This paper brings together considerations of gas leak behaviour and leak detector design and use, with a view to improving the detection of low-pressure natural gas leaks. An atmospheric boundary layer wind tunnel has been used to study ground-based releases of methane at full scale over distances of up to 3 m, under controlled conditions. These scales are relevant to the detection of natural gas leaks from mains and services using hand-portable gas detectors. The mean spatial distribution of the leaking gas plume was determined and used to test and fit a Gaussian dispersion model. This was used for subsequent analysis with respect to the ability of gas leak detectors to confirm and locate a leak. For ground-based leaks, gas concentrations drop rapidly with height such that instruments should ideally sample the air from within 10 cm of ground level. The rapid dilution of gas with distance from the source means that instruments with lower limits of detection, ideally of a few parts per million, have much improved ability to detect a leak from greater distances downwind. Finally, observations showed the variable temporal nature of the gas and the potential for confusion when sampling gas at a single point in time and space.
The micro-scale hydraulic fracturing is a technique which is used to shorten the preheating period by means of controlling water injection to horizontal wells to create homogenously distributed microcracks in the zone between the two wells. Numerous simulation studies have been previously conducted to understand the growth of the steam chamber, however, the model to simulate mini-frac before preheating is left undone. Especially, counterpart research on land facies ultra-heavy oil sand of Xinjiang oilfield is few.
The provides micro-scale hydraulic fracturing model, in this study, is based on experimental analysis in terms of geomechanics theory and mini-frac tests result from field experiment in Xinjiang oilfield. First, laboratory triaxial tests were designed to determinate the mechanical behavior of the oil sand and the mudstone caprock. Drucker-Prager model was used to investigate the two dilation mechanisms, namely, shear dilation and tensile parting, during the simulation of micro-fracturing propagation. Finally, a micro-fracturing model coupled fluid-solid is implemented by with the finite element program ABAQUS, to describe the early SAGD start-up demonstrated on the SAGD well pair in Xinjiang oilfield.
The first-hand target of ABAQUS simulation in this study is to evaluate the performance of micro-fracturing process, particularly, to determine the degree of connectivity of injection and production wells. Thereby to predetermine the bottom hole pressure (BHP) and volume of injection in order to provide a specific guidance for work program carried out in oilfield. As the matter of convenience, this study, according to the simulation results, have discussed how to define the extent of the connected parameters to determine the connectivity of the well pairs. Furthermore, sensitivity analyses have been done to realize the different parameters on proficiency of micro-scale hydraulic fracturing in the field.
Our works are significantly important for the future development and promote the mini-frac tests to enhance the in-situ thermal recovery in Xinjiang oil field. In addition, the model and method can be implemented for any type of heavy oil field which requires early SAGD start-up to increase the oil recovery.
Coplanar perforation is a new perforation technology with unique perforation position that can improve the efficiency of hydraulic fracturing of horizontal wells in low permeability reservoirs which have low porosity, low permeability and are difficult to explore. So it is important to know how fractures initiate and propagate as well as the optimal perforation parameters.
A large-scale true tri-axial hydraulic fracturing simulation experiment is conducted to study fracture initiation characteristic and propagation pattern of coplanar perforation in horizontal wells under different in-situ stresses, compared with conventional helix perforation in horizontal wells under the same in-situ stresses. In the meantime, the three-dimensional finite element method is used to study the influences of different perforation parameter combinations set according to field data on fracturing pressure in horizontal wells, including shot length, shot diameter and shot density.
The experiment results show that the coplanar perforation has simpler fracture propagation pattern under higher stress difference and has following advantages compared to conventional helix perforation: (1) reducing the fracturing pressure, (2) controlling the fracture propagation to reduce the complexity of near-wellbore fracture and (3) reducing the near-wellbore friction resistance. The numerical simulation results show that: (1) the fracturing pressure does not decrease linearly as the shot density increases, it falls rapidly when the shot density increases from 9 to 15 holes per meter, but when the shot density increases from 15 to 21 holes per meter, it falls slowly and finally remains almost constant, (2) the fracturing pressure decreases as the shot diameter increases, and (3) the fracturing pressure stays the same as the shot length increases. Through the conclusions of experimental and numerical simulation, we could provide some valuable suggestions to the hydraulic fracturing on filed. During hydraulic fracturing process in low permeability reservoirs, coplanar perforation technology with high shot density (21 holes/m) and large shot diameter (0.03m) should be used.
The experimental and numerical simulation conducted in this paper show a real process of hydraulic fracturing with coplanar perforation. The advantages and optimal perforation parameters of coplanar perforation are proposed to give guidances for hydraulic fracturing on field.
Understanding the shear properties of joints of rock masses is of great importance for engineering disaster prevention and control. In this paper, a systematic study of the macroscopic shear properties of joints of rock masses with different strengths is carried out using a combination of indoor tests and PFC2D numerical simulations. The results show that (i) the shear stress curve of low-strength rock joints is strain-softening type, while high-strength rock joints are strain-hardening type, and high-strength rock joints are more sensitive to the change of roughness. (ii) With the increase of JRC, the damage mode of different strength rock joints gradually changes from “abrasion” to “abrasion + gnawing,” and the damage characteristics of the surface of high-strength rock joints are more significant. (iii) The contact force between particles is mainly concentrated on the joints. At the beginning of shear, the contact force is mainly distributed on the second-order roughness and gradually concentrated on the first-order roughness as the shear progresses. Compared with the low-strength rock joints, the contact force on the high-strength rock joints is larger and more widely distributed. (iv) Due to the change of contact force, the cracks keep expanding and the particle rotation arc keeps changing. The particles with larger rotational arcs are consistent with the location of crack distribution, and the cumulative number of cracks on the joints of high-strength rock is higher. (v) The total input energy and dissipation energy increase continuously with the shear, and the elastic energy tends to increase at the beginning of shear and then starts to decrease and gradually tends to be constant near the peak of shear stress. The total input energy and dissipation energy of the joints of the high-strength rock are larger, while the peak elastic energy of it is smaller.
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