Prediction Method And Its Application Of The Fractured Oil And Gas Reservoirs

With the rapid development of China’s economy, the more oil and gas resources are exploited and utilized, the less reservoirs of conventional oil and gas become and the more difficult their exploitation turns. As a result, fractured reservoirs will inevitably become a significant field as the focus of oil and gas exploitation are gradually shifted from shallow exploitation to deep one and from conventional reservoirs to unconventional ones.It should be noted that great troubles are encountered in the exploitation and development of fractured reservoirs due to such factors as great lateral variability in lithology and lithofacies, considerable complexity of the types of reservoir space and the physical characteristics of seepage, great changes in the nature, type, shape and size of filling of fractures, and the unclear spatial distribution of reservoirs. So the fractured reservoir has become one of the difficulties in the exploration of oil and gas. Currently, production and research shows that it is very difficult to predict fractures with a single discipline or method. In this case, we need comprehensive prediction of fractures with multi-disciplines, multi-methods and multi-parameters, with the purpose of avoiding multiple solutions and improving the success rate in exploration.The Carboniferous volcanic reservoir in the west of Hashan has a good exploration potential, but a poor knowledge of its fracture characteristics and geophysical response characteristics and a lack of recognition technology to predict this type of fractures call for a comprehensive prediction technology, an effective seismic recognition mode and designation of the favorable development zones in the different tectonic fractures in this region. The study of this subject not only helps nail down the fracture characteristics and establish a comprehensive prediction technology of volcanic fractures, but is also of great referring and guiding significance to similar exploration zones.In this doctoral dissertation, centering around the prediction of Carboniferous volcanic fractures in the west of Hashan, the paper has carried out its study mainly from the following seven aspects, aiming at furnishing a theoretical basis for further exploration there.1. The paper has studied the domestic research status and strike of fracture prediction methods, analyzed the characteristics of the researched regions, and proposed ideas and methods specific for fracture recognition in the west of Hashan.2. The paper has collected, collated, and analyzed a variety of geological, well logging, core, and seismic data. Through observation and description of fractures, it has summed up the types and characteristics of fractures; from the perspectives of structure, fault, reservoir and etc., it has analyzed and summarized the controlling factors of fractures in the west of Hashan; it has summarized fracture segments and completed the statistical tables about the logging response of fracture segments and achievement graphs about seismic response of fractures.3. The paper has completed the seismic geological modeling of six different fracture parameters and wave equation forward modeling of seismic waves, and analyzed the characteristics of seismic wave fields, which underlie the analysis of seismic attributes and extraction of sensitive attributes.4. The paper has calculated and analyzed coherence, dip, azimuth, curvature, attenuation and other data volumes of post-stack seismic attributes, through-well profiles and horizontal slices. And then, based on the neural network algorithm, the paper has predicted the Carboniferous igneous fractured zones in this region according to the seismic attributes; additionally, it has reconstructed characteristic logs of six key wells with their logging data and predicted the post-stack fractures with the method of log restructuring inversion.5. The paper has studied the basic theories of fractured media, i.e. Hudson’s and Thomsen’s theories and make a theoretical analysis of prestack anisotropic characteristics from the aspects of speed, amplitude, attenuation, etc.6. The paper has done a lot of study to do with the prestack seismic gathers in the west of Hashan, such as processing of the prestack preserved amplitude, stacking of the sub-azimuth, migration and forward modeling of the fracture segments and fracture prediction of structural causes based on the analysis of stress fields. The paper has also detected anisotropic fractures according to the prestack azimuth AVO and QVO characteristics in this region and finally predicted the orientation and density map of fractures.7. The paper has explored the applicability of prediction technologies specific to different regions, different strata and different types of fractures, and then designated the relationship between different combinations of technologies and finally optimized the methods applicable to different regions and different strata. Based on the applicability of technologies, it has proceeded to optimize the combinations of different fracture prediction technologies, establish a valid seismic recognition mode of the igneous fractures in the Northwestern Junggar, according to the lithological characteristics, laws of fractures, structural complexity, the quality of seismic data and the amount of drilling wells, and finally guide the delineation of the favorable development zone in the fractured reservoirs.Combining the above research, this paper has mainly proposed the following findings through the research and analysis of data in the western of Hashan.1. The fractures in the studied region are high-angled and nearly parallel in distribution; they run mainly from northeast to southwest; they can reach as many as50strips/meter in density with86%being semi-filled and unfilled fractures. Therefore, the fractures mainly generally characterized as high angled, highly dense and highly effective in the work area will play a positive role in the migration and enrichment of oil and gas in this region. Meanwhile, as the fractures here are mainly high-angledly sheared, the factor which played an important role in the development of fractures is their lithology and regional tectonic position.2. By analyzing the logging response characteristics of fracture segments, the paper has designated the following laws concerning the responses of FMI and conventional logging to fractures.(a) In the FMI images, the high-conductivity fractures are mainly shown as dark sine waves, which are caused by penetration of drilling fluids; the high-resistance are shown as bright sine waves, which are caused by filling of high-resistance minerals with secondary effects in the later periods.(b) The responses of conventional logging to fractures reflect the following characteristics. Firstly, the responses of sonic movements are closely related to the scale and morphology of fractures. If the fractures are horizontal or netted, and relatively developed, the sonic interval transit time will be enlarged in value and lowered in speed. Secondly, when the fracture openings are comparatively large, the "cycle skip" phenomenon may be noted in the p-sonic logs; natural gamma logs are generally low, but may be abruptly increased; the caliper logs may be expanded in diameter; the density of the developed segments may be significantly reduced; as they may be significantly lowered in value due to penetration of drilling fluids, the resistivity logs are characterized as relatively low-valued in the background high-valued resistivity; additionally, due to the relatively great depth in the compensated neutron logging, the developed segments of volcanic fractures are relatively high-valued in porosity, indicating that the greater the porosities are, the more developed the fractures are.3.The seismic responses of the fracture segments are characterized as follows:reflection in the conventional seismic profiles of the developed segments is mainly events which extend over a short distance and tilt to some degrees, but is horizontally discontinuous; the density profiles of reflection are mainly shown as in a beaded and discontinuous pattern in the high-valued section; the instantaneous frequency attenuates fast in the direction of vertical fractures with strong heterogeneity and gas-bearing fractures are more heterogeneous than oil-bearing ones.4.The results from the forward modeling of different random fractures and the actual fractured reservoir models demonstrate the following characteristics.(a) The density and rate of fractures are leading factors in influencing the characteristics of the seismic wave field. The denser the fractures are, the greater the difference between speed and surrounding rocks becomes and the more evident the response of fractures is. The response is generally messy and is shown as partially short slant events.(b) The developed zones of fractures will obscure imaging off the underlying strata.(c) Seismic attributes can best reflect the variations of fracture density and speed, and then length and width, but they can’t reflect the changes in dip. Sensitive attributes include root mean square amplitude, average reflectance intensity, instantaneous frequency, etc.(d) As for the paper’s seismic modeling of the HQ6-HS1well, the results of its forward modeling can well correspond to the actual seismic profiles. The response characteristics of the developed zones are highly similar to those shown by the actual seismic data. The results of attribute analysis demonstrate that those attributes such as coherence, dip, azimuth and curvature are comparatively sensitive to the developed zones, which sets stages for prediction of fractures with the actual data.5. The post-stack technology based on coherence and other attributes facilitates qualitative detection of fractures in the main developed zones, which reveals relatively macroscopic information of faults. The final results of the analysis show that the predicted fracture zones are mainly located in the south of the HQ6-HS1well, and that vertically downward, they tend to be smaller from north to south, but the signaled degree of development goes from weakness to strength.6. Through weighted fitting of multi sensitive well logs, the paper has reconstructed characteristic logs which can reflect the segments of fractures more accurately, and then carried out the log restructuring inversion, thus succeeding in qualitatively predicting the development characteristics of fractures. The results of the prediction show that the high values of fractures’ density logs well correspond to the low values of acoustic impedance inversion profiles.7) The paper has studied fractures from six aspects including influences of fracture parameters on the propagation of seismic waves, azimuthal anisotropic reflection coefficients of P-waves, azimuthal anisotropy of NMO speed, characteristics of single EDA media’s time-distance curves, P-wave movement of orthogonal survey lines, estimation of fracture strike according to the changes of Q-value with azimuth. Besides, the paper has done comprehensive research into methods of prestack azimuthal anisotropy. The findings of the studies are as follows.(a) The velocity and attenuation of three fractured media’s waves have a certain relationship with the fracture density. With the increase of fracture density, S-waves decrease in speech and increase in attenuation.(b) As for EDA media, propagation velocity and attenuation of three waves is evidently oriented; P-waves in dry fractures and the shear waves of fractures containing fluids change significantly.(c) In the near offset (i.e. at small incidence angles), the reflection coefficients change a little at different incidence angles; but in the far offset, they change greatly. Therefore, the data collected in the far offset can better reflect the characteristics of anisotropy.(d) In the observation along the fracture strike, waves are maximum in apparent velocity but minimum in travel time, thus achieving maximum NMO velocity; in the observation perpendicular to the fracture strike, waves are minimum in apparent velocity but maximum in travel time, thus achieving minimum NMO velocity.(e) Along the fractures’ symmetry axis or perpendicular to the fracture strike, the survey lines are maximum in attenuation; but perpendicular to the fractures’ symmetry axis or along the fracture strike, the survey lines are minimum in attenuation. The smaller the incidence angles are, the least the attenuation changes with the observation angles; at the0°incidence angle (i.e. vertical incidence), the attenuation is small and has no difference at different observation angles.8.With the fracture prediction technology based on changes of prestack amplitude with azimuth, the paper has carried out semi-quantitative and quantitative research into the development orientation and density of fractures in the west of Hashan. The results of prediction are as follows. The transition line of fracture density is from HS2well to HQ7well. The density in the south of this line was significantly higher than that in the north; the fracture density in the south of the studied area is not evenly distributed, but with some high-valued parts, such as HQ6 well and HQ101well. The development orientation of fractures is mainly northeast and northeast, between which northeast is dominating.9. Based on the applicability analysis of various fracture prediction technologies, the paper has summed up the typical prediction mode applicable to the different strata, different tectonic zones, and different types of fractures in the studied region. On the whole, the paper has gradually deepened his research from the macro description of fractures, to the qualitative one, then to the semi-quantitative one and finally to the quantitative one. Firstly, it has shown relatively macroscopic information of fractures with the post-stack multi-attribute technology; on this basis, with log restructuring inversion, it has made more accurate qualitative prediction of fractures’ development characteristics and designated relative development degrees of different regions and segments; finally, with the fracture prediction technology of prestack AVAZ and FVAZ, it has pointed out the development density and direction of fractures in different regions and different layers, thus achieving the semi-quantitative to quantitative predictions.The innovations in this dissertation are mainly as follows.1. For the first time the paper has applied the multi-attribute predication technology and the neural network approach to the prediction of the development zones of carboniferous fractures in the west of Hashan. The results of the prediction have great coincidence with the drilling data. In the prediction based on seismic multi-attributes, in order to reduce multiple solutions in prediction, it is necessary to preferably select those seismic attributes which are sensitive to the development of fractures. So in this study, the paper has employed two methods—wave equation forward modeling (including the theoretical models of fracture media and the actual through-well models) and the cross plots analysis of the actual seismic data’s attributes, thus selecting several seismic attributes which can better reflect the development of fractures and laying a foundation for post-stack prediction of fractures. Based on this, with the BP neural network algorithm, the paper has found out non-linear relationship between the fracture density and seismic attributes. Finally the paper has predicted the planar map of fracture’s development density in the whole region, which has achieved good effects.2. Theoretically, the paper has made a comprehensive analysis of the characteristics of seismic waves’azimuthal anisotropy and deduced the relationship between the development azimuth and the parameters such as reflection coefficients, attenuation, movements, which has laid a theoretical foundation for the prestack prediction of fractures. In this thesis, it has focused on deriving the estimation of fracture-strike and velocity-ratio according to the changes of Q with azimuth, and formula calculating the azimuth of fractures with the attenuation parameters of orthogonal survey lines. Finally, based on the technology of azimuthal anisotropy, it has made an integrated prediction of fractures in the west of Hashan, and then worked out the plan of fractures development azimuth and density, whose results greatly coincide with actual drilling data.3. For the first time the paper has analyzed the advantages and application conditions of different fracture prediction technologies, established the comprehensive prediction mode of volcanic fractures in the west of Hashan, and defined the technical processes of the mode. The results of prediction show that this mode achieved satisfactory effects, and is also of great referring and guiding significance to similar exploration zones. The prediction mode is as follows. On the whole, the paper has gradually deepened his research from the macro description of fractures, to the qualitative one, then to the semi-quantitative one and finally to the quantitative one. Firstly, it has shown relatively macroscopic information of fractures with the post-stack multi-attribute technology; on this basis, with log restructuring inversion, it has made more accurate qualitative prediction of fractures’ development characteristics and designated relative development degrees of different regions and segments; finally, with the fracture prediction technology of prestack AVAZ and FVAZ, it has pointed out the development density and direction of fractures in different regions and different layers, thus achieving the semi-quantitative to quantitative predictions.