Search

Natural fractures and wormholes strongly influence the performance of acid fracturing in naturally fractured carbonate reservoirs. This work uses an integrated model to study the effects of treatment parameters in acid fracturing in different reservoir conditions. Hydraulic fracture propagation, wormhole propagation, complex fluid leak-off mediums, and heat transfer are considered in the modeling. The model is validated in several steps by analytical solutions. The simulation results indicated that natural fractures and wormholes critically impact acid fracturing and can change the predicted outcomes dramatically. The high permeability reservoirs with conductive natural fractures or low permeability reservoirs with natural fracture networks showed the highest stimulation potential in applying acid fracturing technology. The optimal acid injection rate depends on natural fracture geometry and reservoir permeability. This study also observed that obtaining a high production index is difficult because natural fractures and wormholes reduce the acid efficiency during acid fracturing. Building an acid-etched fracture system consisting of acid-etched natural fractures and hydraulic fractures may help us better stimulate the naturally fractured carbonate reservoirs. The paper illustrates a better understanding of the effects of the treatment design parameters on productivity. It paves a path for the optimal design of acid fracturing treatment for heterogeneous carbonate reservoirs. [ABSTRACT FROM AUTHOR]

Summary: Hydraulic fracturing (HF) treatment often involves particle migration and is applied for propping or plugging fractures. Particle migration behaviors, e.g., bridging, packing, and plugging, significantly affect the HF process. Hence, it is crucial to effectively simulate particle migration. In this study, a new numerical approach is developed based on a coupled element partition method (EPM). The EPM is used to model natural and hydraulic fractures, in which a fracture is allowed to propagate across an element, thereby avoiding remeshing in fracture simulations. To characterize the water flow process in a fracture, a fully hydromechanical coupled equation is adopted in the EPM. To model particle transportation in fractures with water flow, each particle is treated as a discrete element. The particles move in the fracture as a result of being dragged by fluid. Their movement, contact, and packing behaviors are simulated using the discrete element method. To reflect the plugging effect, an equivalent aperture approach is proposed. Using this method, the particle migration and its effect on water flow are well simulated. The simulation results show that this method can effectively reproduce particle bridging, plugging, and unblocking in a hydraulic fracture. Furthermore, it is demonstrated that particle plugging significantly affects water flow in a fracture and hence the propagation of hydraulic fracture. This method provides a simple and feasible approach for the simulation of particle migration in a hydraulic fracture. Highlights: An EPM/DEM coupled method is developed to simulate particle migration in HF.The hydraulic fracturing process and the particle plugging process can be simulated.The effect of particle plugging on water flow is well captured.An EPM‐based contact search algorithm is developed to accelerate the particle computation. [ABSTRACT FROM AUTHOR]

The existence of faults in deep carbonate reservoirs is characterized by the complex stress field near faults, which has a great influence on the extension of induced fractures and the formation of the fracture network. In this paper a numerical procedure is proposed for the regional boundary stress determination. Fault zone properties were defined by measurement of the samples and were further verified by comparing the calculated stress values with the measured values for the fault zone. The change in the principal stress value and orientation and the perturbation area were quantified. The authors analyzed the boundaries of the areas at the ends of the fault where the horizontal principal stress difference increases and decreases symmetrically. The influence range increases with the increase in the fault length. The stress concentration determined by the fault structure increases or decreases markedly in the area near the fault tip. [ABSTRACT FROM AUTHOR]

Summary: The improved element partition method (IEPM) is a newly developed fracture simulation approach. IEPM allows a fracture to run across an element without introducing extra degrees of freedom. It can also simulate any number of fractures in a prescribed mesh without remeshing. In this study, the IEPM is extended to hydraulic fracture simulation. First, the seepage and volumetric storage matrix of a cracked element are derived using virtual nodes (the intersection points of a crack with element edges). Subsequently, the fully coupled hydromechanical equation is derived for this cracked element. To eliminate the extra degrees of freedom (virtual nodal quantities), the water pressure and displacement of the virtual nodes are associated with their adjacent nodes through least squares interpolation. Finally, the fully coupled equation in terms of nodal quantities is obtained. The verification cases validate the method. By using this method, the field‐scale hydraulic fracturing process is well simulated. The proposed approach is simple and efficient for field‐scale hydraulic fracture simulation. [ABSTRACT FROM AUTHOR]

Subjected to the extremely high pressurization rate, the pulsed fracture (PF) can branch to form a complex fracture network around the wellbore, which is the most desired in the reservoir stimulation. To study the PF behaviors further, the 3D numerical simulation is conducted. The rock formation is modeled by the augmented virtual internal bond method, which needs no separate fracture criterion in fracture simulation since the micro fracture mechanism has been embedded into the macro constitutive relation via micro bond potential. The simulation results demonstrate that the 3D PF always presents an ear-like profile. Some branched fractures advance towards the minor in-situ stress direction. The higher the pressurization rate is, the more complex the generated fracture network is. The longer isolation section makes the fracturing process easier. The PF speed is governed by the rock modulus. Besides these qualitative observations, some quantitative conclusions have been drawn. The findings of this work can deepen understanding on the PF and provide valuable references for PF operation in a reservoir. [ABSTRACT FROM AUTHOR]

The discrete virtual internal bond (DVIB) is a newly developed lattice model, which considers material to consist of bond cells. Through DVIB, both the 3D continuous seepage and mechanical process are reduced to the 1D discrete bond problem. The micro bond functions as both the fluid channel and the mechanical connection between particles. With the calibrated micro hydro-mechanical parameters, the DVIB can simulate the seepage and mechanical behaviors accurately based on a common set of lattice. To model the fractured medium, a fracture is allowed to run through a bond cell, which makes it possible to embed massive fracture into the background meshing scheme. To consider the hydro-mechanical coupling process, the coupling equations are built up, with which the hydro-mechanical coupling process can be well simulated. This method unifies the hydraulic and mechanical process on the micro bond level, providing a micro-macro hydraulic fracture simulation method for the complex reservoir. [ABSTRACT FROM AUTHOR]