The Kenshen tight gas field, located on the northern margin of the Tarim basin, western China, has extreme reservoir conditions of an ultra_depth reservoir (6500 to 8000 m) with low porosity (2 to7%), low matrix permeability (0.001 to 0.5 md), high temperature (170 to 190°C), and high pore pressure (110-120 MPa). Those conditions result in high completion costs and a significant difference in individual well production rates; with only one-third of wells drilled meets expectations. Previous studies focused on natural fracture(NF) and attempted to classify reservoir qualities based on the density of NF. Unfortunately, some NFs were closed or cemented by clay or calcite, and it is hard to distinguish open NF from closed NFs using well images in oil-based mud, which is widely used in this tight gas field for reservoir protection. Thereby, no positive correlation between NFs density and productions has been identified, even with the same stimulation treatment.
In this study, a comprehensive geological study was conducted to find a new way of characterizing the effectiveness of NF. First, the initial and development stages of NFs were recontructed through a tectonic activity study. Two stages were detected and showed different strikes. Second, petroleum system modeling technology was applied to simulate source rock maturation and gas migration, which revealed that gas generated in the Jurassic source rock migrated to the Cretaceous reservoir formation through faults activated in the same period as the late stage of NFs development. NFs developed earlier were closed or cemented by calcite of later deposition; those at late stage were open and effective for gas charge. Also in this study, Advanced analyses of borehole images indicated an alternative way to delineate NFs developed at different stages using geometry (i.e, crossed NFs shall include those ones developed at later stage). Parallel NFs with its development unidentified can be classified through the intersection angle of fracture strike and maximum stress direction. The smaller the intersection angle is, the easier it is for stimulation and alos the higher for the well production. Based on this study, we have divided reservoirs in the study area into three classes: class 1, reservoir with crossed NFs; class 2, reservoir with fractures of small intersection angle; class 3, reservoir with fractures of large intersection angle. This innovative reservoir classification through NF geometry is currently used in the field to determine formation stimulation method. Class 1 reservoir can benefit from acidizing alone with low completion cost. Class 2 reservoir of should be hydraulically fractured with acid. Class 3 reservoir of should be fractured with sand and proppant sand to achieve economical production.
Reservoir classification with NFs geometry had been applied successfully to guide stimulation design in the Keshen tight gas reservoirs. It is a practical and feasible way to choose the most appropriate stimulation treatment method to optimize well performance and avoid restimulation to reduce costs for this extreme type of tight gas field in western China.
Many oil and gas resources in deep-sea environments worldwide are often located in high-temperature/high-pressure (HT/HP) and low-permeability reservoirs. The reservoir-pressure coefficient usually exceeds 1.6, with formation temperature greater than 180°C. Challenges are faced for well drilling and completion in these HT/HP reservoirs. A solid-free well-completion fluid with safety density greater than 1.8 g/cm3 and excellent thermal endurance is strongly needed in the industry. Because of high cost and/or corrosion and toxicity problems, the application of available solid-free well-completion fluids such as cesium formate brines, bromine brines, and zinc brines is limited in some cases. In this paper, novel potassium-based phosphate well-completion fluids were developed. Results show that the fluid can reach the maximum density of 1.815 g/cm3 at room temperature, which makes a breakthrough on the density limit of normal potassium-based phosphate brine. The corrosion rate of N80 steel after the interaction with the target phosphate brine at a high temperature of 180°C is approximately 0.1853 mm/a, and the regained-permeability recovery of the treated sand core can reach up to 86.51%. Scanning-electron-microscope (SEM) pictures also support the corrosion-evaluation results. The phosphate brine shows favorable compatibility with the formation water. The biological toxicity-determination result reveals that it is only slightly toxic and is environmentally acceptable. In addition, phosphate brine is highly effective in inhibiting the performance of clay minerals. The cost of phosphate brine is approximately 44 to 66% less than that of conventional cesium formate, bromine brine, and zinc brine. This study suggests that the phosphate brine can serve as an alternative high-density solid-free well-completion fluid during well drilling and completion in HT/HP reservoirs.
Fan, Junjia (Research Institute of Petroleum Exploration and Development and Peking University) | Zhou, Haimin (Peking University) | Liu, Shobo (Peking University) | Liu, Keyu (CSIRO Earth Science and Resource Engineering)
Tight sandstone gas as one kind of unconventional resources has taken up a significant part in natural gas resource growth in recent years. Tight sandstone gas originated from the definition of U.S. Gas Policy Act of 1978, that regulated in-situ gas permeability to be equal to or less than 0.1 md for the reservoir to qualify as a tight gas formation Gas flow in tight sandstone behaves as non-Darcy flow has been reached a consensus by scholars; however gas-water flow characterization and the main factors for gas-water flow in tight sandstone remain complicated. Scholars proposed that there is a "Permeability Jail?? for water-gas flowing in tight sandstone reservoirs which means both water and gas cannot flow in "Permeability Jail?? range. This may explain why neither gas nor water produced in tight sandstone reservoir in wells of some tight gas field.
In order to get a better understanding on water-gas flow characteristics and to figure out the major affecting factors for gas-water migration in tight sandstone, we analyses pore structure, bulk and clay mineral constitutes and gas-water two phases flow characteristics of five tight sandstone samples with low permeability (<0.1md) and low porosity from the tight gas fields in Kuqa Depression, Tarim Basin by using of micro CT scanning, XRD analysis and physical simulation experiments.
Experimental results indicate that 1) "Permeability Jail?? does not existed in five tight sandstone samples, but flow ranges for gas-water of five samples are different; 2) pore structures and fractures play significant roles for the gas-water flowing, and fractures can improve the gas-water permeability significantly; 3) for permeability, the pore connectivity is more important than total porosity of rocks; 4) content of clay minerals in tight sandstones affected the gas-water migration, the higher the clay minerals contents are, the lower the permeability of rocks is.
Super-high pressure gas fields in Tarim Oilfield mostly lie in the area of Tianshan mountains. the upper cap rock of these gas fields is salt gypsum formation which is deeply imbedded, this salt gypsum formation contains not only many different pressure systems but also soft mudstone and salt-water bed with super-high pressure, 80 percent of all accidents and complex situations take place in this section. Tarim Oilfield arranges well Dina-11 in Dina structure in order to change this adverse situation; all results and achievements during drilling operation are as follows:
Consummate the drilling technology for Shanqian structure and reduce the possibility of accidents and complex situations, quicken the speeds of exploration and development through drilling operation in well Dina-11
Dina area as a main anticipative reservoir of Tarim Oilfield also lies in the structure of Tianshan mountains(Fig.1), as mentioned earlier. It exists great difficulty and risk in drilling operation in this area too. For this reason, Preliminary prospecting well Dina 1 and well Dina 2 arranged earlier in this area were one after the other blowout out of control for drilling accidents.
Well Dina 1, was starting drilling operation in 1999. When the well was drilled at 4440.2m with drilling fluid which density was 1.52g/cm3 on July 14th, 2000. The pump pressure rised from 17.2MPa to 38.1MPa,the safety latch in mud pump was cut. Then, the drill stem was stuck, the upper/down kelly cock and valve in standpipe could not be closed. Eventually, blowout had to performed through this rout way of kelly ?standpipe? kill line manifold? relief line ,the density of ejective brine is 1.24 g/cm3, Cl- is 199000mg/L.The salt crystal block the drill stem. For this reason, this well was abandoned with cement injection at last.
After well Dina 1, Well Dina 2 started drilling operation in 2000.When the well was drilled at 4875.59m with 1.85 g/cm3 mud on April 29th,2001.The kick occurred, after well shutdown, the shut-in casing pressure was 16MPa and the shut-in standpipe pressure was 14MPa.Afterward,the choke valve which installed in the choke manifold was opened up and discharged fluid in hole. Then, shutting in the well again, the caing pressure was 33MPa and the standpipe pressure was 27MPa. During kill operation with fracturing truck,the casing pressure went up 66MPa,the well-control equipment was failed and led to bolw out and fire. After subsequent wrecking and fire fighting operation in 66 days, kick-killing succeed finally.
Hales, John (Halliburton Energy Services Group) | Smith, Ian William (Halliburton Energy Services Group) | Wah, Kee Yong (Tarim Oil Company) | Xun, Liu Jian (Tarim Oil Company) | Yong, Li Ru (Tarim Oil Company) | Qiu, Liu Ming
Tarim Oil Company, a major producer in northwest China, operates the Kela II field in the remote province of Xinjiang. Their completion program required underbalanced perforating for several large-volume gas wells with estimated record breaking flow rates. To support the operational priorities on safety, reliability, and flexibility in the completions, options for perforating were limited to wireline-deployment and modular gun-hangar systems so that gun removal would be possible if any nonconformity occurred. To obtain the maximum well production after perforating, it was imperative that minimal wellbore damage occur. Killing the well was not a viable option since the reservoir properties in the Kela field are fluid sensitive. Therefore, killing the well after perforating underbalanced would increase near wellbore damage and produce a higher skin.
This paper will discuss the two wells completed in this field using a newly developed perforating method. There were four wells developed, but two were completed by another perforating service provider. The expected production from each well was ~4 MMscf/d. Production from the two wells completed with the methods described in this paper exceeded expectations by 46% with a rate of 5.85 MMscf/d at full open choke with a 5 MPA drawdown. The production from the two other wells that were perforated using other methods fell short of expectations with rates of less than 4 MMscf/d.
Based on the third-round resource evaluation, the provable natural gas reserves in Tarim Basin is 7.96×1012m3. The proved gas fields in Tarim Basin main lie in Kuche- Tabei region. The natural gas fields possess the characteristics such as deep buried payzone , large reserve scale, high reserve abundance, good reserve properties, high formation pressure and high production of single well. The single well AOF of Kela 2 ,the main force gasfield of Westto East Pipeline Project (WEPP), is above 1000×104m3/d. Based upon the degree of proved reserve , reserve scale ,single well yield, and the location of gasfield, Kela 2, Yingmai 7, Yangtake, Yudong 2, Jilake and Yaha gasfields in Kuche-Tabei region in Tarim basin have been selected as the supply gas fields with annual transportation of 12.0×109m3.
WEPP is an important project in China. With this project, natural gas in Tarim, Xinjiang will be transported to the termination in Shanghai in the East by way of some provinces and municipal cities including Gansu, Ningxia, Shaanxi, Henan, Anhui and Jiangsu. The gas will supply the enterprise and the residents in the provinces (regions and cities) along the pipeline with a total length of 4,167km. The implementation of West-East Natural Gas Transportation will powerfully accelerate the oil and gas exploration and development in Tarim Basin, enhance the economic development of the Western region, promote the adjustment of China's energy structure and industrial structure, improve the living quality of people along the pipeline and control the atmosphere pollution effectively.
There are abundant oil and gas resources in Tarim Basin. 27 oil and gas fields in total has been found by Tarim Oilfield Co. with proved oil in place (OIP) of 397.526 million tons and original gas in place (OGIP) of 657.93 billion cubic metre(CM) till the end of 2003. A crude oil production base with the production capacity above 5 million tons per year had been established, which favors and starts up the West-East Natural Gas Transportation Project. So far, the proven ratio of oil and gas resources in Tarim Basin is still lower than 10%, which demonstrates a great prospecting potential. The implementation of West-East Natural Gas Transportation will powerfully accelerate the oil and gas exploration and development in Tarim Basin, enhance the economic development of the Western region, promote the adjustment of China's energy structure and industrial structure, improve the living quality of people along the pipeline and control the atmosphere pollution effectively.
Oil and gas exploration and development achievements in Tarim Basin
Tarim Basin lies in the south of Xinjiang Vygur Autonomous Region of China with an acreage of 560×103km2. There are Takelamagan Desert _the floating desert , in the center of the basin and Gobi desert and mountains on the fringe. The exploration and development environment is extremely scurviness . Among 50-years' oil and gas exploration and development experience in Tarim basin, it has demonstrated a zigzag way which called “six ups and five downs”. However, there is no important breakthrough because of the restriction of exploration technologies until 1989.
Pingping, Shen (PetroChina Company Limited, China) | Wenzhi, Zhao (PetroChina Company Limited, China) | Jiayu, Niu (PetroChina Company Limited, China) | Xiaodi, Li (PetroChina Company Limited, China) | Yunhua, Deng (CNOOC Limited, China China Petroleum and Chemical Corporation) | Daqing, Jiao (CNOOC Limited, China China Petroleum and Chemical Corporation)
FRONTIERS AND OUTLOOK OF FUTURE EXPLORATION IN CHINA Shen Pingping PetroChina Company Limited, China Zhao Wenzhi Niu Jiayu Li Xiaodi Deng Yunhua CNOOC Limited, China Jiao Daqing China Petroleum and Chemical Corporation Abstract Oil and gas exploration in China has made great progress in the past 5 years from 1996 to 2000. In eastern China, oil reserves have been increased by extensive exploration in mature exploration basins, with enough discoveries to maintain stable oil production over the past 5 years. In western China, many new large oil-gas fields have been discovered in new plays, which have rapidly increased both oil reserves and production. In the case of gas exploration in west China, several giant gasfields discovered in the Tarim, Ordos and Sichuan basins have doubled gas reserves over the past 5 years. In the Bohai bay area, several new giant oilfields have been discovered in new sequences, and the Bohai bay area will become an important oil production base in the next few years. The future exploration frontiers are foreland basins and new sequences of cratonic basins in western China, deep sequences and subtle traps in eastern Chinese basins, and the offshore area of Bohai Bay. It is estimated that the increasing trend of oil reserves seen during past 5 years will continue through 2015. Oil production in China will keep growing over the next 15 years, and gas reserves are expected to expand rapidly. Domestic production of oil will meet over 60% of domestic demand in China, and gas production and supply will fully meet domestic demand over the next 15 years. Introduction China is an oil-productive country. The annual oil production of China was 1.6x108 tons in 2000, which ranks fifth in the world. Meanwhile, China is also an oil consuming country, consuming 2.1x108 tons of oil in 2000, of which about 6.0x107 tons were imported from other countries. When analyzing the oil-gas supply-demand situation in China for the future 5~10 years is helpful to know the Asian-Pacific petroleum supply-demand trends for the next 10 years. It is predicted that future oil-gas demand in China will be further increased. Based on predictions and analysis of oil and gas resource potential, distribution and favorable exploration targets, various authors have gained detailed insight into the oil and gas supply and demand situation in China over the next 10 years. 1. From 1996 to 2000, Chinese petroleum exploration been significantly stepped up. To meet the New Advances in Oil-Gas new exploration challenges, comprehensive research methods and advanced exploration Exploration in technology have been deployed, resulting in the constant discovery of new large-scale oil-gas fields China such as PL19-3, QHD32-6, Kela-2, Ansai and so forth. Recoverable reserves in China have increased to about 8.0x108 tons of oil and 6.0x1011m3 of gas in the five years from 1996 to 2000, guaranteeing a sustainable increase of domestic oil-gas production. As of 2000, the cumulative
INTEGRATED EXPLORATION TECHNIQUES USED FOR TARIM BASIN, CHINA INTEGRATED EXPLORATION TECHNIQUES USED FOR AND HYDROCARBON DISCOVERIES FOUND IN THE COMPLEX HIGH STEEP STRUCTURES OF KUCHE MOUNTAIN AREA, TARIM BASIN, CHINA Zhao Jianzhang1, Chen Zuchuan1 Jia Chengzao2 and Qian Rongjun1, 1The BGP, CNPC, China; 2The Tarim Oilfield Corp., China Abstract. There are rich gas resources in the mountain area of Kuche, Xinjiang, where the combination of source rock-reservoir bed-cap rock is excellent. The seismic exploration there, however, is very difficult. After several years' effort, an adequate comprehensive exploration technique, suitable for petroleum exploration in mountain area, has been developed by integration of data acquisition, processing and interpretation. The technique includes: (1) the target-oriented acquisition technique in areas with complex surface and subsurface conditions, (2) the precise target-oriented processing technique, (3) the pre-stacked time and depth migration technique, (4) the technique for constructing the precise velocity field of the target zone, and (5) model establishment and the industry mapping technique for constructing the complex structural traps with steep strata. With application of these new seismic techniques, deep traps can precisely be described. Meanwhile, a set of drilling technique for steep dip layers and high-pressure gypsiferous mudstones, logging technique for complex well bore and test technique for high-pressure well in mountain front area has been developed and completed. By use of these techniques, a great achievement of natural gas exploration in Kuche mountain area has been achieved recent three years. fields were discovered. Among them, Kelaka-
2, Yinan-2, Dabei-1 and Tuzi-1 are large-size gasfields and each has gas reserves more than Kuche mountain area is located in the 100 billion m3. The amount of the gases found northern part of Tarim basin, and is an so far shows that the Kuche mountain area is exploration block with Mesozoic hydrocarbon of great exploration potential (Table 1) source rocks. Since shallow oils were found in outcrops in 1958, petroleum exploration in this THE DIFFICULTIES AND TECHNIQUES area has not been conducted almost for 30 OF SEISMIC EXPLORATION IN KUCHE years. The main reasons caused that are as the MOUNTAIN AREA AND THE following. Firstly, the serious offset exists for EXPLORATION EFFECTS upper layer structural high position with lower layer one of a slip structure and in this case the DIFFICULTIES OF SEISMIC deep layer structure high position and shape EXPLORATION can not be reflected by the shallow layer Kuche is a mountainous area with a structure. Secondly, the steep surface number of gullies of different directions. The elongated anticlines with the complex elevation difference of the earth surface is 300- subsurface multiple structures form many 1000 m, and the mountains are mainly in blind areas for seismic survey. From the zigzag or of sin