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Wang, Wenjun (Petrochina Co. Ltd.) | Guo, Hongyan (Petrochina Co. Ltd.) | Huang, Youquan (Petrochina Daqing Oilfield Co. Ltd.) | Yu, Ying (Petrochina Daqing Oilfield Co. Ltd.) | Li, Donggang (Petrochina Daqing Oilfield Co. Ltd.) | Li, Qinggang (Petrochina Daqing Oilfield Co. Ltd.) | Sui, Hongyu (Petrochina Daqing Oilfield Co. Ltd.)
This paper introduces the first successful refracturing example of horizontal well in a tight volcanic gas reservoir of Xushen gas field. The well was fractured and put into production in 2008, with the accumulated gas production of 0.64×108 m3. Due to the limited technical conditions of the horizontal well-stage fracturing process at that time, only four fracturing stages have been carried out. the horizontal section of the volcanic gas reservoirs with more than 600 meters of the well was not fractured, leaving a large potential for increasing production. In 2017, based on the fine research of gas reservoirs, the refracturing optimization design, multi-stage perforation, fracturing and commissioning integrated tubular completion and the diagnosis control of complex fractures in fracturing construction, the refracturing job of the well is implemented successfully with a good result.
Sun, Junchang (Research Institute of Petroleum Exploration & Development, PetroChina) | Zhang, Shijie (Xinjiang Oilfield Company, PetroChina) | Wang, Jieming (Research Institute of Petroleum Exploration & Development, PetroChina) | Guo, Hekun (Research Institute of Petroleum Exploration & Development, PetroChina) | Li, Chun (Research Institute of Petroleum Exploration & Development, PetroChina) | Xu, Hongcheng (Research Institute of Petroleum Exploration & Development, PetroChina) | Zhu, Sinan (Research Institute of Petroleum Exploration & Development, PetroChina) | Zhao, Kai (Research Institute of Petroleum Exploration & Development, PetroChina)
Compared with sandstones and carbonates, volcanic reservoirs are much more complex and heterogeneous due to the special eruption diagenesis mechanism, many types of rock lithology, various mineral compositions and a broad wide of pore sizes according to previous studies. Consequently, accurate characterization of volcanic reservoirs using the powerful nuclear magnetic resonance (NMR) logging requires a comprehensive laboratory NMR investigation of volcanic rock because currently used NMR interpreted methods were only developed for sedimentary reservoirs.
To gain an in-depth understanding of NMR characteristics of volcanic reservoirs with different lithology, a total of 108 low-permeability volcanic reservoir rock plugs from three large volcanic gas reservoirs named Xushen, Changling and Dixi, respectively, were prepared to perform NMR measurements and other related tests including CT scans, thin section petrography, mercury injection and mineral compositions analysis. The selected plugs comprise 9 types of lithology representing the main producing formation lithology of the three reservoirs. Specially, centrifuge tests were conducted with the maximum centrifugal forces up to 500 psi to explore the suitable capillary pressure for
Results indicate that, obviously different from sandstone and carbonate plugs, NMR porosity of volcanic plugs at fully brine-saturated state is strongly dependent on rock lithology. NMR porosities of trachyte, trachytic volcanic and granite porphyry are significantly less than the conventional ones measured by the Archimedes method, which means that, accurate identification of reservoir intervals lithology is a primary prerequisite before correct interpretations of NMR logging. Paramagnetic minerals mainly iron and manganese elements contained in volcanic reservoirs are the fundamental cause resulting in this abnormal phenomenon. The critical values of iron and manganese elements contents are approximately 2% and 0.06% by weight, respectively, above which the NMR porosity will be considerably less than the conventional one suggesting by inductively coupled plasma-atomic emission spectrometry (ICP-AES) tests on 14 representative plugs. Then, a new NMR porosity corrected formula was developed to improve interpreted quality of NMR logging. It was found that the suitable capillary pressure for determination of T2 cutoff of volcanic reservoirs is 400psi, 3 times larger than the commonly recommended standard (100psi) for sandstones. The calculated
The laboratory NMR results were used to interpret NMR logging of the Xushen reservoir of Daqing oilfield in eastern China and aided in detailed reservoirs evaluation. The outcome of beneficial intervals selection and high productivity well completion based on the NMR logging interpretation is very encouraging. This study indicated that a comprehensive laboratory NMR tests is very essential to successful application of NMR logging for complex reservoirs such as volcanic reservoirs.
Carbon dioxide (CO 2) flooding is a mature technology in oil industry, which finds broad attention in oil production during tertiary oil recovery (EOR). After five decade's developments, there are many successful reports for CO 2 miscible flooding. However, operators recognized that achieving miscible phase is one of big challenge in fields with extremely high minimum miscible pressure (MMP) after considering the safety and economics. Compared with CO 2 miscible flooding, immiscible CO 2 flooding demonstrates the great potentials under varying reservoir/fluid conditions. A comprehensive and high-quality data set for CO 2 immiscible flooding are built by collecting various data from books, DOE reports, AAPG database, oil and gas biennially EOR survey, field reports and SPE publications. Important reservoir/fluid information, operational parameters and project performance evaluations are included, which provides the basis for comprehensive data analysis. Combination plot of boxplot and histogram are generated, where boxplots are used to detect the special cases and to summarize the ranges of each parameter; histograms display the distribution of each parameter and to identify the best suitable ranges for propose guidelines. Results show that CO 2 immiscible flooding could recover additional 4.7 to 12.5% of oil with average injection efficiency of 10.07 Mscf/stb; CO 2 immiscible technique can be implemented in light/medium/ heavy oil reservoirs with a wide range of net thickness (5.2 - 300 ft); yet in heavy oil specifically reservoir (oil gravity 25 API) with thin layer (net thickness 50 ft) is better.
Carbon dioxide (CO2) flooding is a mature technology in oil industry that finds broad attention in oil production during tertiary oil recovery (EOR). After about five decades of developments, there have been many successful reports for CO2 miscible flooding. However, operators recognized after considering the safety and economics that achieving miscible phases is one of big challenge in fields with extremely high minimum miscible pressure (MMP). Compared with CO2 miscible flooding, immiscible flooding of CO2 demonstrates the great potential under varying reservoir/fluid conditions. A comprehensive and high-quality data set for CO2 immiscible flooding is built in this study. Valuable guidelines have been concluded, and production prediction models are established to further assist the applicability of new projects for the first time. Results show that along with the current method in literature to find applicability guidelines, prediction models involved with important operation and production parameters help to increase the accuracy of CO2 immiscible applicabilities. Data involved in this study are checked for independence for feature selection before utilization. We also find that support vector machine could predict the enhanced oil production rate and CO2 injection efficiency better than multiple linear regression method based on the data set. Furthermore, the multiple linear regression method build an excellent model for the prediction of enhanced oil recovery with an accuracy of almost 100%.
A prediction model is a tool for decision making and problem solving that has been applied in variety of fields (e.g., medical science [1-3], meteorology , transportation [5, 6], business [7, 8], biology [9, 10], and chemistry [11, 12]) for further applicability evaluation. Eagle et al. built a prediction model to accurately estimate the risk of six month mortality after patients have been hospitalized for acute coronary syndrome (ACS), which provides guidance of the intensity of therapy to clinicians in clinical medicine . Gendt et al. established a numerical weather prediction model to help people to make plans for many activities (e.g., farmers to find the best time for harvest; pilots to schedule the safest path, etc. ). In a prediction model, prediction accuracy mainly depends on the methodology of prediction and the quality of data that fed into the model, which is one of the crucial indicator to evaluate the effectiveness of models that researchers spare no efforts to pursue as high of an accuracy as possible.
The earth environment that human beings rely on is seriously threatened by air pollution and global climate change caused by massive CO2 emissions into the atmosphere. Several studies show that geological storage is an effective way to dispose of CO2. Most oilfields in China have entered middle and late development stages, and a great proportion of them are suitable for CO2-EOR and carbon storage. Enormous economic and social benefits could be brought about by applying CO2-EOR technologies in these oilfields. Therefore, the objective of this paper is to systematically screen, assess, and sort the oilfields in China for their suitability for CO2-EOR and assess potential economic and social benefits for implementing CO2-EOR.
First, the screening criteria of the reservoirs suitable for CO2-EOR used in previous studies by many experts were summarized. Second, the screening of Chinese oilfields suitable for CO2-EOR was first executed based on these screening criteria. Then, a comprehensive assessment and sorting of reservoirs suitable for CO2-EOR was carried out using the fuzzy analytic hierarchy process (FAHP) method. Finally, potential economic and social benefits due to CO2-EOR in China were assessed.
Based on data from 511 onshore oilfields in China, the screening result indicates that 151 oilfields are suitable for CO2 miscible flooding and 1 oilfield is suitable for CO2 immiscible flooding. Using the fuzzy analytic hierarchy process method, the 152 oilfields were synthetically assessed and sorted and their potential benefits were evaluated. Using enhanced oil recovery from 5% to 20%, the resulting incremental production could be 3.45×108t to 13.81×108t, respectively, and the CO2 storage potential could be 8.55×108t to 34.19×108t. Total potential economic benefits could reach $76.15 billion to $710.75 billion, CO2 emission reduction benefits could be $0.35 billion to $64.27 billion, energy security benefits could be $0.84 billion to $3.35 billion, and the total benefits could reach $77.34 billion to $778.37 billion. The implementation of CO2-EOR in China could greatly contribute to the Chinese economy and carbon reduction commitment.
This study conducted a survey of the suitability of Chinese oilfields for CO2 EOR, evaluated and sorting the suitability of different oilfields comprehensively and finally calculated the potential economic and social benefits. This study will provide a foundation for the implementation of a CO2-EOR program in China.
The MH1 well on the western slopes in the northwestern margin of Mahu depression of Junggar Basin obtained high-yield industrial oil flow in the Triassic Baikouquan group, which reveals a good exploration prospect in this area. Studies have shown that the reservoir type is fractured-porous dual medium fault-lithologic reservoir. The use of post stack seismic data can just qualitatively predict the plane distribution of fractures, and the actual drilling condition and prediction results are on the contrary. In anisotropic in HTI medium, the presence of cracks can cause propagation characteristics differences in azimuth of seismic information, such as time difference and amplitude, which can be important information reflecting the presence of cracks from the seismic section. Through high-density wide-azimuth 3D seismic data, this paper analyzes the relationship among the azimuth angle, offset and anisotropic characteristics, predicts the intensity and direction of fractures in the study area, achieves the quantitative characterization of fracture , which correlate well with the drilling result. This study is funded by the research of “Xinjiang-Daqing” and “highdensity and wide-azimuth seismic exploration supporting technology research”.
As Guan Baowen and Su Peidong said that fractures can lead to changes in seismic wave energy, amplitude, frequency, phase and the absorption coefficient when seismic wave passes through the reservoir medium, which causes a change in the reflection characteristics and forms important information reflecting the presence of cracks on the seismic section. Currently ways to carry out pre-stack crack detection through seismic data is mainly based on the property that seismic attributes (time difference, amplitude, etc.) change with azimuth and P-wave azimuthal AVO to detect anisotropic of medium, as Qi Qing (2014) and Wang Peng (2014) discussed. These methods are mostly limited by original data with narrow azimuth, low fold and short offset question so that the reliability of the results of fracture detection is reduced.
The study area is located on the western slopes in the northwestern margin of Mahu depression of Junggar Basin, and it appears south to east monoclinic structure. The depth of target stratum is about 2900-3800m where strike-slip faults were widely developed which extend nearly EW with steep sections and vertical throw ranging from 5-20m. (figure1). The MH1 well have obtained high-yield industrial oil flow in the Triassic Baikouquan group with high reservoir pressure that the pressure coefficient achieved 1.53. The effective porosity of the reservoir was 3-9%, with an average permeability of 0.1-8mD. The comprehensive log interpretation result of MH1 well indicates that relatively developed fractures of the interest interval are the main controlling factor of the high-yield well. According to a general mechanism of fracture formation, it can be speculated that fractures may be more developed as close to the strike-slip faults.
Picking PP and PS sections for registration is required for PP-PS joint inversion of PreSTM gathers. Because of the characteristic of PS-wave propagation asso-ciated with lower velocity and frequency content, weaker reflectivity and possible polarity reversal, manual picking based on data similarities is not relia-ble enough to derive an accurate background Vp/Vs model. To avoid this problem, PS anisotropic PreSDM is employed that takes advantage of non-linear slope tomography. This provides depth-consistent PP-PS imaging, and an initial Vp/Vs model for joint inversion.
In this paper, we describe a successful application of PreSDM-based PP-PS joint inversion in the Northwest of China. We show that a lower Vp/Vs ratio occurs in oil-bearing sandstones compared with dry sands in this area. Also, we conclude that PreSDM-based PP-PS joint inversion offers an efficient workflow and more accurate elastic attributes than PreSTM.
Ji, Bingyu (SINOPEC Exploration & Production Research Institute) | Lv, Chengyuan (SINOPEC Exploration & Production Research Institute) | Wang, Rui (SINOPEC Exploration & Production Research Institute) | He, Yingfu (SINOPEC Exploration & Production Research Institute)
CO2 flooding, which owns better injectivity and displacement effect, becomes one of the most effective EOR methods for low permeability reservoirs. However, there is a big difference for CO2 flooding between low permeability reservoirs and higher permeability reservoirs, which implies the limitations of conventional theories for CO2 flooding in higher permeability reservoirs. In view of large pressure differential and uneven pressure distribution between injection well and production well in CO2 flooding for low permeability reservoirs, the miscible volumetric coefficient, the semi-miscible volumetric coefficient and the immiscible volumetric coefficient were defined to characterize the miscible state of CO2 flooding, and the phase sweep efficiency and the compositional sweep efficiency were proposed to describe the displacement front of CO2 flooding. And then, the influence factors of the miscible volumetric coefficient and the compositional sweep efficiency in CO2 flooding for low permeability reservoirs were obtained through reservoir simulation. Furthermore, the concepts mentioned above were applied in oilfields of SINOPEC, which can reasonably illustrate displacement characteristics of CO2 flooding for low permeability reservoirs. Currently, the pilot tests of CO2 flooding have been going into effect, which indicates good prospects.
Generally, the CO2 flooding processes can be divided into two categories, miscible flooding and immiscible flooding. The minimum miscibility pressure (MMP) used to describe miscibility is generally measured through slim-tube apparatus. Miscible flooding will be achieved at pressures higher than MMP [1,2] . However, miscible or immiscible could not be distinguished by using MMP alone, as the effect of in-situ pressure distribution on miscibility cannot be neglected. Moreover, the obscure boundary between the miscible state and the immiscible state indicates that most of CO2 EOR projects should be considered as near-miscible or partially miscible [3-7] . Especially, the uneven distribution of reservoir pressure and the huge pressure drop from injector to producer in CO2 flooding for low permeability reservoirs will affect miscibility and oil recovery [8-10] . Based on the above understanding, miscible volumetric coefficient, phase sweep efficiency and compositional sweep efficiency were defined and the concept of non complete miscible displacement was proposed for CO2 flooding in low permeability reservoirs, which were used for the design of CO2 EOR projects of SINOPEC.
As a bottleneck problem, borehole instabilities have severely restricted further development and application of gas drilling technology. Here, the mechanisms of borehole instabilities in gas drilling are studied and four instability mechanisms are presented, including mechanical instability of weak rocks or crumbliness rocks because of their low mechanical strength, mechanics-chemical coupling instability caused by production or injection of fluid, rock burst of payzone and dynamically instability of overlying mudstone with pressure during hi-speed production. Because gas drilling has special operating conditions compared with conventional drilling, the experiment simulating field operating conditions is crucial to evaluation of borehole stability in gas drilling and need relate to more evaluating parameters, including formation temperature, pore pressure, overlying pressure, confining pressure, borehole pressure, types and production of formation fluid, velocity of slide wave as well as drilling operation, including dry gas drilling, foam or liquid drilling and conversion process of gas-liquid drilling operation. Meanwhile, deepening study on the mechanisms and evaluation methods of borehole instabilities, improving treating agents and treating process and advancing forecasting technique and sealing technique of formation fluid are thought of as effective approaches of enhancing borehole stability in gas drilling. This will be helpful for understanding the mechanisms of borehole instabilities in gas drilling and improving evaluation methods.
Fracture net pressure examination is a widely used method before hydraulic fracturing. It is very crucial for understanding the characteristics of the formation, the complexity of the fracture as well as improving the success ratio of fracture operation, optimizing the fracturing design, evaluating the operating and the fracturing completion. Due to the nature of volcanic reservoir fracture mechanism, filtration mechanism, percolation mechanism, as well as the complex status of fracture initiation and extension of Daqing reservoir, in order to realize the characteristics of reservoir and fractures, and, to ensure the successful field operation, it is very important to conduct the test fracturing and field examination before major fracturing. After practising fracture tests in nearly 100 wells and analyze the effects of different volcanic reservoir physical characteristics on the fracturing operation, we developed six categories of G function pressure analysis methods and four standardized methods for examining the volcanic fracture complexity level by evaluating the characteristic parameters. We also developed fracturing operation technique for volcanic reservoirs and corresponding theoretical explanation methods, and built the specifications to control the operation risk which is based on the fracture test data, realized the accurate control scheme selection and effective execution, as well as improved the controllability on the fracturing operation of volcanic reservoir. According to the data collected from the filed operations, the success ratio is increased from 36% to 90% after applying this method.