Browsing by Author "Xu, Bin"

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    Finite element simulation of hydraulic fracturing in unconsolidated sands
    (2010) Xu, Bin; Wong, Ronald C. K.
    Fluid injection into reservoirs; such as waste disposal, steam or water flooding and well testing, is a common practice in oil and gas industry. The injection of large volumes of fluid into an unconsolidated sands reservoir can result in significant changes to the in-situ stress distributions which may lead to the hydraulic fractures initiation and propagation. Hydraulic fracturing can be broadly defined as a process by which a fracture initiates and propagates due to hydraulic loading (i.e., pressure) applied by a fluid inside the fracture[l]. Although hydraulic fracturing in hard rock has been comprehensively studied both experimentally and numerically, some fundamental mechanisms of hydraulic fracturing in unconsolidated formation have not been well understood. The hydraulic fracture in unconsolidated sand reservoir can be represented as an anisotropic area of dilation zone or a net of micro-cracks, inside which the formation has low effective stresses and high hydraulic conductivity values. In this thesis, w present a 3D finite element model for simulating the hydraulic fracture initiation and propagation in unconsolidated sands reservoir due to large volumes of fluid injection at high injection rate. To simulate this strong anisotropy in mechanical and hydraulic behaviour induced by fluid injection, a poro-elasto-plastic constitutive model together with a strain-induced anisotropic permeability model are formulated and implemented into a 3D finite element simulator, which is used to match the field injection data. Several case studies, which include the field produced water re-injection into deep unconsolidated formation and well testing in oil sands formation, are conducted using the developed finite element model. The bottom-hole-pressures predicted by the developed finite element codes are used to history match the field bottom-hole-pressures. The numerical calculations clearly show that the presented numerical model can capture the physical mechanism of hydraulic fracture initiation and propagation in unconsolidated sand formation and matches the field pressure versus time curve very well.
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    Mineral nitrogen and phosphorus pools affected by water table lowering and warming in a boreal forested peatland
    (2017-09-14) Munir, Tariq; Khadka, Bhupesh; Xu, Bin; Strack, Maria; Munir, Tariq
    Changes in atmospheric temperature and lowering in water-table (WT) are expected to affect peatland nutrient dynamics. To understand the response of peatland nitrogen (N) and phosphorus (P) dynamics to warming and drainage in a continental wooded-bog of hummock – hollow microtopography, we compared three sites: 1) control, 2) recently drained (2-3 years; experimental), and 3) older drained (12-13 years; drained), during 2013. The WT was lowered at experimental and drained sites to 74 cm and 120 cm, respectively, while a warming of ~1 °C was created at one-half of the microforms using open-top chambers. Responses of peat total-inorganic- nitrogen [TIN = nitrate nitrogen (NO3--N) + ammonium nitrogen (NH4+-N)] and phosphate-P [PO43--P] pools and, vegetation C:N ratio, δ13C, and δ15N to the experimental treatments were investigated across sites/microforms and over time. Peat TIN available and extractable pools increased with deepening of WT and over time, and were greater at hummocks relative to hollows. In contrast, the PO4 pools increased with short-term drainage but reverted to very close to their original (control) nutrient values in the longer-term. The WT and warming driven change in the peat TIN pool was strongly reflected in the vascular vegetation C:N ratio and, shrub δ13C and δ15N, while moss nutrient dynamics did not vary between sites. Therefore, we suggest that atmospheric warming combined with WT deepening can increase the availability of mineral N and P, which then can be reflected in vascular vegetation and hence modify the productivity and ecosystem functioning of the northern mid-latitude continental forested bogs in the long-term.
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    Partitioning Forest-Floor Respiration into Source Based Emissions in a Boreal Forested Bog: Responses to Experimental Drought
    (MDPI, 2017-03-10) Munir, Tariq; Khadka, Bhupesh; Xu, Bin; Strack, Maria
    Northern peatlands store globally significant amounts of soil carbon that could be released to the atmosphere under drier conditions induced by climate change. We measured forest floor respiration (RFF) at hummocks and hollows in a treed boreal bog in Alberta, Canada and partitioned the flux into aboveground forest floor autotrophic, belowground forest floor autotrophic, belowground tree respiration, and heterotrophic respiration using a series of clipping and trenching experiments. These fluxes were compared to those measured at sites within the same bog where water‐table (WT) was drawn down for 2 and 12 years. Experimental WT drawdown significantly increased RFF with greater increases at hummocks than hollows. Greater RFF was largely driven by increased autotrophic respiration driven by increased growth of trees and shrubs in response to drier conditions; heterotrophic respiration accounted for a declining proportion of RFF with time since drainage. Heterotrophic respiration was increased at hollows, suggesting that soil carbon may be lost from these sites in response to climate change induced drying. Overall, although WT drawdown increased RFF, the substantial contribution of autotrophic respiration to RFF suggests that peat carbon stocks are unlikely to be rapidly destabilized by drying conditions
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    UAV Remote Sensing Can Reveal the Effects of Low-Impact Seismic Lines on Surface Morphology, Hydrology, and Methane (CH4) Release in a Boreal Treed Bog
    (Journal of Geophysical Research: Biogeosciences, 2018-02-23) Lovitt, Julie; Rahman, Mustafizur M.; Saraswati, Saraswati; McDermid, Gregory J.; Strack, Maria; Xu, Bin
    Peatlands are globally significant stores of soil carbon, where local methane (CH4) emissions are strongly linked to water table position and microtopography. Historically, these factors have been difficult to measure in the field, constraining our capacity to observe local patterns of variability. In this paper, we show how remote sensing surveys conducted from unmanned aerial vehicle (UAV) platforms can be used to map microtopography and depth to water over large areas with good accuracy, paving the way for spatially explicit estimates of CH4 emissions. This approach enabled us to observe—for the first time—the effects of low-impact seismic lines (LIS; petroleum exploration corridors) on surface morphology and CH4 emissions in a treed-bog ecosystem in northern Alberta, Canada. Through compaction, LIS lines were found to flatten the observed range in microtopographic elevation by 46 cm and decrease mean depth to water by 15.4 cm, compared to surrounding undisturbed conditions. These alterations are projected to increase CH4 emissions by 20–120% relative to undisturbed areas in our study area, which translates to a total rise of 0.011–0.027 kg CH4 day 1 per linear kilometer of LIS (~2 m wide). The ~16 km of LIS present at our 61 ha study site were predicted to boost CH4 emissions by 20–70 kg between May and September 2016.