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Paper Number: 197
In-situ
Fracturing Characterization of Tight Sand: Insight for Reservoir
Stimulation
WU, S.T., ZHU
R.K., CUI J.W. and Yang, Z.
Research Institute of Petroleum Exploration and Development,
PetroChina, No. 20 Xueyuan Rd., Beijing; wust@petrochina.com.cn
___________________________________________________________________________
Nowadays, hydro-fracturing is critical for the effective development
of unconventional tight oil and gas (Zou et al., 2013). Scholars and
engineers have carried out abundant researches on hydro-fracturing and
have concluded several key factors on the fracture generation, including
subsurface stress & pressure, rock mechanics and fracturing fluid
(Rawing et al., 2002; Zhong et al., 2015; Qu et al., 2016). However,
there are still some uncertainties about the distribution of
newly-generated subsurface fractures. Micro-seismic monitoring are not
adequate to provide the high resolution picture of hydraulic fractures
growth, and the noise can have great effect on the interpretation, which
may result in stimulation failure. Thus, it is urgently needed to carry
out precise fracture growth characterization to understand the special
distribution of fractures and to provide reference for on-site hydraulic
fracturing.
The sample is the tight fine-grained arkose from the Upper Triassic
Yanchang Formation in the Ordos Basin, which is the most successful
tight oil play in China. Micro-CT is used to scan the tight sand samples
at different pressures to simulate the whole process of hydraulic
fracturing. The pixel resolution is 2.75μm and the sample is a pillar
with diameter & length of 3mm. The porosity and permeability is 7.8%
and 0.09mD, respectively. The pressure increases along the axial
direction from 0.6MPa to 4.0MPa, 8.0MPa, 12MPa and 14MPa finally.
The new observations and results are as follows
(1) The growth of new fractures is positively related to pressures.
The initial fracture is observed at the pressure around 8MPa, then it
extends to form abundant secondary fractures, and finally forms complex
and connected fracture network. The size of new fractures ranges from
3μm to 250μm.
(2) New fractures are developed along the pressuring direction,
occurring along the boundary between silica minerals and cements (i.e.,
calcite, dolomite). Few fractures could cut through the silica minerals,
such as quartzs and feldspars. No obvious connection between
pre-existing pores and new fractures is observed.
(3) From 4.0MPa to 14MPa, the sample extends 0.05mm, 0.12mm, 0.18mm
and 0.27mm, and the corresponding volume expansion is 3%, 8%, 12%, and
19%, respectively.
In this study, 3D models of fractures at different pressures are
reconstructed by using CT in-situ segmentation, which can show the
dynamic development of fractures directly. The results are helpful to
understand the origin and controlling factors of new fractures, and can
provide reference for on-site hydraulic fracturing.
References:
[1] Zou CN, (2012) ELSEVIER, 978-0-12-397162-3.
[2] Zhong JH, Liu SX, Ma YS, Yin CM, Liu CL, Li ZX, Liu X, and Li
Y(2015) Petroleum Exploration and Development 42(2): 242-250
[3] Rawing GC, Baud P and Wong TF (2002) Journal of Geophysics
Research 107(B10): 2234
[4] Qu GZ, Qu ZQ, HAZLETT RD, FREED D, MUSTAFAYEV R (2016) Petroleum
Exploration and Development 43(1): 1-7