Fujian Zhou

Formation Damage due to Drilling and Fracturing Fluids and Its Solution for Tight Naturally Fractured Sandstone Reservoirs

Drilling and fracturing fluids can interact with reservoir rock and cause formation damage that impedes hydrocarbon production. Tight sandstone reservoir with well-developed natural fractures has a complex pore structure where pores and pore throats have a wide range of diameters; formation damage in such type of reservoir can be complicated and severe. Reservoir rock samples with a wide range of fracture widths are tested through a multistep coreflood platform, where formation damage caused by the drilling and/or fracturing fluid is quantitatively evaluated and systematically studied. To further mitigate this damage, an acidic treating fluid is screened and evaluated using the same coreflood platform. Experimental results indicate that the drilling fluid causes the major damage, and the chosen treating fluid can enhance rock permeability both effectively and efficiently at least at the room temperature with the overburden pressure.

Simulation of coal permeability under non-isothermal CO2 injection

CO2 injection into coal seams is a non-isothermal process, which has significant impact on coal permeability but has not been well studied. In this paper, a non-isothermal model coupled with nonlinear gas flow and matrix deformation was developed. The effects of temperature change on each term of the effective strain during the CO2 injection scenarios, as well as the variations of fluid properties over a range of sub- and supercritical-thermodynamic conditions were investigated. This model involves the balance of thermal energy and the law of heat transfer. Two non-isothermal cases of CO2 injection were studied and compared with the isothermal case. The results show that CO2 injection into coal seams reduces coal permeability for all three cases. The coal matrix expands with temperature increase due to the thermal expansion and shrinks due to the decrease in adsorption amount. However, the final permeability with low-temperature CO2 injection remains lower than that with high-temperature gas injection since the effect of sorption-induced strain on permeability outweighs that of the thermal deformation. The increase in temperature leads to the reduction in coal swelling (with the decrease of CO2 adsorption capacity), resulting in larger cleat aperture and higher coal permeability for the cases studied in this work. [Received: November 29, 2015; Accepted: June 22, 2016]

Impact of the Microstructure of Polymer Drag Reducer on Slick-Water Fracturing

Many studies have focused on the drag reduction performance of slick-water, but the microdrag reduction mechanism remains unclear since the microstructure of the drag reducer and its effect on this mechanism have not been well studied. In this study, the microstructure of the drag reducer in slick-water was effectively characterized by transmission electron microscopy. The viscoelasticity and drag reduction performance of the drag reducer with different microstructures were then investigated. Further, the effects of the microstructure of the drag reducer on the viscoelasticity and drag reduction performance of slick-water were analyzed. The results demonstrated that the viscoelasticity of slick-water is governed by the microstructure of the drag reducer, which exhibits a network structure. In addition, the drag reduction performance is related to the viscoelasticity. At low flow rates, the drag reduction performance is dominantly influenced by viscosity, whereas, at high flow rates, it is governed mainly by elasticity. Furthermore, the drag reducer with a uniformly distributed network structure exhibits the most stable drag reduction performance. This drag reducer was used in a field test and the obtained results were consistent with those of a laboratory experiment.

ESRV and Production Optimization for the Naturally Fractured Keshen Tight Gas Reservoir

The economic gas production in Keshen deep reservoir (7000–8038 m), located in the Tarim basin, as one of the largest gas resources in the north-west China, depends on the optimization of Effective Stimulated Reservoir Volume (ESRV), which was investigated through numerical simulations based on the match of fracture properties and reservoir capabilities under high pressure (>116 MPa) and high temperature (160–170 °C) in this study. An integrated multiphysical model, which couples gas flow with mechanical deformation and describes the gas flow interaction between the matrix, natural fractures and hydraulic fractures, was developed to optimize the ESRV and predict the production. Controlling factors of the ultimate gas recovery were analysed, and the impacts of pressure-dependent permeability of matrix and fracture systems on gas recovery were indicated through two case studies. Results show that if the impacts of effective stress on porosity and permeability were neglected, natural fracture spacing and permeability were the predominant factors affecting the ultimate gas recovery, whereas the half-length of the primary hydraulic fractures and the spacing of the secondary-fracture networks were more important for enhancing gas recovery due to the permeability sensitivity to the effective stress. This study improves the understanding of gas flow interaction among the matrix, natural fractures and hydraulic fractures in tight gas reservoir.

Evaluations of Fracture Injection Pressure and Fracture Mouth Width during Separate-Layer Fracturing with Temporary Plugging

Separate-layer fracturing with temporary plugging (SLFTP) is a potential way to stimulate multiple layer reservoirs due to its low cost, low risk, and high efficiency. In this study, based on the cohesive zone model (CZM), a 3D fully fluid-solid coupling and multiple layer model is established to investigate factors influencing fracture injection pressure and fracture mouth width. The cohesive layer properties are based on the reported study, which have been validated through a series of numerical experiments. Innovatively, the spring model is innovatively proposed to represent the plugging effect of diverting agents and prop the aperture of the previous fractures. Simulation results reveal that the effects of previous fractures in multiple layer formations can be neglected, which is quite different from multistage fracturing for horizontal wells. Fracture injection pressure can be evaluated more accurately by taking the following factors into consideration: the minimum horizontal principal stress, rock tensile strength, injection rate, and pore pressure enhancement. Further, fracture mouth width is strongly influenced by rock tensile strength, Young’s modulus, and injection rate. This study provides a guidance for candidate well selection and diverting agent optimization during SLFTP in multilayer formations.

Study on Fluid-Rock Interaction and Reuse of Flowback Fluid for Gel Fracturing in Desert Area

Hydraulic fracturing requires a large volume of fresh water, which is difficult and expensive to obtain in the desert area such as Tarim Basin. Currently, flowback fluid is typically transported to the sewage treatment plant and then discharged after reaching environmental requirements; however, this is not only costly, but also a waste of water resource. Therefore, it is imperative to understand the potential interactions between fracturing fluid and reservoir rock, and then find solutions to reuse the flowback water for subsequent fracturing. In this study, once flowback fluid was directly collected from the field, its chemical compositions were analyzed; then, filtering, decoloring, and chelating methods were chosen to effectively remove or shield the unfavorable reintroduced components. Moreover, pH value was further tuned during different stages of the recycling process to ensure good gelation and cross-linking properties of guar. Cross-linked guar synthesized with the flowback fluid was evaluated in the lab through shear resistance tests and coreflood tests under the reservoir conditions; results indicated the recycled gel behaved similarly as the original gel, or even better. From this work, a cheap and effective treatment process was proposed to reuse the flowback fluid in the desert area.

The main factors and rules of stress shadow of perpendicular cracks

Based on elasticity theory, we use numerical Galerkin finite element discretization method and implement Matlab finite element code to simulate “stress shadow” distributions of mutual orthogonal fractures. The principal stress and principal distributions have the symmetry characteristic on the intersection (coordinate origin). The relationships between stress shadow and flow pressure ratio, pore pressure, fluid pressure and horizontal stress contract are analyzed, respectively. By these techniques of variable displacement construction, changing the viscosity of the fracturing fluid, exploitation of oil and gas wells changing pump rate and fracturing fluid viscosity, reducing pore pressure and increasing the injection volume, taking the advantages of shadow effect, it is likely to produce a complex fracture network.

Enhancement of the Wettability and Lubrication of Shale Rock via Nanoemulsions

Nanoemulsions have been widely used as additives for drilling fluids in recent years. With the development of nanotechnology, multifunctional nanomaterials have been added to nanoemulsions. The improvement of wettability of the surfaces, alteration of oil-wet on shale rock surfaces, and environmentally friendly conditions are considered as the future development directions of nanoemulsions. In this work, a novel nanoemulsion was prepared by using hydrocarbon-based polyoxyethylene ether, oil (hydrocarbon), distilled water, and formation crude oil as the main raw materials. The shale rocks before and after immersion with as-prepared nanoemulsion were characterized by contact angle measurement, atomic force microscope (AFM), and Fourier transform infrared spectroscopy (FTIR). It is clearly observed that the nanoemulsion greatly improved the wettability of the sandstone and rock surface by forming a layer of active agent film on the surface of the rock. The as-prepared nanoemulsion had good ability to curb the anticollapse and lubricate and protect the oil and gas layer.

Development of a New Multifunctional Cationic Surfactant System with Corrosion Inhibiting Ability

Four alkyl propargyl bis(hydroxyethyl) ammonium surfactants with different lengths in hydrophobic tail (C8–C16) are prepared by direct alkylation of diethanolamine with alkyl bromides. The surfactants exhibit excellent surface properties. The critical micelle concentration (CMC) values of the surfactants 2c and 2d are found to be and mol/L, and the corresponding surface tension () values are 31.83 and 31.69 mN/m, respectively. Furthermore, all of the synthesized propargyl quaternary ammonium salts are found to be very effective in inhibiting the corrosion of N80 steel (IE% = 92–98%). They can be used at a relatively high temperature (≥90°C) and a low concentration (0.039 wt.%). In addition, the apparent viscosity of surfactant 2d is studied at various temperatures, and it can be used as a viscoelastic surfactant up to 100°C. The synthesized surfactants possess multiple functions and have a very good compatibility with VES systems. They can be potentially used in various oil field applications such as enhanced oil recovery, fracturing, acid diversion, and well stimulation.