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

TX 75083-3836, U.S.A., fax 01-972-952-9435. AbstractDuring fracture stimulation treatments, particle-laden slurries may damage pumping equipment and have been shown to erode perforation tunnels. During subsequent flowback operations, minor to severe erosion of surface equipment may be observed if formation sand and/or proppants are produced. This paper compares the erosivity of sand-based and ceramic proppants. This paper will examine the erosion mechanisms and demonstrate that theoretical and laboratory measurements are in agreement that low density ceramic proppants, due to the superior roundness and sphericity, are less erosive than more angular sand products.Real-world results from the oilfield, the waterjet industry, pump and valve manufacturers, and the abrasive industry will be reviewed to substantiate the conclusion that proppant angularity is a dominant parameter that increases erosion in typical pumping and flowback conditions. Impingement tests designed to simulate flowback conditions demonstrate that s ubstituting a spherical lightweight ceramic may reduce wellhead or other impingement surface erosion by up to 95% compared to an angular sand. The highest quality frac sand was nine times more erosive than lightweight ceramic (LWC) in impingement studies. Additional studies utilizing intentionally crushed proppant to increase angularity demonstrate that proppant shape is a dominant factor which controls erosion. Surprisingly, flowback testing has shown that resin coatings applied to proppant have the potential to increase erosion by 15-fold. The authors believe this is partially due to a focusing effect attributed to electrostatic charge. Test alterations with multiphase flow have reduced, but not eliminated, this effect.Slurry abrasion testing has shown that the highest quality frac sands are over 250% as abrasive as LWC. Although frac sands are often specified to withstand several thousand pounds of closure stress, significant sand degradation was observed after exposure to a reciprocating five pound weight. Erosivity and abrasivity of all proppant types were shown to increase with proppant damage. Erosion within chokes was found to be three times higher with sand than with lightweight ceramic.Despite conventional wisdom to the contrary, a number of recent laboratory studies have demonstrated that spherical proppants are less likely to flow back from the fracture as compared to more angular products. All data indicate that equipment erosion occurring during the stimulation treatment and subsequent flowback can be substantially reduced through the use of spherical lightweight ceramic proppants.

Anionic polyacrylamide copolymer friction (or drag) reducers are commonly used in various well stimulation jobs. The effectiveness of friction reduction polymers strongly depends on the compatibility between friction reducing polymers and stimulation liquid to which they are added. Performance of friction reducers can be strongly influenced by the presence of salts, very high or very low pH, or other typical process additives, such as biocides, corrosion and scale inhibitors, hydrogen sulfide (H2S) scavangers, etc. Since most friction reducers are emulsion polymers, an important issue related to their performance is their ability to invert effectively and efficiently upon the addition to the stimulation fluid. A fast enough rate of inversion is especially important, since it determines both the effective onset of optimal friction reduction and its magnitude. Higher friction reduction achieved at the very beginning of a well stimulation will decrease the pumping pressure on a job. We hereby describe novel, fast-inverting friction reducers suitable for effective use in brines and compatible with various additives, such as biocides, clay control agents, and scale inhibitors. Introduction Slickwater fracturing is commonly used nowadays in stimulation treatment of tight gas reservoirs. Common slickwater frac fluids usually contain friction (or drag) reducers, which are used for the purpose of reducing friction pressure during pumping and therefore boosting the efficiency of pumping trucks [1]. Commonly used friction reducers are typically high molecular weight polymers that are believed to cause friction reduction by interacting with eddies of turbulent flow [2]. A number of studies have linked the performance of friction reducing polymers with their rheological behavior in extensional flow fields, and especially their ability to reach an extended conformation and resist degradation of molecular weight due to the action of shear forces [3–6]. More effective friction reduction is typically expected from polymers with larger molecular size in solution (radius of gyration). The latter is known to be strongly influenced by the solution composition, such as the type and concentration of dissolved salts and the presence of various other additives common in frac fluids, such as biocides, oxygen scavengers, scale inhibitors, etc. [7,8]. Although friction reducing polymers may come in many forms, anionic emulsion polymers have lately been friction reducers of choice in most applications. Commercial emulsion polymers typically comprise water-in-oil emulsions of high molecular weight copolymers of acrylamide with different anionic monomers (e.g. acrylic acid). In such emulsions, the active polymer is "packed" inside water droplets dispersed in the continuous oil phase. Upon contact with aqueous-based ambient fluid, emulsion polymers undergo what is commonly known as an "inversion process" during which the high molecular weight polymer is released from the emulsion droplet into the ambient fluid. One can identify several critical steps in the "inversion process" that make performance of friction reducing polymers effective. First, the polymer emulsion has to mix well with the carrier water. Second, the inversion itself must take place. During the actual inversion step, the polymer is released from water-in-oil emulsion droplets into the carrier fluid and the oil originally present as the external phase in the polymer emulsion becomes emulsified forming an oil-in-water emulsion. The released polymer then undergoes swelling (hydration) and subsequent disentanglement. Turbulent friction is reduced once the polymer has migrated to the near-wall boundary layer where its interaction with eddies of turbulence promotes the reduction of friction. Within the content of this paper the term "inversion" would typically imply all of the above described steps. In most applications, emulsion polymers are added to frac fluids "on the fly" which puts certain demand on the ability of polymers to invert efficiently and to reach the desired level of friction reduction in both water and brines rapidly.

Friction reducers have been an integral part of the oil and gas industry for many years. They possess very unique properties of reducing friction pressure associated with the flow of fluid in tubulars. Friction pressure loss or hydraulic characteristics of friction reducers depend greatly on how they are tested and evaluated in the laboratory. Today, there is no standard procedure for their evaluation. American Petroleum Institute (API) oversees the development and publication of industry standard practices for various fluids and materials. Recognizing the need for a standard testing procedure for friction reducers, a Committee made up of members from industry and academia was formed and charged to develop a document outlining the standard procedure. Round-robin tests were conducted by four industry organizations and one academic institution, employing their in-house flow loops and were requested to report the results. Tests were to conduct friction pressure measurements of friction reducers, and to develop and deliver to the industry a standard procedure and method to measure and analyze friction reducers data in straight pipes. The test fluids chosen were two friction reducers: one anionic and the other cationic. Water data were also gathered as base line. Same fluid samples were submitted to all laboratories. The calibration procedure and fluid testing procedure was developed and distributed to all involved in fluid testing. The analysis for data reduction and for reporting results was also developed and distributed to all. It was found that the calibration procedure was more critical than originally thought. The determination of internal diameter of the circular tube is the most important parameter that influences the friction pressure loss results greatly. In this paper, the details of various flow loops, calibration procedure, data analysis procedure, and results obtained with water as base line and two friction reducers are presented and discussed. A standard procedure for testing and evaluating friction reducers for their friction loss properties is outlined. Following this standard procedure and carefully performing the testing with friction reducers will yield very similar results among various laboratories in the industry. This will make it easy when comparing the performance of friction reducers for their friction loss properties from different organizations.