Dong, Jiaxin (Research Institute of Petroleum Exploration & Development) | Ran, Qiquan (Research Institute of Petroleum Exploration & Development) | Peng, Hui (Research Institute of Petroleum Exploration & Development) | Wang, Zhiping (Research Institute of Petroleum Exploration & Development) | Xu, Mengya (Research Institute of Petroleum Exploration & Development) | Wang, Cong (Research Institute of Petroleum Exploration & Development)
The Fluid flow mechanism in unconventional reservoirs is much more complicated than that in conventional reservoirs mainly because of the coexistence of various sized micro-nano pores and natural / artificial fractures in the same reservoir. Due to the various flow patterns and seepage mechanism among differentsized media, and also among different production periods in the same medium, the dynamic production is influenced by both the spatial variable and time variable. The capacity of the tight gas reservoirs is also thus greatly influenced. In this paper, numerical simulation is used to automatically identify the flow pattern according to the critical criteria of flow in different media, and to simulate the dynamic production process with kinetic equations. The simulation is mainly focused on the following characteristics shown in the production process of tight gas reservoirs: I) high-velocity nonlinear, quasi-linear and low-velocity nonlinear flow regimes; II) the slippage effect and diffusion effect under low porosity, low pressure and low permeability conditions; and III) startup pressure gradient, desorption effect and other seepage mechanism.
Li, Ning (RIPED of PetroChina) | Ran, Qiquan (RIPED of PetroChina) | Li, Jiangfang (RIPED of PetroChina) | Yuan, Jiangru (RIPED of PetroChina) | Wang, Cong (Colorado School of Mines) | Wu, Yu-Shu (Colorado School of Mines)
Gas production from unconventional shale gas reservoirs is controlled by multi-scaled fractures, i.e., hydraulic fractures, natural fracture, micro fractures, etc., as flow pathways. On the other hand, shale matrix provides the majority of shale gas storage of both free and adsorbed gas. In addition, field data show that there exists significant amount of micro fractures within low-permeable shale matrix and these micro fractures may connect to large, global-connected fracture network. However, these micro fractures have been either ignored or lumped into the "matrix block?? in the current simulation models. In this paper, we present a physically based multiple-continuum concept to include both natural and micro fractures as well as low-permeability shale matrix. The multiple-continuum conceptual model implemented considers shale gas rock consisting of (1) globally connected natural fractures, (2) micro fractures locally connected between matrix and/or kerogen to global fractures, and (3) low-permeability shale rock matrix. Similar to the classic double-porosity concept, the global fracture continuum is responsible for global gas flow to hydraulic fractures or wells, while low-permeability shale matrix, providing main storage space, is locally connected to micro-fractures and interacting with globally connecting fractures. In addition, our model formulation also includes the following processes: (1) nonlinear adsorption/desorption effect, (2) Klinkenberg effect, (3) non-Darcy flow (at high flow rate and low flow rate), and (4) pressure-sensitive rock deformation. We use a hybrid modeling approach to describe different types of fractures, including hydraulic fractures, natural fracture network, and micro fractures. We will demonstrate model application to quantify flow behavior in fractured shale gas reservoirs.
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.
Natural gas of volcanic gas reservoirs in China is rich in resources, and its exploration and development is great potential, but the study of the volcanic gas reservoir is a brand new field, there are a lot of difficulties in this study field. Due to the key problem of describing the formation framework of the volcanic gas reservoir hierarchically, this paper break the study method of sedimentary reservoir, take XS 21 block volcanic gas reservoir in XS Gas Field, north Songliao Basin as the example, use outcrop??core?? logging??seismic data and so on, according to the idea of "the control of magma source??, present the "Volcanic edifice -
volcanic facies- effective accumulating and filtrating body?? volcanic formation hierarchical describing plan. Stressly, illustrate the Volcanic edifice identify and classify idea of confirm magma source-confirm surface-confirm body, and expound the concrete way to confirm volcanic crater by seismic section??seismic attribution and microtectonics, confirm surface of the volcanic edifice by outcrop??logging and seismic data, confirm body by 3D seismics. Though this idea and way, we identify and dissect the volcanic edifices in XS 21 block, and describe them quantitatively. This study is the base of the volcanic reservoir description and prediction, at the same time, it create a new way to study the volcanic reservoir more scientific than before.
Quite a few large-scale volcanic gas reservoirs have been discovered in deep zone in Songliao Basin of North-East China in recent years. The consequent issue is how to produce this kind of gas reservoir effectively. Reservoir characterization is essential for the effective production of volcanic gas reservoir, which provides the geological basis for gas reservoir development. The geological scenarios of volcanic gas reservoirs are extremely complex, which is featured by quick changes of lithofacies and its thickness,
distributions of fractures and pores/vugs, strong heterogeneity, changes of gas well productivities among different wells, etc.
The difficulties of reservoir characterization pose a great technological challenge on effective production of volcanic gas reservoir. In this paper a systematic methodology is proposed for characterization of fractured volcanic gas reservoir based on a large amount of laboratory experiments, theoretical studies and field practices. The proposed methodology is a multi-discipline integration, including
identification of lithologies and lithofacies, eruption period analysis and stratigraphic correlation, identification and evaluation of fractures, identification of volcanic body and reservoir prediction, well log interpretation for the dual porosity volcanic reservoir, discrimination of gas and water layer, reservoir appraisal and geological modeling, etc. These studies provide a strong base for making decisions on the
reasonable well pattern and development plan.
As a special type of oil and gas reservoir, volcanic reservoirs may occur in various oil- and gas-bearing basins all over the world, which has become an increasing point for the oil and gas production, of course, an important exploration target1-3. With the improvements of exploration technologies, many volcanic oil and gas reservoirs have been discovered. Quite a few volcanic gas reservoirs have been found in Songliao Basin since 1995, such as Wangjiatun, Changde, Shengping, Xingcheng, Changling reservoirs, etc. The consequent issue is how to produce these reservoirs effectively.
Compared to other volcanic oil and gas reservoirs, the volcanic reservoirs in Songliao Basin are typical of deep burials, high temperature and pressure, large reservoir thickness and highly tight rocks, high reserve abunance and high productivity4. However, due to the complexities of volcanic lithofacies, rapid changes of thickness, strong heterogeneity and weak seismic reflection, reservoir characterization comes to be a technological bottle-neck for effective development of volcanic gas reservoir.
After a large amount of laboratory work, theoretical studies and field practices, the latest research results of geology, seismic and well logging have been integrated. The systematic studies in this paper include lithological identification of volcanite, eruption period analysis and stratigraphic correlation, identification and evaluation of fractures, identification of volcanic body and reservoir prediction, log
interpretation for the dual-porosity volcanic reservoir, discrimination of water and gas layer, reservoir appraisal and geological modeling, etc.
Identification of lithologies and lithofacies of volcanic reservoir
Core analysis shows that the major chemical component in the target volcanic rock is SiO2, and the average content is 74.4%. Al2O3 and Na2O+K2O are minor, and their average contents are 10.4% and 8.4%, respectively. Hence the volcanite is mainly silicic volcanic rock, with minor neutral and basic volcanic rock. According to the naming rule of "component + structure + origin??, the volcanite can be classified into two major types, i.e. lava and volcaniclastic rocks, which can be further classified into 38 sub-types. In the area of interests, the lava is mainly rhyolite and dacite, and the main volcaniclastic rocks include crystal tuff, welded tuff, welded breccia,
volcanic breccia and breccia lava.