Cavernous Reservoirs Research Articles

Oil-and-gas potential of the northeast Predpatom trough

This article discusses the geological structure and prospects of the oil and gas potential in the northeastern part of the Predpatom trough. The materials of the study included geological findings from deep drilling and data from direct geochemical studies conducted in recent years. It describes the specific features of the structural plans of the upper versus the lower part of the geological cross-section. It is assumed that deep drilling in the area of the Ulugur structure has not reached the target productive horizons in the autochthonous part of the section. After acquiring more recent seismic data, there is a proposal to drill a deeper exploratory well in the area. As another priority target for oil and gas exploration, a potential zone of Vendian clastic deposits wedging on the southwestern slope of the Suntar uplift has been identified. A forecast has been made for the distribution of lithological traps stacked with well-sorted granular Vendian reservoirs in the northern parts of the Ergedzheysky and Ulugursky license areas. In the allochthonous part of the section, the potential for oil and gas content in carbonate deposits from the Vendian and Lower Cambrian periods is highlighted in reservoirs formed by secondary processes. It is assumed that thrust deformations could lead to the circulation of aggressive solutions, resulting in the formation of secondary fractured cavernous reservoirs. Subsequently, hydrocarbon accumulations may migrate and accumulate within these reservoirs. Good gas prospects were also noted in the upper part of the section, where gas deposits were confirmed in newly discovered fields. The results of direct geochemical testing of the Ulugur and Ergedzheysky license areas are presented, enabling the preliminary delineation of promising zones and areas in the studied territory. At the same time, narrow elongated geochemical anomalies are associated with the prospects of the allochthonous part of the section, while isometric anomalies are linked to the autochthonous part of the section. The need to consolidate all available geological and geophysical information on the Pre-Atomic deflection within the framework of case studies is noted.

Investigation on Water Invasion Mode and Remaining Oil Utilization Rules of Fractured-Vuggy Reservoirs: A Case Study of the Intersection Region of S99 Unit in Tahe Oilfield

Fractured-vuggy reservoirs are a new target in carbonate oil and gas exploration and development. Because of well-developed reservoir bodies, including fractures and caverns, bottom water invasion can be observed in oilfield development, with low utilization efficiency of crude oil in the reservoir. Accordingly, this study focused on the intersection region of the S99 unit of the Tahe fractured-vuggy reservoirs. Based on seismic data, the reservoir bodies can be divided into three types—caverns, fractures, and broken solution pores. Using the same location condition assignment algorithm, four single-type models are fused into a multi-scale discrete three-dimensional geological model of fractured and cavernous reservoirs, and the corresponding fractured-vuggy reservoir model was established for numerical simulation. The single-well historical fitting precision exceeded 85%. Furthermore, the development can be divided into four stages—initial stage of production, peak production stage, liquid control and oil stabilization stage, and scale gas injection stable. Streamlining sweep analysis determined the utilization and distribution characteristics of the remaining oil in the reservoir. It can be concluded that structure, caverns, and fractures were the main controlling factors affecting the remaining oil distribution in the fractured-vuggy reservoir. The fluid exchange among single-well reserve zones was calculated using streamline-based quantitative sweep analysis and interwell flow quantitative analysis method. Through source-sink quantitative analysis, interwell flow relations were derived, and three water breakthrough modes were further concluded: violent flooding, slow ascending of water cut, and low cut or intermittent water production.

The combining different-scale studies in a reservoir simulation model of a deposit with a fractured-cavernous type of carbonate reservoir

When constructing reservoir simulation models of complex reservoirs, it is necessary to take into account the zonal heterogeneity of the filtration and reservoir properties and the influence of secondary processes on the filtration characteristics of the deposit. The underestimation of complex mining and geological conditions negatively affects the forecast for technological and economic indicators of the field development. The paper presents an approach to creating a reservoir simulation model of a complex reservoir based on the integration of different-scale research that allow us to judge the alternation of different types of reservoirs along the lateral and vertical lines. As a result, a refined reservoir permeability cube is constructed which allows us to take into account the influence of the fracturing and cavernous reservoir on the technological indicators of development.

ИЗУЧЕНИЕ ЗОН АНОМАЛЬНОГО ПЛАСТОВОГО ДАВЛЕНИЯ С ПОМОЩЬЮ АНАЛИЗА АТРИБУТОВ СЕЙСМИЧЕСКИХ ПОЛЕЙ НА ПРИМЕРЕ МЕСТОРОЖДЕНИЙ ЗАПАДНОЙ СИБИРИ

Бурение глубоких скважин во многих районах мира, особенно в трещинно-кавернозных коллекторах сталкивается с рядом проблем, часть из которых имеет геологическую природу, а часть – техническую. В частности, осложнения могут вызывать аномально высокие пластовые давления. В настоящее время зафиксирован рост числа скважин (в том числе и аварийных) с установленным аномально высоким пластовым давлением. По этой причине актуальность исследований в области прогноза зон с аномально высоким пластовым давлением, в частности для предотвращения аварийных ситуаций, возросла. Повышенное внимание к изучению зон аномально высокого пластового давленияи формированию механизма их прогноза начало уделяться в последней трети 20-го столетия. Изучалась геологическая природа и механизмы возникновения этих зон, пространственные закономерности их локализации. Кроме того, были предложены способы прогноза их появления, которые можно было бы использовать при проектировании скважин и в процессе бурения. Авторами приведён анализ известной геологической информации о формировании и пространственной приуроченности зон аномально высокого пластового давления.Кроме того,показано, что с зонами аномально высоких пластовых давлений могут быть связаны значительные изменениясейсмических атрибутов. Обобщенный анализ приведённых данных позволяет нам сказать, что зоны аномально высокого пластового давления достаточно широко распространены в осадочном чехле, в том числе и на участке нашего исследования. Существующая проблема связи аномально высокого пластового давления с сейсмическими атрибутами далека от окончательного решения. Правильный прогноз оценки пластовых давлений дает возможность вести бурение в оптимальных режимах. Задача прогнозирования зон аномально высокого пластового давления, дренирующих толщи коллекторов на этом месторождении насущна и актуальна, поскольку в настоящее время его разбуривание продолжается по сети эксплуатационных скважин Цель: использование сейсмические атрибуты для контроля и уточнения динамической модели резервуара;выявление зон аномально высокого пластового давленияв разрезах месторождения углеводородов по данным сейсморазведки (сейсмическим атрибутам). Объекты: породы со сложным строением пустотного пространства, обусловленным повышенными трещиноватостью и кавернозностью, которые образовались за счет тектонических дислокаций в осадочном чехле. Методы: теоретическое и экспериментально обоснование связи зон аномально высокого пластового давленияи сейсмических данных. Результаты. Были построены карты вероятностей распределения пластового давления по площади в межскважинном пространстве на изучаемое месторождение.

ПРОБЛЕМА ОБНАРУЖЕНИЯ НЕСТРУКТУРНЫХ НЕФТЕГАЗОНОСНЫХ ОБЪЕКТОВ ПО ДАННЫМ СЕЙСМОРАЗВЕДКИ

In sedimentary rocks, zones of excessive fissuring are often superimposed on porous and cavernous reservoir types, creating and complicating traps of non-structural hydrocarbons. Traps of this kind are hard to find and usually not detectable with standard CDP seismic survey methods. Non-standard approaches are needed in the implementation of their successful forecast. For this it is possible to use the properties of both scattered and specular reflected waves, on the basis of which a number of unique techniques have been created. In particular, these include the Prony filtration method, that allows for the frequency-dependent analysis of the wavefield, on the basis of which it is possible to successfully predict oil-and-gas bearing features of any complexity. The article provides an example of application of the Prony filtration method from the practical experience of the authors.

ИНФОРМАТИВНОСТЬ ГЕОМЕТРИЧЕСКИХ АТРИБУТОВ ДЛЯ ПРОГНОЗИРОВАНИЯ ТРЕЩИНОВАТОСТИ КОЛЛЕКТОРОВ НА ПРИМЕРЕ МЕСТОРОЖДЕНИЯ УГЛЕВОДОРОДОВ ТОМСКОЙ ОБЛАСТИ

В последние годы, в связи с истощением традиционных коллекторов углеводородов, значительно возрос интерес к коллекторам, проницаемость которых обусловлена главным образом степенью их трещиноватости. Перспективы картирования зон трещиноватости связывают с сейсморазведкой. По существу, она является единственным наземным геофизическим методом, который позволяет получить детальное трехмерное изображение геологической среды. Однако возможности сейсморазведки ограничены ее разрешающей способностью и строением интервалов коллекторов. Исходя из результатов выполненного нами ранее сравнительного анализа новейших методик и технологических приемов изучения трещинно-кавернозных коллекторов, мы протестировали все методы анализа сейсмических данных в той последовательности, в которой они излагаются в обзорах. Анализируемые методики ранжированы по широте использования и доказанной эффективности. По убывающей эффективности, методы изучения трещиноватости располагаются следующим образом: 1. Азимутальный анализ анизотропии с получением эффективной модели азимутальной анизотропии; 2. Анализ геометрических атрибутов; 3. Специальные технологии миграционных преобразований (дуплексная миграция и др.); 4. Выделение из сейсмической записи поля рассеянных волн. Так как проведенная на участке работ сейсморазведка является стандартной (не широкоазимутальной, не мультиазимутальной, не многоволновой), то для исследования трещиноватости из списка вышеперечисленных методов нам доступны методы анализа геометрических атрибутов. Необходимо отметить, что в этом случае прогноз трещиноватости мог быть ограничен выделением зон трещиноватости (качественный анализ). Это актуальность. Цель: изучение возможностей атрибутного анализа сейсмических данных для прогноза выделения тектонических нарушения и зон повышения трещин. Объекты: породы со сложным строением пустотного пространства интенсивной кавернозности и трещиноватости, за счет которых образовались тектонические дислокации в осадочном чехле. Методология. Был рассчитан ряд объемных сейсмических атрибутов, таких как атрибуты когерентности (дисперсии) и кривизны (и ее разные варианты), хаоса. Полученные результаты были визуализированы в программном интерфейсе Petrel® компании Schlumberger с помощью имеющихся 3D МОГТ сейсмических данных для исследования потенциальных структурных и стратиграфических контролей и выделения зон с интенсивностью развития трещин в пределах исследуемой области. Аналогичным образом для горизонтов были рассчитаны поверхностные атрибуты с целью создания карт атрибутов и получения полного понимания строения целевых объектов и признаков для выделения зон интенсивного развития трещин. Результаты. Проведен анализ геометрических атрибутов для выделения зон повышения трещин, также была рассмотрена калибровка сейсмических атрибутов по данным скважин (керна).

A physical modelling study of the reservoir scale effect on amplitude variation with incidence angle

Abstract The 3D pre-stack seismic data from a physical modeling experiment were employed to investigate the effect of reservoir scales on AVO (amplitude variation with offset or incidence angle). Eight cuboid samples simulating cavernous reservoirs with different widths and the same thickness and elastic parameters were set within a 3D model. 3D seismic data acquisition and processing were conducted. To get the AVO responses of the samples with different widths, trough amplitudes corresponding to the sample tops at different incidence angles were extracted from the pre-stack angle gathers. Amplitude calibration for transducer radiation patterns was conducted on the extracted amplitudes at different angles. AVO analysis was conducted to quantitatively demonstrate the effect of sample scales on AVO characteristics. The effect of sample width was weak when the width was less than 60 m, which was 3/5 of the wavelength. When the width was larger than 60 m, both AVO intercept and gradient gradually increased with the sample width. The AVO gradient peaked at 150 m, which was 1.5 times the wavelength. Cross-plot analysis of AVO intercept and gradient showed the samples were aligned in a straight line when the sample width was less than twice the seismic wavelength. The result in this study partially verified the conclusions of reservoir scale effect on AVO responses drawn from previous numerical modelling studies. For a heterogeneous rectangular reservoir, the effect of reservoir scales on AVO responses could potentially be used to quantitatively estimate reservoir scale.

Response simulation and corresponding analysis of dual laterolog in cavernous reservoirs

Abstract To figure out the response characteristics of dual laterolog logging in carbonate and volcanic cavernous reservoirs, the parameters of laboratory formation model and tool were optimized on the basis of numerical simulation, then, the experimental data was used to calibrate the numerical simulation results. Finally, the response characteristics of cavernous formations in the original formation condition were acquired. The results show that the radial radius and height were set to 1 and 2 meters, meanwhile the length of dual laterolog in vertical and radial direction were reduced to 1/20 and 1/6, which can satisfy the need of laboratory experiments and there is a good match between experimental data with numerical results under the same circumstance. The numerical simulation parameters calibrated by physical modeling experiment were used to find out the dual laterolog response characteristics of cavernous formations. The results show the apparent resistivity on the dual laterolog by the well bore is the lowest at the cave center, and the apparent resistivity could not accurately reflect the infill resistivity of the cave; the larger the radius of the cave, the lower of the infill resistivity and the smaller the distance between the cave and the well bore, the more sensitive dual laterolog will be to caves in formations. The distance between the cave boundary and well wall that could be recognized by deep laterolog is less than 0.5 m, as for shallow laterolog, the largest distance is 0.3 m.

High-density point-receiver seismic survey of a deep carbonate cave reservoir system: a case study from the Tarim Basin, Western China

In the northern rim of the Tarim Basin, western China, oil is being produced from an extensive system of interconnected caves, faults and fractures in Ordovician carbonates buried about 6000 to 7000 m below the surface. Although onshore, drilling costs are high due to the depth of the field, so, to produce the field economically, wells must be accurately located within the cavernous features to access as large a drainage area as possible. However, imaging fine details of the reservoir structures presents challenges due to a complex overburden. While the surface terrain and near-surface geology are relatively benign for land seismic operations, the subsurface exhibits large lateral velocity changes related to widely distributed Permian volcanic formations around 1000 and 1200 m above the target zone. These formations are considered to have led to errors in the imaging of the target layers in legacy datasets due to incorrect velocity modelling of the overburden. Improved imaging of detailed features of the deep cavernous carbonate reservoir beneath the complex overburden would require an accurate velocity model in addition to seismic data with both high temporal and spatial resolution. To determine whether new land seismic acquisition and processing technologies could provide a significant uplift in imaging quality, in 2013 PetroChina Tarim Oil Company (TOC) acquired a high-density full-azimuth 3D seismic survey using a point-source and point-receiver geometry. The dataset was recorded using an advanced high-channel count acquisition system and processed with high-end processing techniques designed to address the various challenges presented in the area. Three approaches were used over different depth ranges to develop an accurate velocity model for prestack depth migration (PSDM). Model velocities were constrained using available well data, providing confidence in the accuracy of the resulting image. Relative amplitude preserving (RAP) methods were used for noise attenuation and signal processing to maximise the reliability of subsequent inversion and interpretation work.

Numerical Simulation of Dual Laterolog in Cavernous Carbonatite Reservoir

Zhao, J.; Wang, D., and Gu, H., 2015. Numerical simulation of dual laterolog in cavernous carbonatite reservoir.The cavernous reservoir has become one of the main carbonate reservoirs for stable and increasing production. The degrees and the types of fillings materials are the key factors which affect the characteristics of accumulation and flowage. However, due to the complexity of fillings in the caves, the response laws of logging of the cavernous reservoir can not be understood clearly and its effective evaluation has been severely hampered. This paper uses for the first time numerical simulation to study the responses in the curve of dual lateral log of cavernous reservoir. By establishing a continuous equation of the dual lateral log and its boundary factors and using the electric field superimpose principle, a finite element method is used to study the cave size, different filling agents and degrees are simulated. The result indicates that the size, filling agents and degrees show a clear response, and they have a clear correspondence. Summarizing and analyzing the response laws on the dual lateral logging curves of the cavernous reservoir can provide theoretical evidence for the logging evaluation of the carbonate cavernous reservoir.

Accumulation process and model for the Ordovician buried hill reservoir in the western Lunnan area, the Tarim Basin

The Ordovician buried hill reservoir in the western Lunnan area, a type of dissolved fracture and cavernous reservoir, is mainly composed of heavy oil. The oil is the mixture sourcing from the Middle-Lower Cambrian and Middle-Lower Ordovician, with three stages of pool forming process: (1) the destruction and parallel migration/accumulation during the late Caledonian to early Hercynian; (2) the oil and gas accumulation during the late Hercynian characterized by adjustment upward along faults and parallel migration/accumulation; (3) the formation of heavy oil during the latest Hercynian. The Ordovician buried hill reservoir is affected by the diffusion of light oil and gas but had no hydrocarbon charging during the late Yanshan period to Himalayan period, but in this period, formed the association of heavy oil and dissolved gas cracked from crude oil with dry coefficient of 0.91–0.96. The study on accumulation process of the Ordovician buried hill reservoir has important implications for the exploration potential of early oil and gas accumulation in the cratonic area of the Tarim Basin.

Nature of the scattered seismic response from zones of random clusters of cavities and fractures in a massive rock

ABSTRACTThe scattering of elastic energy by random clusters of fractures and/or cavities in a massive rock is studied. The interpretation of the scattered seismic response reveals crucial information about the clusters of inhomogeneities (fractures/cavities), which may correspond to reservoirs. The study is based on a new two‐dimensional numerical‐modelling method that relaxes the constraints on the location and orientation of the inhomogeneities, accounts for inhomogeneities that have almost no volume but a finite surface area (fractures) and improves the accuracy of the calculation when the size of the inhomogeneities is comparable to the mesh size.It is shown that the nature of the seismic response of zones of diffuse fracturing and/or cavities is associated with the non‐uniformity of micro‐inhomogeneities in such zones; accumulations of these micro‐inhomogeneities are known as clusters.The relationship between the non‐uniformity of micro‐inhomogeneities and the strength of the seismic response has been established and measured.Considerable differences in the structure of the seismic response of zones of diffuse fracturing and diffuse cavities have been identified. Converted PS‐waves dominate in the scattered wavefield associated with fractures. This is explained, as the modelling results show, by a greater transparency of fluid‐filled fractures, which reduces the reflected energy of compressional waves. The wavefield associated with cavities is characterized by the predominance (in terms of strength) of compressional PP‐waves. The strength of converted PS‐waves in the scattered wavefields for both media is approximately the same.On the whole, according to the results of the modelling, the energy of the scattered response of fractured reservoirs is considerably less (about two times) than that of cavernous reservoirs.