Abstract
Pressure transient behaviors of water injectors with the influence of Waterflood-Induced Fractures (WIFs) in unconventional tight oil reservoirs are much more complicated than hydraulic fracturing producers due to the variation of the WIF properties in the field-testing period. In this paper, we present the interpretation on five different types of Bottom-Hole Pressure (BHP) responses for water injectors by combining a commercial simulator tool and our proposed models, where the Shrinking Fracture Length (SFL) and Decreasing Fracture Conductivity (DFC) effects are captured for single long WIF while the multiple fracture closures effect is characterized for multiple WIFs. Results show that the commonplace phenomena of prolonged storage effect, bi-storage effect, and interpreted considerably large storage coefficient suggest that WIF(s) may initiate by long-time water injection. Based on this interpreted large storage coefficient, the fracture half-length can be estimated. The upward buckling of pressure derivative curve may not necessarily be caused by closed outer boundary condition, but the Decreasing of Fracture Conductivity (DFC). As for multiple WIFs, they would close successively after shutting in the well due to the different stress conditions perpendicular to fracture walls, which behaves as several unit slopes on the pressure derivative curves in log–log plot. Aiming at different representative types of BHP responses cases in Huaqing reservoir, Changqing Oilfield, we innovatively analyze them from a different perspective and get a new understanding of water injector behaviors, which provides a guideline for the interpretation of water injectors in tight oil reservoirs.
Highlights
Waterflooding is one of the most commonly used development methods in the field of petroleum industry and there are growing evidences showing that water injection may induce formation fracturing [1,2,3,4,5]
We find that 3/2 slope recognized by Mohamed et al [38] in Mini-Frac analysis would not appear in waterflooding, but unit slope instead, which can be interpreted as fracture storage effect
This work intends to show a comprehensive analysis of pressure responses in water injection wells considering the influence of Waterflood-Induced Fractures (WIFs)(s)
Summary
Waterflooding is one of the most commonly used development methods in the field of petroleum industry and there are growing evidences showing that water injection may induce formation fracturing [1,2,3,4,5]. These WIFs may be beneficial in improving injectivity or injection rates. Quick water breakthrough, accompanied with low oil rate and high water cut, occurs in producers along the WIF direction (Fig. 2b) while long-term low production rate is sustained for producers that perpendicular to WIF (Fig. 2c). Such dynamic behaviors of WIFs need to be timely and carefully evaluated in the waterflooding program. We divide the whole water injection process into four stages as following (Fig. 3)
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