No abstract

Summary Hydraulic-fracture initiation and propagation in the presence of multiple layers with different mechanical and flow properties are investigated experimentally using a novel fracturing cell. Mixtures of plaster, clay, and hydrostone are used to cast sheet-like and porous test specimens in layers with different configurations and properties. The layered specimens are hydraulically fractured under varying far-field differential stress. Fracture growth is recorded using a high-resolution digital camera. Key frames are subsequently analyzed using digital image correlation (DIC) to reveal microcracks, measure strains, and show other features such as shear-failure events that are difficult to detect with the naked eye. The problem of a hydraulic fracture induced in a soft layer bounded by harder layers is considered. We demonstrate numerous laboratory experiments that reveal a clear tendency for induced fractures to avoid harder bounding layers. This is seen as fracture deflection or kinking away from the harder layers, fracture curving between the harder bounding layers, and fracture tilt from the maximum far-field stress direction. These observations appear to be more pronounced as the contrast in Young's modulus and fracture toughness between the layers increases and/or the far-field differential stress decreases. Moreover, when a fracture is induced in a relatively thin layer, the fracture avoids the harder bounding layers by starting and propagating parallel to the bounding interfaces. Fracture propagation parallel to the bounding layers is also observed in relatively wide layers when the far-field stress is isotropic or very low. A fracture approaching a dipping, harder layer tends to curve away from the hard layer by kinking toward the high side of the interface. Nonplanar fracture trajectories are observed even in homogeneous materials when the far-field differential stress is relatively low. Furthermore, various other fracture behaviors in layered specimens are demonstrated and discussed, such as fracture offsetting at material interfaces, fracture branching and complex fracture trajectories, and shear failure of weakly bonded interfaces.

No abstract

No abstract

Summary In well stimulation operations, the ability to sustain long-term conductivity of hydraulic/acid fractures defines an efficient and effective hydrocarbon production operation. However, it is challenging to keep the fracture conductive in the soft and weak carbonate formations due to many challenges. For example, the plastic deformation of rocks causes proppant embedment or asperities failure, resulting in fracture conductivity reduction. Consolidating chemicals, particularly diammonium hydrogen phosphate (DAP), have shown to be effective in rock consolidation and could reduce the decline in fracture conductivity if applied to carbonate formations. The previous research tested DAP at ambient conditions, whereas this work involves studying the hardening properties of DAP at reservoir conditions. The solutions with two initial concentrations (1 and 0.8 M) were tested at 77°F (ambient), 122°F, and 176°F. Furthermore, a post-treatment analysis was conducted to compare the performance of the chemical under different conditions. The analysis included understanding the changes in carbonate rocks’ (limestone and chalk) hardness (impulse hammer test and indentation test), porosity (helium porosimeter), permeability (steady-state and unsteady state nitrogen injection), and mineralogy [X-ray diffraction (XRD) and scanning electron microscopy (SEM)]. Results demonstrated that both rock lithologies reacted efficiently with the DAP solution, presented in terms of the noticeable improvements in their hardness. The elevated temperatures positively affected rock hardness, leading to a more than 100% increase in hardness for most samples. After obtaining successful results from experiments at various temperatures, the pilot American Petroleum Institute (API) conductivity experiments were conducted, testing the conductivity sustenance through the rock hardening concept. Preliminary API conductivity experiments have demonstrated that treated rock samples with DAP provided higher conductivity values than the untreated samples at high stresses. The results shown in this study provide a good foundation for further studies on the implementation of DAP in actual acid/hydraulic fracturing field operations.