Jim D. Weaver

New Guidelines for Applying Curable Resin-Coated Proppants

Both curable-resin precoated proppants (CRCPs) and on-site liquid-resin-coating (LRC) systems are used in hydraulic-fracturing treatments to reduce proppant flowback after fracture-stimulation treatments.

Fracture-Related Diagenesis May Impact Conductivity

Rapid loss of fracture conductivity after hydraulic fracture stimulation has often been attributed to the migration of formation fines into the proppant pack or the generation of fines derived from proppant crushing. Generation of crystalline and amorphous porosity-filling minerals can occur within the proppant pack because of chemical compositional differences between the proppant and the formation, and the compaction of the proppant bed because of proppant pressure solution reactions. Findings presented in this paper suggest that diagenesis-type reactions that can occur between proppant and freshly fractured rock surfaces can lead to rapid loss of proppant-pack porosity and loss of conductivity.

A Study of Proppant-Formation Reactions

Based on long-term API conductivity data, proppants are generally chosen to optimize fracturing cost versus fracture conductivity. This long-term data is acquired by measuring conductivity for two days at simulated conditions of closure stress and reservoir temperature. It is recognized that the majority of the conductivity damage to a proppant pack occurs rapidly during the first day at simulated conditions. It is clear on examination of raw data that conductivity has not reached equilibrium after two days of flow, but rather it is still declining. Investigators who have studied the effect of longer flowtime on fracture conductivity of proppants have all concluded that the decline in conductivity continues for a much longer time. Much attention has been given to the mechanical properties of proppants to aid in the selection of the best material for a particular fracture treatment. These properties include: size, proppant-size distribution, sphericity, hardness, crush resistance, and acid solubility. Other than acid solubility, little information about the chemical reactivity of the proppants has been noted. Recent work has demonstrated that, at some reservoir conditions, certain proppants become significantly chemically reactive. This paper presents the proppant as starting material for geochemical reactions with various aqueous fluids saturated in typical formation minerals. Methods have been developed for rapid testing of these interactions, and test results clearly demonstrate how important proppant chemical reactivity is as losses in proppant-pack permeability of 50–90% were obtained, owing to the genesis of cementatious and porosity-filling minerals. In addition, some proppant chemistries were found to be much more susceptible to these types of reactions, losing much of their mechanical strength in a surprisingly short period of time. Results from this study indicate that operators who rely solely on API test methods in evaluating the suitability of proppants for particular downhole conditions might miss significant damage potential from geochemical reactions occurring between the proppant and formation fluids.

Understanding proppant flowback

Proppant production from hydraulically fractured wells can cause severe operational problems, increase safety concerns, and dramatically reduce economic returns on well-stimulation investments. Methods that have helped eliminate or minimize proppant flowback include modified completion designs, the use of controlled fracture closure for obtaining early closure on the proppant pack, and the use of materials designed to reduce proppant production. Curable resin-coated proppants, chopped fiberglass. thermoplastic strips, and chemicals that modify the surface of the proppant are all accepted methods for minimizing flowback This paper presents the results of both physical and numerical modeling of proppant flowback recorded during the development of a chemical designed for modifying the proppant surface. The goal of this study was to develop an understanding of the mecha nisms that control proppant flowback. Laboratory experiments performed in slot models with no closure stress helped establish the interaction of proppant size, proppant distribution, and fluid velocity. Additional studies of the impact of closure stress, fracture width, and fluid rate on proppant flowback were performed with modified API linear conductivity cells. Data obtained from the physical modeling were used to calibrate a numerical model that predicts proppant flowback. In this model, fluid flow in the proppant pack is described by Darcys equation for flow through porous media. The resulting velocity distribution allowed local stability to be assessed along the free surface between the proppant pack and the continuous fluid phase. Repeating these steps allowed evaluation of the interface that develops over time.

Remediation of Proppant Flowback-Laboratory and Field Studies

This paper presents the results of laboratory studies and field case histories of a remedial treatment technique using a lowviscosity consolidation fluid system that is placed into the propped fractures by coiled tubing (CT) or jointed pipe coupled with a pressure pulsing tool. The treatment fluids are designed to provide consolidation (for previously placed proppant) near the wellbore to glue the proppant grains in place without damaging the permeability of the proppant pack. Laboratory flow testing indicates that the proppant pack in a fracture model under closure stress only requires lowstrength bonds between proppant grains to withstand high production flow rates. The consolidation treatment transforms the loosely packed proppant in the fractures and the formation sand close to the wellbore into a cohesive, consolidated, yet highly permeable pack. Field case histories are presented and the treatment procedures, precautions, and recommendations for implementing the treatment process are discussed. One major advantage of this remedial treatment method is the ability to place the treatment fluid into the propped fractures, regardless of the number of perforation intervals and the length of the perforated intervals without mechanical isolation between the intervals. The fluid placement efficiency of this process makes remediation economically feasible, especially in wells with marginal reserves.