5%-2.5%.(2) Based on the TOC content results obtained from the single well modeling method and analysis of correlation relationship between TOC content and logging data of the various parameters, the gamma spectral element method was modified by adding density and Gamma-ray (GR) to Uranium content to establish a new TOC predicting model.(3) From analysis of logging curve characteristics and petrophysical parameters, petrophysical simulation is processed for TOC and porosity of JY1 well. Study revealed two main results: ①The variation of logging curve showed "three parts"characteristics which can be recognized from the curve shape and the crossplot of density versus p-wave velocity and densiy versus s-wave velocity. ② Petrophysical simulation characteristics of TOC and porosity expressed were: firstly, density, p-wave and s-wave velocity decreased wholly with the increase of TOC, while Poisson ratio increased with the increase of TOC; secondly, density,p-wave and s-wave velocity gradually decreased with the increase of porosity, while Poisson ratio also increased with the increase of TOC.(4) The result of AVO modeling of target section in study area obtained were as follows: ① Reflection coefficient of the top boundary of shale layer had negative polarity, and deceased with the increase of incident angle. AVO intercept P shows negative and slope G shows positive. ② VO response of shale layer varied with TOC, porosity and thickness of the layer, and showed three major characteristics in the quadrants:firstly, intercept P decreased and slope G increased with increase of TOC, and location of P-G intersection points vary from the Ⅲ quadrant to the Ⅱ quadrant; secondly, intercept P decreased and slope G increased with increase of porosity, and location of P-G intersection points vary in the Ⅱ quadrant; thirdly, location of P-G intersection points vary from the Ⅳ quadrant to the Ⅱ quadrant through the Ⅲ quadrant with the increase of thickness, in which the variation route of P-G intersection points appear in a reverse as the thickness increased up to some numerical value.(5) Lateral distribution prediction of the thickness of shale layer and gas enrichment parameters using seismic inversion and attribute analysis method were as follows: ① The thickness of shale layer is 80-118 m, in which it is larger in JY3 well peripheral area than other areas. ② TOC distribution is 0.86%-4.2%, which is vertically larger on the 3rd and 4th single layer than others and is horizontally larger in JY2 well peripheral area and area between JY2 well and JY3 well than other areas. ③ orosity distribution is 1.87%-7%,which is vertically larger on the 3rd and 8th single layer than others and is horizontally larger in JY2 well peripheral area and area between JY2 well and JY3 well than other areas. ④ Ro distribution is 1.05%-3.4%, which is vertically larger on the 3rd and 8th single layer than others and is consistent with porosity distribution characteristics.(6) The delineation of gas-bearing favorable enrichment zone of gas shale by overlapping display analysis of multi-parameters and comprehensive evaluation of gas-bearing properties obtained were as follows: ① It is effective to take TOC and Ro as the overlapping parameters with the threshold standard (TOC>2.15%, Ro>1.82%) for delineation of shale gas gas-bearing favorable enrichment zone. ② Gas-bearing distribution regularity exist in study area including: firstly,the nearer to JY3 well area,gas-bearing properties is better;secondly,gas-bearing properties of the 1st section of lower Longmaxi Formation is vertically better than that of the 2nd and 3rd section;thirdly, gas-bearing properties is horizontally best in the area between JY2 well and JY3 well.In above, the conclusion of single well evaluation, petrophysical modeling and AVO simulation is mainly obtained from JY1 well, which might have some differences with that of JY2, JY3 area and others, In addition, comparison analysis between well data and seismic data express that the prediction of thickness of shale layer,enrichment parameters and gas-bearing properties generally has a good effect, and only has some error in partical area and single layer.