Dynamic prediction and influencing factors analysis of gas and water co-production stage in tight sandstone reservoir

AbstractAiming at the problem of complicated occurrence and flow state of the fluid in tight sandstone reservoir, this paper takes Chang 7 reservoirs in Southwestern of Ordos Basin as an example to analyze the occurrence characteristics of movable fluids by nuclear magnetic resonance experiment, while takes a series of microscopic experiments to analyze the influencing factors of difference of movable fluids. The results show that the T2 spectrum curves of fluid-saturated samples from Chang 7 reservoirs in the study area are dominated by the unimodal shape and the left-high-peak-right-low-peak bimodal shape. After centrifugation, the T2 spectrum curves are dominated by the left-high-peak-right-low-peak bimodal shape. The average movable fluid saturation is 33.27%, and the average T2 cutoff value is 13.61 ms. The movable fluids are mainly distributed in medium and large pores, and a small amount is distributed in small pores. The occurrence characteristics of movable fluids in tight reservoirs are complex and not controlled by a single factor. The size of throats and the connectivity of pore-throat have obvious effects on the saturation of movable fluids. The small size of throats and poor connectivity of pore-throat in tight reservoirs not only restrict the fluids in micropores, but also make the fluids in macropores difficult to flow under the control of small throats. The development of clay minerals will make the pore throats smaller, more complex and have poorer connectivity, and increase the fluid seepage resistance. On the other hand, it will make the specific surface area larger, which will cause a large number of fluids adsorbed on the clay surface and difficult to flow, resulting in the reduction of movable fluid saturation.

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A reservoir quality evaluation approach for tight sandstone reservoirs based on the gray correlation algorithm: A case study of the Chang 6 layer in the W area of the as oilfield, Ordos Basin

Energy Exploration & Exploitation ◽

10.1177/0144598721998510 ◽

2021 ◽

pp. 014459872199851

Author(s):

Yuyang Liu ◽

Xiaowei Zhang ◽

Junfeng Shi ◽

Wei Guo ◽

Lixia Kang ◽

...

Keyword(s):

Quality Evaluation ◽

Comprehensive Evaluation ◽

Ordos Basin ◽

Reservoir Quality ◽

Pore Throat ◽

Tight Sandstone ◽

Evaluation Approach ◽

Correlation Algorithm ◽

Gray Correlation ◽

Sandstone Reservoir

As an important type of unconventional hydrocarbon, tight sandstone oil has great present and future resource potential. Reservoir quality evaluation is the basis of tight sandstone oil development. A comprehensive evaluation approach based on the gray correlation algorithm is established to effectively assess tight sandstone reservoir quality. Seven tight sandstone samples from the Chang 6 reservoir in the W area of the AS oilfield in the Ordos Basin are employed. First, the petrological and physical characteristics of the study area reservoir are briefly discussed through thin section observations, electron microscopy analysis, core physical property tests, and whole-rock and clay mineral content experiments. Second, the pore type, throat type and pore and throat combination characteristics are described from casting thin sections and scanning electron microscopy. Third, high-pressure mercury injection and nitrogen adsorption experiments are optimized to evaluate the characteristic parameters of pore throat distribution, micro- and nanopore throat frequency, permeability contribution and volume continuous distribution characteristics to quantitatively characterize the reservoir micro- and nanopores and throats. Then, the effective pore throat frequency specific gravity parameter of movable oil and the irreducible oil pore throat volume specific gravity parameter are introduced and combined with the reservoir physical properties, multipoint Brunauer-Emmett-Teller (BET) specific surface area, displacement pressure, maximum mercury saturation and mercury withdrawal efficiency parameters as the basic parameters for evaluation of tight sandstone reservoir quality. Finally, the weight coefficient of each parameter is calculated by the gray correlation method, and a reservoir comprehensive evaluation indicator (RCEI) is designed. The results show that the study area is dominated by types II and III tight sandstone reservoirs. In addition, the research method in this paper can be further extended to the evaluation of shale gas and other unconventional reservoirs after appropriate modification.

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