Improving Proppant Placement Efficiency Using the Self-generated Gas Floating Technique
Creators: Li, Yikun
Publisher: name: University of Alberta Library
Year: 2024

<p>Hydraulic fracturing plays an essential role in producing unconventional hydrocarbon resources. The success of hydraulic fracturing operations and the performance of developing unconventional hydrocarbon resources strongly depend on the distribution of proppants in the induced fractures. Proppants establish effective flowing paths with high conductivity for reservoir fluids by preventing the closure of the induced fractures. However, due to the gravitational force, proppants tend to settle down rapidly in the induced vertical fractures. This behaviour leads to the closure of the unpropped fracture at the higher sections after the hydraulic fracturing operation and a lower filling efficiency. Many studies on proppants have been made to improve the proppant filling efficiency. However, they are limited to the methods mainly focused on reducing proppants&#39; density or increasing the buoyancy force of fracturing fluid. One particular study suggests utilizing gas-suspended proppants to increase the proppant placement efficiency at the higher sections of fractures (Wang et al., 2017). The authors recommend injecting nitrogen along with proppants and fracturing fluid during the slurry stage of hydraulic fracturing operations. The nitrogen gas bubbles attach and bring proppants to the higher section of the fracture. To minimize the cost of nitrogen and the complexity of hydraulic fracturing operations, we desire to examine if gas bubbles can be generated inside fractures to improve proppant placement efficiency. Here, the self-generated gas floating technique is proposed in this study for the first time. This technique adds an external lifting force to proppants from the reaction-generated CO2 bubbles. It incorporates the proppants’ surface characteristics to bring the proppants to a higher fracture location and increases the proppant filling efficiency. </p>

<p>In this study, we first alter the wettability of ceramic proppants using a siliconizing chemical called SurfaSil and measure the contact angles of three type of proppants (i.e., ceramic proppants, resin-coated proppants and SurfaSil-treated proppants). Then, we evaluate the adhesion forces between resin-coated proppants and CO2/air. Furthermore, we apply the self-generated gas floating technique in a transparent fracture model to study the proppant placement efficiency at the laboratory condition. We study the effects of proppant size, proppant wettability, reaction rate, fracture width and gas bubble types on the proppant placement efficiency (i.e., the ratio of the area occupied by proppants to the total fracture area). The proppant placement efficiency is quantified through the MATLAB image analysis codes. In conclusion, there is a proportional relationship between the reaction rate and proppant placement efficiency. A larger fracture width also tends to increase the proppant placement efficiency. There is an inverse relationship between the size of proppants and the proppant placement efficiency. Among the different proppant types, resin-coated proppants are proven to yield the highest proppant placement efficiency in the fracture model. </p>

<p>Next, we analyze the actual wellhead pressure recorded by one field hydraulic fracturing operation and find that the pressure decline rate is about 40 MPa/min after the pumping of proppant slurry. We then adopt pressure decline rates  similar to the field decline rate of  40 MPa/min in high-pressure experiments to examine whether the self-generated gas floating technique can actually work. In our high-pressure experiments, we first pressurize the reactor that is filled with resin-coated proppants and sodium bicarbonate solution to 10 MPa using CO2. Subsequently we inject acetic acid into the reactor. Lastly, we decrease the pressure in the reactor under different rates to simulate the pressure decline stage after the hydraulic fracturing operation. During the experiments, we initiate the acid and base reactions under two different pressure decline rates and visually evaluate whether the self-generated CO2 bubbles can lift the resin-coated proppants that are originally located at the bottom of the reactor. From the high-pressure experiments, we can conclude that the chemical reaction cannot be initiated if we maintain high-pressure conditions. If we decline the pressure at a high rate, the reaction rate becomes faster to generate a larger amount of CO2 bubbles that push proppants upward. If the pressure decline rate is low, the reaction rate is low, and only a few CO2 bubbles are generated, which can barely attach and bring the proppants to the higher location in the reactor.</p>