Critical Evaluations of Additives Used in Shale Slickwater Fracs

Abstract

Slickwater fracturing has increased over the past decade with the advent of shale gas plays. Horizontal wells are now the standard with up to 1 million gallons of water in as many as 6 to 9 frac stages per well. The objective is to create as much contact with the reservoir as possible and many times a secondary goal is to prop open the created fractures. Additive packages have been minimized to save money. Due to environmental concerns and fresh water availability, the flowback and produced water is collected and used for subsequent fracture treatments. The purpose of this work is to examine water treatment techniques and critically evaluate the performance of additives that are employed in slickwater fracs of shale reservoirs and give guidelines for selecting additives that will optimize performance during pumping, fluid recovery and production. Comments will be made on the topic of proppant selection. Following the proppant, the major additive in most jobs will be the friction reducer which is required to reduce the friction pressure while pumping at the extreme rates of 50 to 120 barrels per minute (bpm). The second concern should be additives to treat bacteriological activity. The injection of water will ultimately result in the cultivation of sulfate reducing bacteria which produce Hydrogen sulfide (H2S) and biproducts such as black iron sulfide on the surface if not treated properly. Scale inhibitors become vitally important as water dissolves salts from the formation. Shales have sub microDarcy matrix permeability with natural fractures and cleats providing avenues for gas desorption and flow to the wellbore. Shales can have as much as 50% clays. Are additives necessary to stabilize clays? Finally, the use of surfactants can be beneficial in promoting the flowback of injected fluids to restore the relative permeability to gas. Which surfactant types are the most beneficial? Introduction Revitalization of slickwater fracs over the last decade have increased due to higher natural gas prices and more experience in fracturing with lower cost fluids. Slickwater fracs have been employed in low permeability and large net pays, and require large amounts of water to obtain adequate fracture half-lengths. Before Barnett Shale was fractured in 1997, many fracs were carried out with a cross-linked fluid and large amounts of proppants. The difficulty in cleaning the wells and the low return made many wells uneconomical. Some wells were even treated with slickwater and no proppant. Initial production was higher but declined rapidly. Eventually, the state of the art has evolved to high rate slickwater fracs with various additives. The question to be addressed is how do the various additves perform in shale and how do we select which additves are necessary particularly in light of the fact that most fracs are now conducted with produced and/or flowback water from previous fracs. Selecting a method of extracting the gas is crucial in how one should stimulate the shale pay. The mechanical properties indicate that horizontal wells may be a viable option. Whether vertical or sub-vertical wells are drilled, there will be a variety of stimulation options available, with the selection of the fluid and additives being based upon the mineralogy. Fluid additive selection needs to take into account the:Tubulars and pumping rate and pressuresHigh percent of clays.Potential generation of fines both siliceous and organicAcid solubilityMicrobiological activityPotential for scale generationProblem with recovering injected fluids

A review on hydraulic fracturing of unconventional reservoir

“…The evolvement of the fractures around the main fracture would increase the regional permeability, but it also could lead to significant leakoff, which could limit the development of hydraulic fracture. However, it is hard to determine whether these fractures are in open state and stay as viable flowpaths [47,72]. The key factor that can blunt the fracture propagation is shear sliding along the interface [28] and Anderson et al [6] found that if the frictional properties changed along the interfacial surface close to hydraulic fracture, the path of the fracture could be alerted.…”

A critical evaluation of unconventional gas recovery from the marcellus shale, northeastern United States

“…Resins are used near the end of the stimulation process to hold proppant in place; ceramics are used when a stronger proppant is required. Ceramics also have a lower specific gravity than sand, allowing them to remain buoyant in the slickwater (Kaufman et al, 2008). Gels are used to increase the viscosity of the water, allowing it to carry proppant more effectively, but these are not recommended for brittle shale stimulation.…”

Effect of polymer on gas flow behavior in microfractures of unconventional gas reservoirs

“…Friction reducers that are mainly polyacrylamide based serves as one of the primary additives in slickwater fracturing fluid to reduce the friction drag related to high pump rate. The friction reducer could change the turbulent flow to laminar flow by interacting with turbulent eddies, and the reduction of energy loss could reach as much as 20%e80% in laboratory experiments and 30%e90% in field applications compared with fresh water (Kaufman et al, 2008;Rimassa et al, 2009;Shah and Kamel, 2010;Sun et al, 2010;Lindsay et al, 2011;Paktinat et al, 2011). The concentration of polymer friction reducer is about 1 wt% in slickwater fracturing fluid, but a great amount of polymer would remain in microfractures after flowback considering the large volumes of fracturing fluid required in slickwater treatments (Carman and Cawiezel, 2010;Zhou et al, 2011;Sun et al, 2014).…”