Factors Affecting the Stability of Proppant in Propped Fractures: Results of a Laboratory Study

Abstract

Factors controlling the stability of proppant in propped fractures were identified and investigated experimentally. The results indicate that the absolute size, distribution and type of proppant may affect stability, and hence the likelihood of proppant flowback. The extent of embedment of the proppant into the rock determined by closure stress and rock hardness) was found to play a key role in stabilising the pack. in addition, channels, which may form due to proppant settling before fracture closure, were found to significantly reduce stability. These are, therefore, favourable sites for proppant flowback. A numerical study has been used to determine lengths of unpropped fractures which may be maintained open due to the rock stiffness. The implications of these results for well productivity and the understood mechanisms of proppant flowback, in the field, are discussed. Introduction The flowback of proppant from hydraulically fractured wells is of significant concern to the industry. In addition to problems of valve, line and choke erosion, there is the potential for a loss in near wellbore fracture conductivity. Furthermore, expensive separation facilities are required to filter out proppant from the hydrocarbons, and these may necessitate manning the rig. While substantial work has been carried out to identify means of reducing the amount of proppant flowed back, far less effort appears to have been directed towards understanding the mechanisms which determine whether or not a given well will back produce proppant. Hence it is difficult to decide a priori what preventative measures should be taken. The uncertainty has been concisely put by Daneshy (1) who said of techniques to reduce flowback "None of these methods works all the time, and no criteria are available to know when to do what". Various methods have been devised to reduce proppant flowback. These include resin-coating the proppant, installing mechanical screens and modifying the completion design. Resin-coated proppants have been used successfully in certain environments. P. 569^

Mechanics of light weight proppants: A discrete approach

“…Formation of a stable pack is easier with softer rock platens which permits deeper particle embedment [15] iii) Presence of softer deformable particles in the pack increases flowback resistance, but will result in loss of porosity and subsequent loss of fluid flow rate [49][50][51].…”

“…Choking is observed while emptying of silos due to the formation of arches. Conversely for the proppants a stable arch has been found to be essential for preventing flowback and maintaining a stable pack [14,15]. Also the linear relation between the filling height and draining time in case of an hourglass filled with fine sand is due to interparticle friction.…”

“…A modification to the conductivity test wherein an arrangement is made to collect loose proppant and measure their flow rate is used to study proppant flowback. Several researches have focused on understanding the factors affecting proppant flowback and finding ways to reduce it [14,15,[49][50][51][52]. Apart from the DEM (discrete element method) simulations by Asgian [14] the rest focus on experimental approach.…”

Fracturing Fluid Characterization Facility

“…Previous numerical and experimental investigations of the flowback phenomena concluded that a proppant pack is stable when the range of the ratio of the fracture width to proppant diameter is between the values from three to six. [27][28][29] But these studies did not observe that these ratios are dependent on the cleanup flow rates; on the other hand, the FFCF results 25 show that the proppant flowback can occur within the recommended ratios, albeit at higher fluid rates as compared to the rates for the higher ratios.…”

Compaction of granular material inside confined geometries

“…Such an accumulation process is known to occur in the petroleum industry, where sand is liberated from the host rock during extraction, altering the underground morphology of cracks [5,6]. This may also be relevant for understanding proppant flowback in propped fractures [7]. Additionally, this geometry is representative of a number of recent experimental studies in Hele-Shaw cells [8][9][10][11] where the validity of Janssen stress redirection has not been ascertained.…”