Estimation of the pore space structure of tight gas reservoirs of the Moscovian stage locates in DDB Eastern part using a limited well logging dataset

The paper concerns the analysis of the world practice in the study of tight gas reservoirs and some of their lithological and petrophysical properties are demonstrated on the example of Svyatogorska Area perspective intervals. The purpose of this work is to establish the features of petrophysical properties of tight gas reservoirs from the upper and middle Carboniferous strata of Svyatogorska area according to laboratory and well logging data and matching the obtained geophysical results with lithological composition and mineral structure of rocks to assess gas prospects. The laboratory investigation on core samples included petrographic and petrophysical analysis of rocks. The effective porosity cutoff value for tight gas reservoirs of the studied area is 4,5 % under reservoir conditions and 4 % in the laboratory settings. 130 core samples were selected, for which the effective porosity is larger than the cutoff value. The histogram of effective porosity and absolute permeability distribution for tight gas intervals shows that PHI is in the range of 4–7 % and K – 0.01–2 mD (the largest number of samples has K – 0.01 mD). Conducted analysis of the micropetrographic study results of the core material showed that the matrix of the Moscowian stage rocks takes an average of 70 % from the volume, represented by 41-47 % quartz, 3 - 10.5 % – feldspar and 2.1–6 % – mica minerals. Cement is represented mainly by illite clay component (14–17.5 %) with calcite content (3–8.75 %). According to the formation evaluation of the well logging data clay volume, effective porosity and gas saturation were calculated for the well N10 Svyatogorska area. The intervals: 3836.4–3866.6 m and 4107.6–4140,4 4 m were indicated as tight gas reservoirs. The effective porosity of promising formations varies in the range - 4.5–9.5 %, and the gas saturation in 50–65 %. According to the results of well testing – 19.0 thousand m3/d flow rate of gas was obtained in the interval 4042–4150 m of the well N10 (before intensification). Summarizing the obtained results and comparing with the world oil and gas practices in the field of unconventional sources of hydrocarbons one suggests that the rocks of the Moscow strata of Svyatogorska Area are tight gas reservoirs.

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Controls on Pore Structures and Permeability of Tight Gas Reservoirs in the Xujiaweizi Rift, Northern Songliao Basin

Energies ◽

10.3390/en13195184 ◽

2020 ◽

Vol 13 (19) ◽

pp. 5184

Author(s):

Luchuan Zhang ◽

Shu Jiang ◽

Dianshi Xiao ◽

Shuangfang Lu ◽

Ren Zhang ◽

...

Keyword(s):

Clay Minerals ◽

Pore Space ◽

Fractal Theory ◽

Songliao Basin ◽

Tight Gas ◽

Gas Reservoirs ◽

Tight Gas Reservoirs ◽

Pore Networks ◽

Pore Structures ◽

The Impact

As significant components of tight gas reservoirs, clay minerals with ultrafine dimensions play a crucial role in controlling pore structures and permeability. XRD (X-ray diffraction), SEM (scanning electron microscopy), N2GA (nitrogen gas adsorption), and RMIP (rate-controlled mercury injection porosimetry) experiments were executed to uncover the effects of clay minerals on pore structures and the permeability of tight gas reservoirs, taking tight rock samples collected from the Lower Cretaceous Dengloukou and Shahezi Formations in the Xujiaweizi Rift of the northern Songliao Basin as an example. The results show that the pore space of tight gas reservoirs primarily comprises intragranular-dominant pore networks and intergranular-dominant pore networks according to fractal theory and mercury intrusion features. The former is interpreted as a conventional pore-throat structure where large pores are connected by wide throats, mainly consisting of intergranular pores and dissolution pores, and the latter corresponds to a tree-like pore structure in which the narrower throats are connected to the upper-level wider throats like tree branches, primarily constituting intercrystalline pores within clay minerals. Intragranular-dominant pore networks contribute more to total pore space, with a proportion of 57.79%–90.56%, averaging 72.55%. However, intergranular-dominant pores make more contribution to permeability of tight gas reservoirs, with a percentage of 62.73%–93.40%. The intragranular-dominant pore networks gradually evolve from intergranular-dominant pore networks as rising clay mineral content, especially authigenic chlorite, and this process has limited effect on the total pore space but can evidently lower permeability. The specific surface area (SSA) of tight gas reservoirs is primarily derived from clay minerals, in the order of I/S (mixed-layer illite/smectite) > chlorite > illite > framework minerals. The impact of clay minerals on pore structures of tight gas reservoirs is correlated to their types, owing to different dispersed models and morphologies, and chlorite has more strict control on the reduction of throat radius of tight rocks.

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