Liu, H. (Research Institute of Petroleum Exploration and Development, PetroChina) | Meng, S. (Research Institute of Petroleum Exploration and Development, PetroChina) | Zhao, Z. (Research Institute of Petroleum Exploration and Development, PetroChina) | Yan, J. (Research Institute of Petroleum Exploration and Development, PetroChina) | Yao, Z. (Research Institute of Petroleum Exploration and Development, PetroChina)
Over-developed oilfields in East China have gone through suffering situations induced by high cost in recent years. The trend of resource deterioration is irreversible and the deterioration leads to the increasing difficulties in oil and gas exploration and development. The rate of return-on-investment continually decreases resulting from over-rapidly increasing properties, and ever-rising facility depreciation and damage. The space for increasing income and seeking profit is further narrowed by low oil price. These oilfields now move into the period that has huge obstacles to increase profit, confronted with more difficulties in obtaining economic reserves and profitable production. Now the development in over-developed oilfields has stepped into double-high phase, meaning that to avoid ineffective or poorly effective measures and control the fundamental indices like the rise of watercut, natural decline, and so on play the key role in improving development profit. It is necessary to accelerate the build-up of production capacity and enhance reserve, production, and the rate of return-on-investment by actively reforming techniques for the formation of a series of new ones on enhancing oil recovery, as well as advancing the four-unified-into-one management mode of merging production and management, investment and cost, reserve increase and production build-up, and research and production. The concept of focusing on reserve and production must be further switched, meanwhile profit must be realized in all the ingredients of exploration and development during their whole processes, and efficiencies must be maximized. The burden of production and management is relieved, and the focus is switched to profit in an overall and complete way by further recognizing that investment today is tomorrow's cost and strictly controlling property scale to prevent its excessive increase.
Chen, Zhide (Exploration and development research institute of Daqing oil field company P.R.China) | Zhang, Liyan (Exploration and development research institute of Daqing oil field company P.R.China) | Ang, Li (Exploration and development research institute of Daqing oil field company P.R.China) | Tong, Yuanping (Exploration and development research institute of Daqing oil field company P.R.China) | Wang, Guishui (Exploration and development research institute of Daqing oil field company P.R.China) | Wang, Jianmin (Exploration and development research institute of Daqing oil field company P.R.China)
CO2 geo-sequestration, as an effective method to reduce Green House Gas emissions, is a front subject in China. In order to identify the potentials of implementing large scale CO2 sequestration projects according to China's current development tendency, this paper analyzes the general formation conditions of the main oilfields in China, which are manifested in complex formation structure with strong heterogeneity, low or ultra-low permeability and low porosity.
In China, the main application mode of CO2 sequestration, also the most profitable method, is to inject CO2 into oil and gas reservoirs to enhance recovery. Meanwhile, 60%-80% of the injected CO2 could be stored into the formation. Jilin oilfield in northeast of China, the dominant test field for CO2 sequestration, has been listed into the National Key Program in 2006. Pilot tests reveals formation here is featured with well developed sand body, good connectivity, well defined cap rock, undeveloped fractures, and independent hydro-geological system, which contribute to a considerable and secured storage for CO2. Though still on the initial stage, studies here would shed light on the further researches in China.
Some unique advantages of conducting CO2 sequestration projects in China have been summarized and analyzed in this work too. Most of the new-found oil and gas reservoirs belong to low-permeability reservoirs, wherein CO2 flooding shows its superiority over conventional water injection in tight formations. Moreover, both main CO2 sources (coal-fired plants & power plants) and injecting sites(low-permeability reservoirs) locate in northern and eastern China, which would meet the demand of CO2-EOR and shorten the distance of transportation to reduce corrosion risk and operation cost. Although being not regulated by Kyoto Protocol, positive measures of CO2 sequestration in China possesses a significant and far-reaching meaning to releive the pressure of global environment.
Key words: Low-carbon, Geo-sequestration, CO2-EOR, Pilot test in Jilin Oilfied
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
CO2 gas has low viscosity, flows very freely and is corrosive. This paper presents two innovative and economical anti-corrosion techniques of injecting (flooding with) CO2 under conditions of high temperature, high pressure and the CO2 cause corrosion. The down hole packer was fabricated with the CO2 corrosion resistant chrome-plated alloy steel. Chromium is one of the most hard metal and corrosion resistant material, the equipments and devices down hole were chrome-plated that can be also wear-resistant effect. and the general rubber design. The casing tubing annulus injected the imidazoline type corrosion inhibitor to keep the pressure balancing, anti-corrosion situation to protect tubing pipes and casing pipes.This anti-corrosion technique of injection well in CO2 flooding ensured the packer seal long-term security and achieved the CO2 gas safety injection.
Key words: Anti-corrosion, the piping design technique of injection well in CO2 flooding, corrosion inhibitor
The GaoTaiZi oil region and YuShuLin oil region of the Daqing oil field is a low permeability reservoir, the waterflood development wasn't effective. In 2003, the CO2 flooding pilot test had been developed to research effective exploitation technique about the low permeability reservoir and evaluated CO2 flooding commercial efficiency deeply, created the CO2 flooding support equipment. In 2007, the CO2 flooding test had been expanded. There are 45 pilot well now, 2.4km2 producing oil area, 96×104t producing geologic reserve. The 14 injection well and 30 oil well had been put into production in the CO2 flooding test regions, the cumulative injection of CO2 was 94786t.
1. Anti-corrosion Technique
The CO2 oil displacement technique was that the low permeability reservoir was injected with CO2 to expand the crude oil and reduce its viscosity. The reservoir hence was qualified for CO2 miscible flooding to increase the oil recovery factor. The injection system had the corrosion risk when injected the CO2 flooding followed by water flooding because the water and CO2 fixed together to formed carbon acid.
The oil tubes and casings' corrosion had been researched under high pressure, high temperature in the Daqing's deep gas well regions. This paper analyzed CO2 corrosion environment's feature and principle in the lab and on the deep gas well field to grasp the CO2 corrosion rules and develop optimizing evaluation for the various casing pipes and corrosion inhibitors in the deep gas well, had been finished optimizing the completion pipe string design and technique to be most reasonable economical. These techniques supported the anticorrosive scheme design for various CO2 deep gas wells, established the reasonable productivity and provided the integrated various economic rational CO2 anticorrosive technologies for the development of the Daqing's deep gas well regions.
All the drilled gas well which had the industrial natural gas in the Daqing's deep gas well regions were high temperature, high pressure characteristics, there were 41.3% gas well was serious corrosion which the CO2 partial pressure was more than 0.2MPa. At the beginning of exploratory development, mostly materials of gas wells' oil tubes and casing pipes were made of the common carbon steel, the CO2 corrosion was very serious on these oil tubes and casing pipes. Although the exploitation oil tubes and casing pipes were made of 13Cr alloy steels can obtain the better anticorrosive effect but the cost of these tubes and casing' price was very high which cost was 5 ~ 6 times to the ordinary carbon steel casing pipes. Use lower investment price with the anticorrosive infection to invent and create a corrosion inhibitor control technique of economic rationality and auxiliary project.
The Anticorrosive Technology Research
1. The research of CO2 corrosion rules
This research was base on the CO2 corrosion environment's details in the deep gas well, was passed the lab experiment to research the temperature, CO2 partial pressure, velocity of flow, PH, Cl- concentration and analyzed these five corrosion ingredients above by the grey correlation theory, the result was CO2 partial pressure was the most important reason to induce CO2 corrosion and the temperature > velocity of flow > H+ concentration > Cl- concentration.
The monitoring data of corrosion section from the lab and the gas field were calculated to find out the concentrative corrosion was in the middle-upper part of the gas well, the Figure.1 was showing this opinion.
The practices have shown that the premise for successful implementation of gas drilling is stable borehole wall, formation containing less water or no water, not high pressure formation and clear reservoir bed relation, etc. Therefore, accurately evaluating borehole wall stability, predicting formation water production and selecting reasonable well intervals for gas drilling to effectively prevent occurrence of complex conditions such as blocking, sticking and fracture of drilling tools, etc. caused by formation water production and borehole wall instability are the keys for success of gas drilling. The paper, aiming at the problems of formation water production, long interval reaming, bit freezing and drilling tool failure occurring in deep zone gas drilling of Daqing oilfield, has analyzed difference of formation water production between gas drilling and conventional drilling, sifted logging response parameters of water zone under the deep zone gas drilling conditions of Daqing oilfield by borrowing the reservoir fluid identification methods under conventional drilling conditions and combining the practical gas drilling data, and established qualitative water zone identification method for gas drilling; in addition, based on Darcy's law, the paper has optimized and formulated quantitative calculation method for unsteady flow formation water production under dynamic condition and established borehole wall stability prediction model under gas drilling conditions in Daqing deep zone volcanic rock formation.
The formation water production prediction method and borehole wall stability model established in the paper, as important basis for feasibility analysis, engineering design and operation of gas drilling, have been 100% popularized in engineering design of deep zone gas drilling in Daqing, China. The coincidence rate of water-producing horizon predicted by using this method, comparing with the practical water-producing horizon, is up to above 80%, and that of borehole wall stability evaluation result, comparing with practically measured borehole diameter data, is up to above 85%.
In the volcanic rock gas reservoir in Xujiaweizi fault depression structural belt of Daqing oilfield, China, Xushen gas field, with the scale over 1000*108 m3, was formed since the beginning of 2005. To continue to expand the exploration result and prove the reserves scale earlier, it is necessary to accelerate natural gas exploration, increase amount of drilled exploration wells, enhance drilling speed and shorten drilling period on the premise of protecting reservoir beds. However, the rocks below Daqing gas field deep zone Q2 Member are of bad drillability, high hardness and low drilling rate. To enhance drilling speed, several speed increasing tests for conventional drilling, such as bit optimization and parameter optimization, etc., were carried out in Daqing oilfield in 2004. Although certain effect has been achieved, the requirement for accelerating exploration still can not be satisfied. It is urgently necessary to apply new drilling technologies to make a breakthrough in enhancement of ROP (rate of penetration).For this reason, studies and field tests of gas drilling supporting technologies have been performed in Daqing oilfield since 2005 and the drilling operation of 14 gas wells has been finished, in which the ROP, 3 to 5 times of that of conventional drilling, has been increased significantly. While in gas/foam drilling practice, the complex conditions such as borehole wall instability and blocking, sticking and fracture of drilling tools, etc., caused by formation water production occurred frequently, and the drilling operations of several wells were completed in advance since the designed well depths were failed to be reached due to formation collapse in crushed zone of volcanic rock. The problems of borehole wall instability and formation water production seriously influence the efficiency of gas/foam drilling, thus restraining the advantages of gas/foam drilling technology.