Surfactant and Friction Reducer Interaction in High Salinity Slickwater Fracturing

Slickwater fracturing has been phenomenally successful in unconventional shale formations due to their unique geomechanical properties. Nevertheless, these treatments consume large volumes of water. On average, hydraulic fracturing treatments use up to 13,000,000 gallons of water in unconventional wells. In an effort to reduce the use of freshwater, research has focused on developing friction reducers (FR) that can be used in high salinity brines such as seawater and produced water. However, commonly used friction reducers precipitate in high salinity brine, lose their friction reduction properties, and cause severe formation damage to the proppant pack. Consequently, this work proposes the use of common surfactants to aid the FR system and achieve salt tolerance at water salinity up to 230,000 ppm. This paper will (a) evaluate five surfactants for use in high salinity FR systems, (b) evaluate the rheological properties of these systems, and (c) evaluate the damage generated from using these systems. Four types of tests were conducted to analyze the performance of the new FR at high salinity brine. These are (a) rheology, (b) static proppant settling, (c) breakability, and (d) coreflood tests. Surfactants with ethylene oxide chain lengths ranging from 6 to 12 were incorporated in the tests. Rheology tests were done at temperatures up to 150°F to evaluate the FR at shear rates between 40-1000 s-1. Proppant settling tests were performed to investigate the proppant carrying capacity of the new FR system. Breakability and coreflood tests were conducted to study the potential damage caused by the proposed systems. Rheology tests showed that using surfactants with high ethylene oxide chain length (>8) improved the performance of the FR at water salinity up to 230,000 ppm. Anionic surfactants performed better than cationic surfactants in improving FR performance. The ammonium persulfate was used as a breaker and showed effectiveness with the proposed formula. Finally, the retained permeability after 12 hours of injecting the FR was over 95%. This shows that after using this system, the productivity of the formation is minimally affected by the new FR system. This research provides the first guide on studying the impact of using different ethylene oxide chain lengths of surfactants in developing new FR systems that can perform well in a high salinity environment. Given the economic and environmental benefits of reusing produced water, this new system can save costs that were previously spent on water treatments.

An Optimal Design Algorithm for Proppant Placement in Slickwater Fracturing

Slickwater fracturing technology is one of the significant stimulation measures for the development of unconventional reservoirs. An effective proppant placement in hydraulic fractures is the key to increase the oil production of unconventional reservoirs. However, previous studies on optimizing proppant placement are mainly focused on CFD numerical simulation and related laboratory experiments, and an optimization design method that comprehensively consider multiple influencing factors has not been established. The objective of this study is to establish an optimal design algorithm for proppant placement based on the construction characteristics of slickwater fracturing combined with Back Propagation (BP) neural network. In this paper, a proppant placement simulation experimental device was built to analyze proppant placement form data. We established a BP neural network model that considers multiple influencing factors and used the proppant placement form data to train and calibrate the model, which the proppant placement form prediction model is finally obtained. Using the proppant placement form prediction model, we designed an algorithm that can quickly select the three groups of construction schemes with the best proppant-filling ratio based on the massive construction schemes. The results indicate that the prediction results of the algorithm for proppant placement form are consistent with the CFD simulation results and experimental results, and the numerical error of the balanced height and the distance between the front edge of the proppant sandbank and the fracture entrance is within 5%. After using this algorithm to optimize the design of the fracturing construction scheme for the C8 oil well in Changqing Oilfield, the stimulation performance of the C8 oil well after fracturing is 2.7 times that of the adjacent well. The optimal design algorithm for proppant placement established in this paper is an effective, accurate, and intelligent optimization algorithm. This algorithm will provide a novel method for hydraulic fracturing construction design in oilfields.

Dry Powder Deliver of Friction Reducers: A Step Change in Slickwater Fracturing

Since the early development of unconventional resource plays, slickwater fracturing fluids have expanded rapidly and are now the most common type of fluid system used in the industry. Slickwater and viscosifying friction reducer (VFR) fluids consist of polyacrylamide (PAM) polymers and are typically delivered to location in a liquid form such as a suspension or emulsion in a hydrocarbon-based carrier fluid. Recently, advances in dry powder delivery operations have provided unique advantages over the liquid versions of FRs including cost savings and improved health, safety and environmental (HSE) aspects. This paper describes the dry powder delivery process and describes the advantages that this new technology has brought to field operations. The method involves delivering polyacrylamide powder for slickwater fracturing treatments directly into the source water on location, thereby eliminating the use of liquid polymer slurries or emulsions. Liquid friction reducers typically contain 20-30% active polymer loading, with the remaining volume being the carrier fluid to keep the polymer in suspension. By delivering 100% powder, several benefits are gained including elimination of truck deliveries of FR liquids to location, reduction of total chemical volumes by 70-80%, reduction of spill hazards, and lower overall chemical costs. Different powders are available for various applications including the use of fresh or produced water, and viscosifying or non-viscosifying polymers. The key technology for "dry on the fly" (DOTF) operations is the powder delivery equipment. Due to the different molecular structures between polyacrylamide and guar polymers, delivering PAM is more technically challenging than guar and requires much higher mixing energy to achieve proper dispersion and hydration. The delivery system described in this paper uses a unique technology which creates the necessary conditions for powder mixing and has been successfully applied on over 350 wells since early 2019, with over 7,000 tons of polymer delivered.

Research on the Characteristics of Microscopic Damage to Different Pore Types by Slippery Water Fracturing Fluid in the Chang Member Tight Sandstone Reservoir, Ordos Basin, NW China

This paper examines the damage caused by slickwater fracturing fluid to the microscopic pore structure of tight sandstone in the Chang 7 member in the Ordos Basin. A submicron CT in situ displacement system was used to analyze and graphically represent changes in pores in core samples following fracturing fluid damage. The results show the following: (1) the damage caused by slickwater fracturing fluid to tight sandstone fractures mainly occurs in the early stage of fluid incursion. The damage is characterized by a decrease in the effective pore volume, increase in the number of pores, and insignificant subsequent damage. The main causes of pore damage by slickwater fracturing fluid are retention of slickwater in the liquid phase and hydration swelling of clay minerals in the pores. (2) After the high pressure intrusion of slickwater fluid, the pore size of large-size intergranular pores increases, and there is no obvious damage after water flooding. However, fractures and small dissolution pores in the cores are the main areas of fluid retention after fracturing fluid invasion due to their small flowing radius and complex structure. These are the locations where damage mostly occurs.

Experimental and Numerical Study on Proppant Transport in a Complex Fracture System

Slickwater fracturing can create complex fracture networks in shale. A uniform proppant distribution in the network is preferred. However, proppant transport mechanism in the fracture network is still uncertain, which restricts the optimization of sand addition schemes. In this study, slot flow experiments are conducted to analyze the proppant placement in the complex fracture system. Dense discrete phase method is used to track the particle trajectories to study the transport mechanism into the branch. The effects of the pumping rate, sand ratio, sand size, and branch angle and location are discussed in detail. Results demonstrate that: (1) under a low pumping rate or coarse proppant conditions, the dune development in the branch depends on the dune geometry in the primary fracture, and a high proportion of sand can transport into the branch; (2) using a high pumping rate or fine proppants is beneficial to the uniform placement in the fracture system; (3) sand ratio dominates the proppant placement in the branch and passing-intersection fraction of a primary fracture; (4) more proppants may settle in the near-inlet and large-angle branch due to the size limit. Decreasing the pumping rate can contribute to a uniform proppant distribution in the secondary fracture. This study provides some guidance for the optimization of proppant addition scheme in the slickwater fracturing in unconventional resources.