The Median Ratio and Near Wellbore Friction: Useful Proppant Admittance Criteria for Design and Placement of Safe and Effective Propped Hydraulic Fracture Treatments

Abstract

Screenouts of Propped Hydraulic Fracture (PHF) treatments have numerous failure causes, namely, Near-Wellbore Friction, Deviatoric stress, Non-compliant geologic formations, Multiple fractures, Segmented en-echelon fractures, Backstress due to pressure depletion, and, Fracture-tip dilatancy. This paper focuses on the newly-introduced parameter of the Median Ratio (MR) of the Rate Step-down Test (RST) and Near-wellbore (NWB) friction, both of which must be used concurrently as Proppant Admittance (PA) criteria, because screenout causes are not failure diagnosis methods, therefore, not useful in predicting, and/or avoiding screenouts. Each of the PA criteria, while necessary for diagnosis, is not sufficient for accurate prediction of screenout potential, because, when each PA criterion is considered separately it is accurate in 40–45% of the cases, whereas, when both of the PA criteria are used concurrently prediction accuracy increases to over 95%. Therefore, both PA criteria are necessary for accurate Fracture Entry Friction (FEF) analysis, and, prediction of screenout potential. The MR can be determined easily, rapidly, and accurately with the proposed four-equal-step RST procedure. The MR is an empirical function defined as: MR=DP4 / DP1. Concurrent occurrence of: 1) a MR value greater than 0.5, and, 2) a NWB friction value greater that 30 bar (435 psi) is considered: a) an anomaly, b) it is indicative of higher than normal NWB friction, and, c) it is the threshold for PA problems. Both the MR and NWB friction are calculated accurately with enhanced FEF analysis of the RST. The RST has a very short duration, during which, all parameters remain constant: wellbore configuration, perforation configuration, fluid parameters, and fracture dimensions (length, width and height). In addition, pressure loss due to friction is a function of flowrate; hence, progressively smaller pressure reduction steps should be noted as the rate is reduced during the RST. Because all parameters are constant, any deviation from the expected pattern of progressively decreasing pressure loss steps is a strong indication of hindrance to fluid flow, and can only be caused by a restrictive NWB area, and the associated NWB friction. Therefore, the MR and NWB friction are powerful diagnostic criteria of PA, which are useful for the successful design and placement of PHF treatments. The methodology of concurrent usage of the MR and NWB friction, and of the specific four-step RST procedure, has been tested extensively on numerous PHF treatments, in both geologically and geographically diverse conditions. We demonstrate that they provide a high-level of confidence required for pre-mainfrac redesign and modifications to the completion, the treatment procedure, and the treatment schedule, and also, for on-the-fly, real-time decision and control. Utilized wisely, the methodology increases the probability of achieving safe and effective placement of PHF treatments.

Screenout Detection and Avoidance

“…The TW methodology also calls for equipment to be connected to the wellhead, Thus, these requirements tend to increase the cost of most methodologies, so, this chapter presents in detail only three simple methods, two of which are very simple to use and have low cost or no cost at all. The three methods are: Enhanced FEF analysis, Massaras et al [9], Median Ratio (MR), Massaras et al [10] and Inverse Slope (IS), Massaras et al [12]. The first two methods are classified as post minifrac (pre-frac), and the third as real-time, and are presented in detail in Section 4, entitled "Simple Screenout Detection Methods.…”

“…This is explained in more detail in the subsequent sections entitled "fracture tip dilatancy" and "fracture toughness". Extensive discussions on both are provided by Massaras et al [9,10].…”

“…Many theories have been developed and presented regarding the mechanics of fracture tip propagation and fracture tip pressure behavior, which have been summarized by Massaras et al [10]. Fracture tip dilatancy (FTD) is among these theories; however, it is more than a theory-one could say it is a property-as it can be observed in nature (beach), and, modeled, observed, and measured in the lab.…”

“…FTD essentially is influenced by fracture toughness, which dictates that a lot of pressure is required to reopen a very small closed fracture as discussed in the previous section entitled "Fracture Toughness." Massaras et al [9,10] provide an extensive and detailed discussion on FTD.…”

“…Both Conventional and Enhanced FEF analysis require software, a computer, and a trial-and-error solution procedure, as there are several unknowns which prevent a direct solution. To further simplify the analysis and enable rapid on-location determination of appropriate and useful SO potential, or SO onset information, Massaras et al [10] developed the post-minifrac Median Ratio (MR) technique, which uses data from the SDT and stipulates that the SDT must be performed according to a prescribed four equal-step procedure. Massaras et al [11] also developed the proprietary Screenout Index (SOI) technique which is a specialized version of the MR technique used when the SDT is not performed as per the prescribed four equal-steps procedure.…”