Petroleum Exploration and Development

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Innovations and challenges of sedimentology in oil and gas exploration and development

Sun Longde; Fang Chaoliang; Li Feng; Zhu Rukai; Zhang Yunhui; Yuan Xuanjun; Jia Ailin; Gao Xingjun and Su Ling

. 2015, 42(2): 129-136.

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Based on the development of sedimentology and its application effects in practice, research demands on the sedimentary reservoirs in the oil and gas exploration and development in the future are summarized, and key research fields and directions of the future are put forward as well. Recently, sedimentology in China has achieved a number of accomplishments: Lithofacies paleogeography of depositional basins during significant tectonic movements and a new model of sandbody in shallow water delta have been built, expanding the fields of oil and gas exploration. New knowledge of deep-water sedimentary sandbody distribution has sustained significant discoveries in (ultra-)deep water area and lake basin center. New cognition of (ultra-)deep reservoir mechanism extends the depth of oil and gas exploration and development. New progress of development pattern of organic-rich shale and study of unconventional reservoirs make a series of major breakthroughs in unconventional oil and gas exploration and development. Multi-scale development geological modeling predicts effectively the distribution of remaining oil. New techniques and methods of sedimentary reservoirs provide a foundation for the development of the sedimentology theory and its industrial application. In the future development of sedimentology, it is necessary to make progress in traditional sedimentology, to innovate fine-grained sedimentology and unconventional reservoir geology and to provide a foundation for sedimentary prototype basin restoration, organic-rich shale area evaluation, favorable reservoir prediction and favorable target area optimization, promoting the continuous innovations on sedimentology.

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Accumulation conditions and exploration directions of natural gas in deep subsalt Sinian-Cambrian System in the eastern Sichuan Basin, SW China

Gu Zhidong; Yin Jifeng; Yuan Miao; Bo Dongmei; Liang Dongxing; Zhang Hang and Zhang Li

. 2015, 42(2): 137-149.

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Based on comprehensive analysis of outcrops and drilling data, and interpretation of more than 6×104 km2 seismic data, this paper delineates the regional distribution of the gypsum-salt layers in Middle-Lower Cambrian in eastern Sichuan Basin, and discusses accumulation conditions for natural gas in deep subsalt Sinian-Cambrian System in eastern Sichuan Basin, and points out favorable exploration directions. There are two sets of regional gypsum-salt layers in the Middle-Lower Cambrian in eastern Sichuan Basin, which are regional detachment layers, with fault-related folds occurring above and broad anticlines occurring below. There are 6-7 rows of NNE and NE trending large traps developed under the salt. Good space configuration of source rocks, reservoirs and cap rocks provide favorable accumulation conditions for natural gas in the deep subsalt in this region: three sets of hydrocarbon source rocks including Sinian Doushantuo Formation, the third Member of Sinian Dengying Formation, and dominant Cambrian Shuijingtuo Formation; three sets of weathering crust karst reservoirs and grain beach reservoirs including the second and fourth Members of Sinian Dengying Formation, and Cambrian Shilongdong Formation; and the regional cap of gypsum-salt in the Middle-Lower Cambrian and shale in Shuijingtuo Formation. The top target in deep subsalt exploration in eastern Sichuan Basin should be the fourth Member of Dengying Formation, followed by Shilongdong Formation and the second Member of Dengying Formation, and areas on migration direction with appropriate burial depth and being far away from the edge of the basin should be the first choices in exploration. Therefore, the favorable hydrocarbon exploration areas in the near future are the Honghuadian-North Huaying Mountain, Liangshuijing-Pubao Mountain-Leiyinpu, and Datianchi structural belts.

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Prediction of gas production in carbonates based on CT analysis and nuclear magnetic resonance (NMR) logging

Li Ning; Wang Kewen; Zhang Gong; Wu Hongliang; Zhao Luzi; Wu Lihong; Feng Qingfu and Feng Zhou

. 2015, 42(2): 150-157.

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Based on the analysis of 3-D CT data of full diameter carbonate cores and gas-test results of the coring formations, it is found there exists a quantitative relation between daily gas production per meter in effective intervals and CT70 porosity (according to the CT resolution of full diameter core, CT70 porosity is defined as the percentage of the volume of pores whose diameter is larger than 70 μm in the core), and a new method of gas production prediction in carbonates is put forward using CT70 porosity. Firstly, the exponential relationship between gas production and CT70 porosity is proved by effective medium approximate theory (EMA) in dual media and planar radial flow formula. Then the physical significance and varying pattern of each parameter in prediction formula are determined through theoretical analysis and calculation. The method of proportional transformation between CT and NMR data is put forward, therefore the application of gas production prediction based on NMR logging can be realized. The method has been used for the gas production prediction of 16 wells of the key exploration area in Sichuan Basin. The gas-test results prove the correctness and practicability of the method presented in this paper.

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Genesis and evolution of Lower Cambrian Longwangmiao Formation reservoirs, Sichuan Basin, SW China

Zhou Jingao; Xu Chunchun; Yao Genshun; Yang Guang; Zhang Jianyong; Hao Yi; Wang Fang; Pan Liyin; Gu Mingfeng and Li Wenzheng

. 2015, 42(2): 158-166.

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Based on observation of outcrops, cores and thin sections and analysis of logging data and experiment, the features, main controlling factors, evolution and distribution of the Longwangmiao Formation reservoirs in the Lower Cambrian, Sichuan Basin, are examined carefully and the distribution of favorable reservoirs is predicted. Mostly fracture-pore type, the Longwangmiao Formation reservoirs are dominantly comprised of residual dolarenite, oolitic dolomite and crystal dolomite, with dissolution cavities and dissolution pores as the main storage space, an average porosity of 4.28%, and average reservoir thickness of 36 m. The formation of the reservoirs is controlled by grain shoal facies, parasyngenetic dissolution and parasyngenetic dolomitization. The reservoirs have experienced four evolution stages, the period of pore formation laid the foundation for storage space types and physical conditions of the reservoirs; the reservoir physical properties were improved further in the hypergene karstification period; the porosities were decreased by minerals filling during the hydrothermal period; and the reservoirs became denser in the burial dissolution and asphaltic filling period. Based on the main controlling factors of Longwangmiao reservoirs, the high geomorphology area between Huayingshan fault and Longquanshan fault is predicted as the most favorable reservoir zone. Exploration breakthroughs will possibly be made in Guang’an-Nanchong-Jiange area.

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Characteristics of lacustrine shale porosity evolution, Triassic Chang 7 Member, Ordos Basin, NW China

Wu Songtao; Zhu Rukai; Cui Jinggang; Cui Jingwei; Bai Bin; Zhang Xiangxiang; Jin Xu; Zhu Desheng; You Jianchang and Li Xiaohong

. 2015, 42(2): 167-176.

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With low mature Triassic Chang 7 Member shale samples from the Ordos Basin as study object, the 3-D porosity evolution with temperature increase and its main controlling factors are analyzed based on the physical modeling under high temperature & pressure and nano-CT scanning data. More and more nano-pores were developed in Chang 7 Member organic-rich shale with the increase of maturity. The porosity calculated from the nano-CT scanning model increased from 0.56% to 2.06%, more than 250% times larger, when temperature increased from 20 ℃ to 550 ℃. The process of porosity evolution can be divided into three phases. Firstly, porosity decreased rapidly from immature to low mature stage because of weak hydrocarbon generation and strong compaction; Secondly, porosity increased rapidly when the maturity increased from low mature stage to mature and post-mature stage, organic matter cracked into hydrocarbon (HC) massively, and clay minerals transformed intensively; Thirdly, porosity system kept stable when the shale entered into post-mature stage and the intensity of both HC generation and clay mineral transformation decreased. Organic matter thermal evolution, clay mineral transformation and brittle mineral transformation make different contribution to the porosity of shale, and the ratio is 6:3:1 respectively. It is inferred abundant organic matter pores occur when Ro is over 1.2%.

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Fracture characteristics under the coupling effect of tectonic stress and fluid pressure: A case study of the fractured shale oil reservoir in Liutun subsag, Dongpu Sag, Bohai Bay Basin, Eastern China

Luo Yang; Zhao Yanchao; Chen Honghan and Su Hui

. 2015, 42(2): 177-185.

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Based on the integrated study on core description, fluid inclusions and basin modeling, and considering the coupling effect of tectonic stress and fluid pressure, the characteristics of fractures were analyzed in the inter-salt fractured shale oil reservoirs of strong overpressure in upper Sha 3 member (Es3) in Liutun subsag, Dongpu Sag, Bohai Bay Basin. The results show that: (1) Most of fractures in this reservoir are structural or hydraulic, with scattered occurrence, even horizontal hydraulic fractures in strong overpressure area; (2) Overpressure changes the differential stress and principal stress direction by changing the poroelastic response of rock and the seepage force of pore fluid, and thus influencing the type and occurrence of fractures; (3) Structural and hydraulic fractures can be divided into seven types according to their stress states. The development of those fractures includes 3 stages. The transverse tension and longitudinal tension fractures are mainly developed in the first stage, tectonic shear fractures mainly in the second and third stages, and hydraulic fractures mainly in the early phase of the second stage before tectonic uplift. When tectonic stress and overpressure are both strong, the type and occurrence of fractures are diverse.

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Controlling factors of Jurassic Da’anzhai Member tight oil accumulation and high production in central Sichuan Basin, SW China

Chen Shijia; Zhang Huanxu; Lu Jungang; Yang Yueming; Liu Chaowei; Wang Li; Zou Xianli; Yang Jiajing; Tang Haiping; Yao Yitong; Huang Youlin; Ni Shuai and Chen Yingying

. 2015, 42(2): 186-193.

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Based on observation of cores and casting thin sections, reservoir fluorescence, hydrocarbon geochemical characteristics and oil production test data, the factors controlling the tight oil accumulation and high production in the Da’anzhai Member in the Jurassic Ziliujing Formation in central Sichuan Basin are studied. The shelly limestone, muddy shell limestone and tight limestone are all oil-bearing in the Da’anzhai Member. The dissolved pores and fractures in shelly limestone and the matrix pores (microfractures, intercrystal pores) in tight limestone are all oil-bearing in large area, which is the reason why oil wells can keep low production for a long period of time. Owing to the extensive oil-bearing, natural gas from the Upper Triassic Xujiahe Formation provided the major power for oil migration and accumulation in the Da’anzhai low amplitude, water-free tight limestone reservoirs, driving the dispersed oil from the matrix pores into an accumulation, so the oil is more enriched in the western structures of central Sichuan Basin where oil wells are higher in gas-oil ratio. In contrast, in the east of central Sichuan, the Huayingshan major faults generally cut through Da’anzhai Member, allowing gas to migrate to the formations above Da’anzhai Member along fractures to accumulate or dissipate, without gas as driving force, as a result, the oil is less enriched than that in the west part of central Sichuan.

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Compound hydrocarbon accumulation and enrichment in southwestern Huizhou area, Pearl River Mouth Basin, Southern China

Chen Weitao; Du Jiayuan; Shi Hesheng and He Min

. 2015, 42(2): 194-199.

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Aimed at four major issues in the oil and gas exploration of the southwestern Huizhou area in Pearl River Mouth Basin, in-depth study on geologic features and oil and gas reservoir forming pattern has been carried out to find out exploration potential and direction of compound oil and gas reservoirs in this area. There develops the compound formation structure of basement, rifting and depression layers in this area, and a composite hydrocarbon transport system comprised by faults, sandstone layers and the sequence boundaries, which are connected to the several hydrocarbon generation subsags in the adjacent Huixi half graben. Controlled by the subsag-controlling boundary faults, long inherited paleo-uplifts and the ancient structural ridges, oil and gas accumulate in multiple series in vertical direction, multiple connected traps horizontally, forming compound reservoirs of various traps. According to the three types of structural zones controlling oil and gas enrichment, the southwestern Huizhou area can be divided into seven compound oil and gas accumulation zones in three categories, which combine into a large compound hydrocarbon accumulation province.

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Effect of bioturbation on reservoir rock quality of sandstones: A case from the Baram Delta, offshore Sarawak, Malaysia

Ben-Awuah Joel and Padmanabhan Eswaran

. 2015, 42(2): 200-208.

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With the Baram Delta in Malaysia as research subject, the effect of bioturbation on porosity and permeability of reservoir sandstones is evaluated and analyzed based on core and thin section analysis, EDX (energy-dispersive X-ray), FESEM (Field Emission Scanning Electron Microscope), mercury porosimetry and spot permeametry measurement. Samples are from cored intervals of two wells in the Baram Delta, W-1 and W-6. Analysis results indicate that the cored intervals in well W-1 are dominated by Diplocraterion ichnofabrics, intensely bioturbated, sediment packing activity is observed, and fine grade materials (clays and organic matter) from the host sediment are incorporated into burrow fills and linings, thereby decreasing isotropy and sorting of the sediments and reducing the local porosity and permeability in the burrows. The cored intervals in well W-6 are dominated by Ophiomorpha ichnofabrics, highly to intensely bioturbated, clays and mud are cleaned from the burrow fill and host sediment and concentrated in the burrow linings, sediment cleaning activity by burrowing organisms results in cleaner and well sorted burrow fill materials, thereby increasing the local porosity and permeability in the burrows. Porosity and permeability of reservoir rocks in the Baram Delta have therefore either been enhanced or reduced by bioturbation depending on the type of burrow, fill material and burrowing activity.

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Application and enlightenment of carbon dioxide flooding in the United States of America

Qin Jishun; Han Haishui and Liu Xiaolei

. 2015, 42(2): 209-216.

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The application and characteristics of CO2 flooding in the USA were analyzed and summarized. Complementary techniques of CO2 flooding were generalized by an application example, and then the enlightenment was expounded for CO2 flooding in China. The development process and forming reasons of CO2 flooding technology were analyzed and summarized based on the systematic tracking of EOR survey data all over the world and the sufficient investigation of CO2-EOR technology application. The current situation of American CO2-EOR technology was evaluated using the quantity, scale and EOR production of projects as indexes. The characteristics and development-driving force of the projects were summed up. The characteristics of American CO2 miscible flooding were emphatically outlined about reservoir properties, crude oil properties and project timings. Meanwhile, the scale and reservoir adaptability differences between American CO2 miscible and immiscible flooding were comparatively analyzed. A series of complementary techniques were illuminated with the development of SACROC CO2-EOR project as an example. The challenges, technical bottlenecks and suggestions were analyzed and proposed for the promotion of CO2 miscible flooding technology in China.

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Development modes of Triassic Yanchang Formation Chang 7 Member tight oil in Ordos Basin, NW China

Li Zhongxing; Qu Xuefeng; Liu Wantao; Lei Qihong; Sun Hualing and He Youan

. 2015, 42(2): 217-221.

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The Triassic Yanchang Formation Chang7 Member tight oil reservoir in Ordos Basin, featuring complex pore-throat structures, low porosity, low permeability, rich micro-fractures and low pressure coefficient, is difficult to produce by advanced water-flooding from cluster vertical wells with low individual-well producing rate. With Block A as an example, the material balance calculation, numerical simulation and field practical analysis showed that the horizontal well production would increase significantly using volumetric fracturing. But the well pattern of water injection in vertical wells and oil production in horizontal wells has high risk of injection water breakthrough in horizontal wells, and the proportion of water breakthrough wells reached 65%. In contrast, the formation energy decreased in depletion development with horizontal wells, in which the oil production kept stable in early period, but decreased quickly after 12 months, the cumulative decline of oil rate in 4 months amounted to 50.3%. Therefore, a development scheme of depletion production from volumetric fracturing horizontal wells at first and then water-flooding huff and puff after producing energy is deficient was proposed. Following this scheme, the daily oil rate of single well increased by 78.3% after the first cycle of water huff-and-puff than before the treatment, marking the initial success of the measure.

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Dynamic fractures are an emerging new development geological attribute in water-flooding development of ultra-low permeability reservoirs

Wang Youjing; Song Xinmin; Tian Changbing; Shi Chengfang; Li Jiahong; Hui Gang; Hou Jianfeng; Gao Chunning; Wang Xiaojun and Liu Ping

. 2015, 42(2): 222-228.

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Based on the dynamic characteristics of water-flooding in ultra-low permeability oilfields, the concept of dynamic fracture is proposed and its formation mechanism and impacts on oilfield development are discussed. Dynamic fractures refer to the new-generated fracture channels when bottom hole pressure exceeds rock breakdown pressure and propagation pressure, or the effective fracture channels generated when the originally closed or filled natural fractures are reactivated, due to high pressure in near wellbore area of injection well in a long term water injection of the ultra-low permeability reservoir. Dynamic fractures, controlled by the current stress field, constantly extend toward the direction of the oil well with the rise of the injected water volume and bottom hole pressure, until connecting with the oil well hydraulic fractures. The analysis of the sealed coring well groups L76-60 and L88-40 located in the same block in the JA Oilfield, Changqing Oilfield, NW China shows that the dynamic fractures changed the seepage characteristics of water displacing oil in the ultra-low permeability reservoir, greatly aggravated the reservoir heterogeneity, which led to the reduction of profile producing degree and the distribution of remaining oil on both sides of the fractures in continuous or discontinuous belts. The generation and propagation of dynamic fractures are closely related to water injection pressure, injection-production ratio, oil and water well stimulation measures. When the porosity-permeability condition of the matrix allows effective oil displacement, high pressure water injection or oil and water well stimulation measures should be avoided as far as possible.

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Application of the transient rate decline analysis for determining average reservoir pressure in naturally fractured reservoirs

Daryasafar Amin; Amareh Iman; Fathi Nasab Mohammad; Kharrat Riyaz and Jalilian Milad

. 2015, 42(2): 229-232.

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Based on the fracture-pore dual medium model proposed by Warren and Root et al, the solution of the pseudosteady-constant production fractured system by Mavor and Cinco-Ley and the solution of the pseudosteady-constant pressure matrix system by Daprat are extended, and a new method is presented for estimating average reservoir pressure of naturally fractured reservoirs using the transient rate decline analysis. The specific method is taking logarithm of the matrix production equation and fracture production equation to obtain two straight lines; then the slope and intercept of the straight lines are used to determine storage capacity ratio, total storage capacity and reservoir drainage area, and average reservoir pressure at last. Examples show that the new method is easy and practical, and gives result similar to that of the material balance method and direct Tiab synthesis method.

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Sliding risk of jack-up platform re-installation close to existing footprint and its countermeasure

Mao Dongfeng; Zhang Minghui; Zhang Laibin; Duan Menglan and Song Linsong

. 2015, 42(2): 233-237.

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Analysis on the sliding risk of jack-up platform re-installation close to the existing footprint is conducted with finite element method. An effective countermeasure to reduce the sliding risk is suggested and an example analysis for an existed jack-up platform re-installation in a typical block with two layer soil within design driving depth in China sea area is carried out. Taking into consideration fluid seepage in pore structure, sliding contact friction, and the material and geometric nonlinear properties, a fluid-solid coupling finite element model for the spudcan-soil interaction is constructed. By comparing the numerical simulation result with the experimental result published in the literature, the reliability of the finite element model is verified. With the model, the failure process, the movement pattern of foundation and the spudcan-soil interaction under the impact of the footprint are analyzed. The study shows: in pitching pile the plastic failure zone appears at the low corner close to the spudcan first, then the area beneath the spudcan, and the plastic area becomes larger to form a connecting region finally as the loading increases continuously; the migration pattern of soil around the spudcan changes sharply with the distance between the spudcan and the footprint increasing; at the same time the peak value of the horizontal sliding force increases first then decreases gradually. In the final pitching pile position, ‘stomping’ in advance in appropriate distance and depth can reduce the sliding risk of a jack-up platform re-installation effectively.

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Evaluation of horizontal wellbore single-phase pressure drop models based on large-scale experiment

Wang Zhiming; Yang Jiankang; Zhang Quan; Wang Xiaoqiu; Gao Hong; Zeng Quanshu and Zhao Yanlong

. 2015, 42(2): 238-241.

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The prediction accuracy of the five horizontal wellbore single-phase drop models (Siwon, Asheim, Su, Yuan, Ouyang) is evaluated and compared based on the high-quality experimental data measured by a self-developed experimental system with large size horizontal wellbore (ID: 139.7 mm). The results show that the pressure drop prediction accuracy of the five models is, from high to low, Ouyang, Siwon, Asheim, Yuan, and Su. Generally, the Su and Yuan models have high predictions and the Siwon and Asheim model have low predictions. The Ouyang model has good prediction performance, its average relative error is only 5.5%, and the example also proves its good prediction effect, but it can’t represent complex mechanisms such as the resistance effect caused by wall perforation. The models for predicting the pressure drop of single-phase flow in a horizontal wellbore need to be further improved to completely represent complex mechanisms of the variable mass flow in the horizontal wellbore, such as the resistance effect caused by wall perforation and lubrication flow drag reduction effect caused by wall influx.

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Macro-fracture mode and micro-fracture mechanism of shale

Zhong Jianhua; Liu Shengxin; Ma Yinsheng; Yin Chengming; Liu Chenglin; Li Zongxing; Liu Xuan and Li Yong

. 2015, 42(2): 242-250.

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Through the study of fabric characteristics, mechanical characteristics, crack and microstructure characteristics of shale, the micro-mechanism, controlling factors and fracture modes of shale macro-fracture are revealed. The macro-fracture of shale is affected by fabric characteristics, coring direction, confining pressure etc, and occurs when the pores and micro-cracks extend, merge and link up. Macroscopic failure modes include shear fracture, tensile fracture and slip rupture. Microscopically, the micro-crack failure mainly includes shear slip type (type II) and tensile type (type I). In essence, the shear rupture and slip fracture of shale are all caused by shear slip rupture of micro-cracks, but due to different shale structure, the failure takes on different modes. The tensile fracture is caused by the tensile extension of micro-cracks. In addition, during the macro-fracture, the shear and tensile fracture of micro-cracks co-exist, but make different contribution to the macro-fracture. Finally, shale fracture patterns, namely shear, tensile rupture and slip burst are set up, and the microscopic mechanism of macro-fracture is explained from the microscopic viewpoint.

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A reserve calculation method for fracture-cavity carbonate reservoirs in Tarim Basin, NW China

Zhao Kuanzhi; Zhang Lijuan; Zheng Duoming; Sun Chonghao and Dang Qingning

. 2015, 42(2): 251-256.

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To avoid big deviation in reserve estimation and make scientific and reasonable reserves to guide the exploration and development of fracture-cavity carbonate reservoirs, a new three-dimensional reservoir space description and reserve calculation method for layered, fracture-cavity carbonate reservoirs is used to calculate the reserves of X block in the Halahatang oilfield in the Tarim Basin. In the method, the reserve calculation parameters are worked out by quantitative spatial delineation based on high precision three-dimensional seismic data; the high confidence well-seismic inversion and sensitive attributes and threshold value are used jointly to delineate the oil-bearing area, effective thickness and effective porosity of the cavity, vug and fracture reservoirs. The reserves are calculated with the volumetric method, and the dynamic recovery of different types of reservoirs is calibrated by actual drilling data. This method is suitable for reserve estimation and development program making of fracture-cavity carbonate reservoirs with strong heterogeneity.

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23 April 2015, Volume 42 Issue 2