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
