Experimental Study on Proppant Transport of Different Proppant Pumping Schedule in Hydraulic Fracturing of Unconventional Reservoir

Abstract

ABSTRACT The sand-carrying capacity of slickwater and the proppant transport capacity are the focus of unconventional reservoir fracturing research, but little attention has been paid to the effect of pump schedule on the proppant transport in fractures. In this paper, through the visual proppant transport experiment, the effects of two pump schedules (proppant concentration stepped increasing and proppant concentration constant) on the sand dune shape under different proppant concentration are systematically studied in straight fracture and complex fracture. The results show that two pump schedules have little effect on the sand dune in straight fracture and complex fracture, and the influence of pump schedule can be ignored when the proppant concentration is increased to more than 320 kg/m3. During the combined injection of different size proppant, the pump schedule has little effect on the dune shape. Constant sand addition will make fracturing construction easier, less risk, lower cost, and better adapt to large-scale simultaneous fracturing or multi-well 3D fracturing. INTRODUCTION Hydraulic fracturing is a necessary reservoir stimulation way to develop the unconventional oil and gas commercially. Different from conventional reservoirs, unconventional reservoirs are more sensitive to fracturing fluid damage and require complex fracture networks, so slickwater has become the first choice for unconventional reservoir fracturing. However, the slickwater with low viscosity is easier to form complex fracture, but also loses a lot of capacity to suspend proppant, so the proppant transport has become a very critical problem. Therefore, the relevant studies are mostly focused on the improvement of the sand-carrying capacity of slickwater and the transport capacity of proppants, including fracturing fluids and proppants with different properties, combined injection of proppants with different sizes, etc. Few studies (Singh et al., 2021, 2022) have been done to optimize the proppant pump schedules. At present, the pump schedule of proppant concentration stepped increasing (PCSI, the proppant concentration increases at regular intervals during the sand-carrying fluid injection process) is commonly used in unconventional reservoir fracturing, which follows the pump schedule of high viscosity fracturing fluid. However, increasing the proppant concentration in the later stage of fracturing increases the difficulty of operation. In order to avoid abnormal pressure rising, linear gel is usually used to carry high concentration proppant, but this not only increases the amount of chemical agent. but also increases the complexity of operation. The pump schedule of proppant concentration constant (PCC, rapidly increasing the concentration of proppant to a certain level in the early stage of sand-carrying fluid injection, and maintaining a constant concentration of proppant during the subsequent injection process) can avoid these problems. Chevron Company (Singh et al., 2021) compared the production of 26 PCSI pump schedule wells and 68 PCC pump schedule wells, and found that the difference in production was very small. In the application of PCC pump schedule, it is found that it has the following advantages: the irregular pressure peak value is significantly reduced, which reduces the risk of screen-out and sand plugging; it can maintain a constant friction pressure drop and hydrostatic; reduce or even avoid the use of linear gel and chemical additives, reduce friction and reduce costs.