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.
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.
He, Liu (Research Inst. of Petroleum Exploration and Development, PetroChina) | Yingan, Zhang (Jilin Oilfield Company, PetroChina) | Honglan, Zou (Research Inst. of Petroleum Exploration and Development, PetroChina) | Yang, Gao (Research Inst. of Petroleum Exploration and Development, PetroChina)
The formations of DaQing gas field are mainly volcanic reservoirs which have the characteristics of low permittivity and complex formation structures. Most of these wells need fracturing remodeling to meet the standards of industrial gas stream, and also, the gas productivity tests as well as the pressure recovery tests conducted on these wells are different from other regular gas reservoirs. Considering the nature of the volcanic reservoir, such as dissolution pores, karsts caves, natural fracture development, we built two mathematical models of dissolution pores development and natural fracture development under both of the Darcy flow conditions and Non-Darcy flow conditions separately to predict the production of triple porosity reservoir after gas reservoir well fractures. By using the Laplace transform and numerical inversion, the equation to calculate production of complex volcanic gas wells is obtained. Based on these researches, law of volcanic reservoir productivity is investigated. The theoretical data are compared with the practical data collected from the field operation. The comparison results reveal how the parameter of the fluid volume, proppant indexes, and conductivity of artificial fracturing induced fractures and length of fractures change affect the productivity. The research work reported in this paper provides theoretical support on the optimization method of fractured wells design of volcanic gas reservoir.
Nowadays, as the deep gas reservoirs in Daqing are explored, the complex volcanic reservoirs have been the major reservoirs in deep natural gas exploration and production. The reserves of volcanic gas reservoirs take up 88% of the total gas reserves. However, the deep complex gas reservoirs may cause heavy pollution during the drilling completion, and some of the barriers between target zones of the wells are very thin, leading to a poor stability. Additionally, because of the complex water/gas relations in the formation, such as appearance of bottom water and water and gas sharing the same formation in some wells, the fracturing operations will induce water channeling. All these facts may cause the failure of the fracturing operations.
Especially, when the fractured formation is close to the water/gas interface, the fractures will easily extend into the water layer. The existence of water in the gas wells directly leads to the reduction of production and recovery rate of the gas reservoirs, or even kills the gas reservoir in the worst cases. For these types of gas wells, acidization technology is a promising solution. It not only avoids the pollution near the wells of volcanic formation, but also chemically dissolves the fillings in the fractures and pores, improves formation seepage flow environment, increases fluid mobility, and finally optimizes the productivity. Acidization technology also has the advantages of low investment and quick payback.
This paper reports the volcanic reservoir acidization technology we developed. The lab test results show that this technology solves the problems of high erosion rate of the oil strings under high temperatures (125-160 degree of Celsius). The acidization technology is applied in three wells, and the productivity of those increases profoundly.
The function of the fracturing strings is to transfer mechanical energy to crack the formations, and conduct proppant to the formations. Because the deep volcanic reservoir has the properties of deep burial, tight and complex formation lithology, the fracturing operations in such reservoir may encounter the issues of high downhole temperature, high operation pressure and high operation friction. The conventional fracturing strings are not capable of overcoming these problems. The fracturing strings are facing the following challenges:
1. Deeply buried volcanic reservoirs, which are normally deeper than 3000 meters, have very high temperatures, which are higher than 150 degrees Celsius. How to improve the temperature adaptability of the fracturing rubber packer is technically challenging. 2. The formation lithology of volcanic reservoirs is tight and complex, the initiation and extension of the fractures are relatively difficult. The operation pressure is high, so that the fracturing strings and other downhole equipment have to be able to sustain this high pressure. 3. Due to the high discharge rate and large scale operation in the process of volcanic reservoir fracturing, the fracturing strings should have low streamwise friction, strong anti-proppant erosion ability. In order to meet these challenges, we conduct the research on the fracturing equipment and fracturing strings used in the volcanic reservoirs. By studying the theoretical mechanics, optimizing packer structure, improving the temperature and pressure bearing ability of rubber packers, we design special tools such as anchoring tools to improve the adaptability of the fracturing technique, and also generate the safety specifications and operating regulations for the fracturing tools stripping in and out in the deep gas reservoir wells. All these achievements make the large scale fracturing remodeling of the deep gas volcanic reservoir possible. After the field tests, the success ratio is 97.5%.
Zhao, Bangliu (Exploration and Production Department) | Wang, Daxing (CGGVeritas Petroleum Research Institute) | Shi, Songqun (CGGVeritas Petroleum Research Institute) | Shen, Liang (CGGVeritas) | Miao, Xiaogui (CGGVeritas) | Wang, Pu (CGGVeritas)
The 1'st members of Yingcheng formation, lower Cretaceous in Xingcheng development area, Songliao Basin mainly have rhyolite gas reservoirs. The 1'st member of Yingcheng is belong to the low-porosity and low-permeable reservoir, the reservoir space are mainly fracture-pore. By the means of field out crops observation, core description, thin section analysis, well logging identification and the seismic forecasting, the fractures of 1'st member of Yingcheng volcanic reservoir was analyzed in three aspects: the genetic, the attitude and the width. The dominate fracture type of 1'st member of Yingcheng are the tectonic related fractures(78.5%), then the diagenetic related fractures(17.0%), corroded fractures(4.2%) are the last. The high angle fractures and the vertical fractures are most common in tectonic related fractures; the oblique fractures are the following. The dominate type of diagenetic related fractures are level fractures, followed by high angle and oblique fractures. The fractures in the reservoir were mainly wide fractures (width>1mm) and narrow fractures (0.1mm<width<1mm), the fractures of middle width are the minority. The distributions of the fractures were mainly controlled both by the regional stress field and the activity of the faults, the fractures are displayed by belts in coincident with the strike of the main faults.
Keywords: volcanic gas reservoir, reservoir fracture, structural fractures, diagenetic fractures, high angle fracture
The development and exploration of volcanic gas reservoir is now the hot spot in petroleum fields. The volcanic gas reservoirs have been discovered in Songliao basin, Chaidamu basin, Sichuan basin and so on[1-2]. It is quite difficult to study the fracture characteristics of volcanic gas reservoir. This paper takes the volcanic gas reservoir of the Yingcheng Formation of Xushen area in Daqing oilfield for example to study the fracture of volcanic reservoir.
The volcanic gas reservoir of the Yingcheng Formation in Xingcheng area is located in Shengping-Xingcheng structural belt of Xujiaweizi Fault-Depression in the north of Songliao basin, mainly developed in the first member of Yingcheng Formation in Lower Cretaceous Series. The buried depth of reservoir is 3000~3500m. The volcanic gas reservoir is mainly the rhyolite. The types of reservoir are mainly fracture-porosity style, which belongs to low porosity and low permeability reservoir.
1. Lithology and lithofacies characteristics
The lithology and lithofacies characteristics control the development of fractures and cavity, which is the dominant factor to make the types of reservoir space and reservoir heterogeneity complicate[3-4]. By the chemical analysis of core gathered from 7 wells, the chemical composition of volcanics is mainly the SiO2(74.24% on average) and next is Al2O3(10.4% on average) and K2O+Na2O(8.42% on average). The figure of alkaloid-SiO2 is made by the analysis ,which suggests that the lithology in the area is mainly acidic volcanic rock and the next is the intermediate lava. The type of rocks is mainly the rhyolite and others are the mixpah, trachydacite, dacite, andesite and necrolite.
This paper mainly focuses on the igneous rock reservoirs of Yingcheng group in Xushen gas field, Daqing. According to plenty of data in field (Core description, conventional logging, FMI, drilling data), this work presents the general development characteristics of core fractures, the response feature of fractures in conventional well logging and FMI, then, an quantitative fracture-identification criteria has been established with the method of curve unit-principle and algorithm.
Compared with the identification results of conventional well logging, FMI shows a significant advantage of fracture characterization in most cases. In some local low-resistivity formations, conventional well logging could serve as an assist method in collating the results of FMI to distinguish the low-resistivity caused by fractures.
Results provided through practices and research in this paper demonstrates that combining FMI with the conventional well logging can lead to a creative and feasible method in identifying fractures in the igneous rock, and shed some light on related researches on other complex reservoirs, such as metamorphic rock, carbonate rock, and tight sand oil gas reservoirs.
keywords: ingenous rock reservoir, fracture, FMI (fullhole image logging), curve unit-principle
Feng, Zhiqiang (Daqing Oilfield Co. Ltd.) | Huang, Wei (Daqinge Research Institue of Petroleum Exploration and Developmeny, Daqing Oilfield CompanyLtd, PetroChina) | Feng, zihui (CNPC) | Zhang, Erhua (CNPC) | Liu, Chuanping (E&D Research Institue, Daqing Oilfield Compant Ltd, PetroChina)
Lithology identification of complex volcanic rock, reservoir prediction with seismic data and identification of volcanic rock lithology and prediction of fluid property in reservoirs using wireline logging are key techniques for volcanic gas reservoir exploration. Up to now, many exploration wells and a large amount of 3D seismic are performed in volcanic gas reservoirs. Practice and theoretical research has establish a series of effective methods of lithology identification and reservoir and fluid prediction by seismic and wireline logging, with much achievement in gas field exploration evaluation and gas reservoir characterization of Xushen Gas Field.
Key words: Songliao Basin, volcanic gas reservoir, reservoir prediction, complex litholgy, lithology identification, reservoir parameter.
Songliao Basin, located in northern China, is the largest and most prolific non-marine oil-bearing basin. It develops two sets of different Cretaceous formations. The lower rifted formation has alluvial fan to deep lake siliclastic sediments and coal beds, and widely developed volcanic rock. The upper depression formation is mainly fluvial-lucustrine sediments. Being a huge oilfield, Daqing Oilfield has oil reservoirs discovered in sandbodies in fluvial and deltaic facies of upper depression formation in 1959, and large gas reservoirs discovered in volcanic rock in lower rifted formations in 2002. Volcanic gas reservoirs have burial depth of 3000m~5500m. High pressured fluid history of the basin causes difficulty for tight glutinite in this depth to form gas reservoir. Compared with siliclastics, volcanic rock has not only variety of lithology types and pore types, but also little research result. Study on field crops and modern volcanoes process establishes structural patterns for different types of volcanoes. Based on core analysis data of large amount of volcanic rock, and combined with wireline logging and gas testing data, a series of exploration techniques including lithology identification and reservoir and fluid prediction are developed for volcanic gas reservoirs.
1 Seismic identification technique for volcanic rock body
Difference of eruption periods and scales cause variety of lithology and complexity of depositional structures. Weathering and host rock also cause variety of seismic structures of volcanic rock.