The Median Ratio and Near Wellbore Friction: Useful Proppant Admittance Criteria for Design and Placement of Safe and Effective Propped Hydraulic Fracture Treatments
Authors: Massaras Leon V. , Massaras Dimitri V. , Al-Subhi Salim 
Published: May 2011

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.