Performance Evaluation of Multistage Fractured Horizontal Wells in Tight Gas Reservoirs at Block M, Ordos Basin

Block M of the Ordos Basin is a typical low-permeability tight sandstone gas accumulation. To develop these reservoirs, various horizontal well fracturing technologies, such as hydra-jet fracturing, open-hole packer multistage fracturing, and perf-and-plug multistage fracturing, have been implemented in practice, showing greatly varying performance. In this paper, six fracturing technologies adopted in Block M are reviewed in terms of principle, applicability, advantages, and disadvantages, and their field application effects are compared from the technical and economic perspectives. Furthermore, the main factors affecting the productivity of fractured horizontal wells are determined using the entropy method, the causes for the difference in application effects of the fracturing technologies are analyzed, and a comprehensive productivity impact index (CPII) in good correlation with the single-well production of fractured horizontal wells is constructed. This article provides a simple and applicable method for predicting the performance of multi-frac horizontal wells that takes multiple factors into account. The results can be used to select completion methods and optimize fracturing parameters in similar reservoirs.

Fractal characteristics of the micropore throats in the shale oil reservoirs of the Chang 7 Member of the Yanchang Formation, Ordos Basin

With the development of the global shale oil and gas revolution, shale oil became an important replacement field to increase oil and gas reserves and production. The Chang 7 Member of the Yanchang Formation in the Ordos Basin was an important shale oil exploration series in China. To study the micropore-throat structure characteristics of the Chang 7 Member, we launched nuclear magnetic resonance (NMR) and high-pressure mercury injection (HPMI) experiments to analyze the pore-throat structure features of the Chang 7 reservoir, and we considered fractal theory to study the fractal characteristics. The NMR results indicated that the T2 spectral morphology of the Chang 7 reservoir could be characterized by three main patterns encompassing early and late peaks with different amplitudes: the type 1 reservoir contained mostly small pores and few large pores, and the porosities of the small and large pores range from 4.16% to 9.04% and 0.70% to 2.40%, respectively. The type 2 reservoir contained similar amounts of small and large pores, and the type 3 reservoir contained few small pores and mostly large pores, while the porosities of the small and large pores range from 1.81% to 2.74% and 3.32% to 5.64%, respectively. The pore-throat structure parameters were obviously affected by the pore size distribution, which in turn influenced the reservoir seepage characteristics of the reservoir. The micropore-throat structure of the reservoir exhibited obvious piecewise fractal characteristics and mainly included dichotomous and trilateral fractals. The type 1 reservoirs were dominated by dichotomous fractals, and these two fractal types were equally distributed in the type 2 and 3 reservoirs. The fractal dimension of the pore throats of different scales exhibited a negative correlation with the corresponding porosity, but no correlation was observed with the permeability, indicating that the size of the reservoir determined by pore throats imposed a strong controlling effect on their fractal characteristics.

Characteristics and Genetic Mechanism of Chang Eight Low Permeability and Tight Reservoir of Triassic Yanchang Formation in Central-East Ordos Basin

In this article, the characteristics of Chang 8 reservoir of Triassic Yanchang Formation in northern Shaanxi are studied by using polarizing microscope, field emission scanning electron microscope, image particle size, X-ray diffraction analysis of clay, and constant pressure mercury intrusion. The study shows that the target layer is in a relatively stable and uniform sinking burial period after deposition, and the lithology composition in the area is relatively complex, mainly composed of debris–feldspar sandstone and feldspar sandstone, with the characteristics of fine grain and high content of interstitial material. The porosity of the reservoir is generally between 4% and 12%, with an average of 8.05%. The permeability is generally between 0.03 × 10−3 and 0.5 × 10−3 μm2, with an average of 0.16 × 10−3 μm2. Strong compaction and well-developed cementation of calcareous, siliceous, and authigenic illite are important reasons for the formation of extra-low porosity and extra-low permeability reservoirs. But at the same time, because of the protective effect of chlorite film, some residual intergranular pores are preserved, which makes the some reservoirs with relatively good physical property, forming a local relatively high-porosity and high-permeability section of the “highway.”

From Technical Evolution to Production Performance: A Technical Review of 700 Wells Over 8 Years in the South Sulige Tight Gas Field

Unlike many unconventional resources that demonstrate a high level of heterogeneity, conventional tight gas formations often perform consistently according to reservoir quality and the applied completion technology. Technical review over a long period may reveal the proper correlation between reservoir quality, completion technology, and well performance. For many parts of the world where conventional tight gas resources still dominate, the learnings from a review can be adapted to improve the performance of reservoirs with similar features. South Sulige Operating Company (SSOC), a joint venture between PetroChina and Total, has been operating in the Ordos basin for tight gas since 2011. The reservoir is known to have low porosity, low permeability, and low reservoir pressure, and requires multistage completion and fracturing to achieve economic production. Over the last 8 years, there has been a clear technical evolution in South Sulige field, as a better understanding of the reservoir, improvement of the completion deployment, optimized fracturing design, and upgraded flowback strategy have led to the continuous improvement of results in this field. Pad drilling of deviated boreholes, multistage completions with sliding sleeve systems, hybrid gel-fracturing, and immediate flowback practices, gradually proved to be the most effective way to deliver the reservoir's potential. Using the absolute open-flow (AOF) during testing phase for comparative assessment from South Sulige field, we can see that in 2012 this number was 126 thousand std m3/d in 2012, and by 2018 this number had increased to 304 thousand std m3/d, representing a 143% incremental increase. Thus, technical evolution has been proved to bring production improvement over time. Currently, South Sulige field not only outperforms offset blocks but also remains the top performer among the fields in the Ordos basin. The drilling and completion practices from SSOC may be well suited to similar reservoirs and fields in the future.

Diagenesis Evolution and Pore Types in Tight Sandstone of Shanxi Formation Reservoir in Hangjinqi Area, Ordos Basin, Northern China

Diagenesis and pore evolution of tight sandstone reservoir is one of the most important issues surrounding clastic reservoirs. The tight sandstone of the Shanxi Formation is an important oil and gas producing layer of the Upper Paleozoic in Ordos Basin, and its densification process has an important impact on reservoir quality. This study determined the physical properties and diagenetic evolution of Shanxi Formation sandstones and quantitatively calculated the pore loss in the diagenetic process. Microscopic identification, cathodoluminescence, and a scanning electron microscope were used identify diagenesis, and the diagenesis evolution process was clarified along with inclusion analysis. In addition, reservoir quality was determined based on the identification of pore types and physical porosity. Results show that rock types are mainly sublitharenite and litharenite. The reservoir has numerous secondary pores after experiencing compaction, cementation, and dissolution. We obtained insight into the relationship between homogenous temperature and two hydrocarbon charges. The results indicated that there were two hydrocarbon charges in the Late Triassic–Early Jurassic (70–90 °C) and Middle Jurassic–Early Cretaceous (110–130 °C) before reservoir densification. The quantitative calculation of pore loss shows that the average apparent compaction, cementation, and dissolution rates are 67.36%, 22.24%, and 80.76%, respectively. Compaction directly affected the reservoir tightness, and intense dissolution was beneficial to improve the physical properties of the reservoir.

Pore Structures of the Lower Permian Taiyuan Shale and Limestone in the Ordos Basin and the Significance to Unconventional Natural Gas Generation and Storage

In the Ordos Basin, multiple sets of coal seams, organic-rich shale, and limestone are well developed in the Permian Taiyuan Formation, which are favorable targets for collaborative exploration of various types of unconventional natural gas resources, including coalbed methane, shale gas, and tight gas. In this study, core samples from the Permian Taiyuan Formation in the eastern margin of the Ordos Basin were used to carry out a series of testing and analysis, such as the organic matter characteristics, the mineral composition, and the pore development characteristics. In the shale of the Taiyuan Formation, the total organic carbon (TOC) content is relatively high, with an average of 5.38%. A thin layer of black shale is developed on the top of the Taiyuan Formation, which is relatively high in TOC content, with an average of 9.72%. The limestone in the Taiyuan Formation is also relatively high in organic matter abundance, with an average of 1.36%, reaching the lower limit of effective source rocks (>1%), being good source rocks. In the shale of the Taiyuan Formation, various types of pores are well developed, with relatively high overall pore volume and pore-specific surface area, averaging 0.028 ml/g and 13.28 m2/g, respectively. The pore types are mainly mineral intergranular pores and clay mineral interlayer fractures, while organic matter-hosted pores are poorly developed. The limestone of the Taiyuan Formation is relatively tight, with lower pore volume and pore-specific surface area than those of shale, averaging 0.0106 ml/g and 2.72 m2/g, respectively. There are mainly two types of pores, namely, organic matter-hosted pores and carbonate mineral dissolution pores, with a high surface pore rate. The organic matter in the limestone belongs to the oil-generation kerogen. During thermal evolution, the organic matter has gone through the oil-generation window, generating a large number of liquid hydrocarbons, which were cracked into a large number of gaseous hydrocarbons at the higher mature stage. As a result, a large number of organic matter-hosted pores were generated. The study results show that in the Ordos Basin, the shale and limestone of the Permian Taiyuan Formation have great potential in terms of unconventional natural gas resources, providing a good geological basis for the collaborative development of coal-bearing shale gas and tight limestone gas in the Taiyuan Formation.

Development Characteristics and Finite Element Simulation of Fractures in Tight Oil Sandstone Reservoirs of Yanchang Formation in Western Ordos Basin

Structural fractures have a significant control effect on the large-scale accumulation of hydrocarbons in the Yanchang Formation. Previous studies have affirmed the important role of fractures in hydrocarbon accumulations in strongly deformed zones. However, for low-amplitude structural areas, the degree of fracture development is relatively low, and their control on sweet spots of hydrocarbons has not yet formed a unified understanding. In this paper, taking the Upper Triassic Yanchang Formation in the western Ordos Basin as an example, the development characteristics, prediction method, and the distribution of fractures in tight sandstone reservoirs in low-amplitude structural areas have been systematically studied using a large number of cores, thin sections, paleomagnetism, FMI logging, acoustic emission, productivity data, and finite element method. The research results showed that the Yanchang Formation in the study area mainly develop high-angle and vertical fractures, which were formed by regional tectonic shearing. Fractures are mainly developed in the fine-grained and ultra-fine-grained sandstones of the distributary channel and estuary bar microfacies, while the fractures in the medium-grained sandstones of the distributary channel and the mudstones of the distributary bay are relatively underdeveloped. The core fractures and micro-fractures of the Yanchang Formation all have the regional distribution characteristics, and the fracture strikes are mainly between NE50° and NE 70°. Moreover, the finite element method was used to predict the fractures in the target layer, and the prediction results are consistent with the actual distribution results of the fractures. The coupling analysis of fractures and tight oil sandstone distribution showed that the existence of fractures provided conditions for the accumulation of hydrocarbons in the Yanchang Formation. The confluence and turning areas of the river channels were repeatedly scoured by river water, and the rocks were brittle and easy to form fractures. The thickness of the fractured sandstone in these areas is usually greater than 0.4 m. Moderately developed fracture zones are prone to form hydrocarbon accumulation “sweet spots,” and the fracture indexes of these areas are usually distributed between 0.8 and 1.2. However, when the fracture index exceeds 1.2, over-developed fractures are unfavorable for the accumulation of hydrocarbons.