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Abstract

The procedures of the Rate Step-down Test (RST) also referred to as “Step-Down Test” (SDT), and of Conventional Fracture Entry Friction (FEF) Analysis have been used worldwide for over 15 years to separate the Measured Total System Friction (MTSF) into constituent components. The magnitude of the FEF components has served as guidance to design and/or modify the placement procedure and treatment schedule, in order to reduce unplanned terminations (screenouts) of propped hydrofrac treatments.

Conventional FEF Analysis has inherent limitations which produce non-unique (somewhat erroneous) results that can only be overcome with Enhanced FEF Analysis methodology; which bypasses these limitations with innovative methods: appropriately corrected fluid friction factors, non-uniform perforation discharge coefficient, application of Maximum Drag Reduction (MDR) asymptote, and, matching of MTSF with Calculated Total System Friction (CTSF) at all rates of the RST. Several RST examples are analyzed with FEF Analyzer to illustrate and document the success in placing safe and effective propped hydrofrac treatments.

Real-world analogies are used as visual aids to explain the complex processes of FEF, along with RST design methodology, RST execution procedure, and detailed calculation procedures for performing Enhanced FEF Analysis.

Net-pressure history matching is significantly improved, due to the increased accuracy of calculating the magnitudes of the wellbore friction and of the FEF components.

Introduction

FFF is the most serious impediment to placing safe and effective propped hydraulic fracture treatments. The advent and widespread use of the RST, along with application of conventional FEF Analysis methodology reduced the abnormally high percentage of screenouts prevalent since the inception of hydraulic fracturing technology in 1947.

The major limitations of conventional FEF Analysis have been: over or under estimating both the MTSF and the CTSF, and, difficulty in achieving a unique (correct) solution. These limitations result from: 1) limited ability to separate wellbore friction from perforation friction, 2) application of incorrect correction to fluid friction factors, 3) absence of plotting and matching feature to compare MTSF with CTSF, 4) ignoring the effect of restriction(s) in the flow path.

FEF continues to adversely affect the safe and effective placement of propped hydraulic fracturing treatments in some regions of the world, a condition which can be improved with Enhanced FEF Analysis, which provides unique (correct) results by: 1) correctly extrapolating fluid friction data for a given conduit diameter to various conduit diameters, 2) applying correct corrections to the fluid friction factors, 3) correctly calculating friction in the perforations (by applying non-uniform perforation Discharge Coefficient) and in the near-wellbore region, 4) accounting for pressure losses due to restrictions in the flow path, 5) applying Maximum Drag Reduction (MDR) methods for improved prediction of friction factors for turbulent flow, 6) matching of MTSF to CTSF at all flow rates of the RST, 7) accurately separating MTSF into wellbore friction and FEF, and, 8) accurately separating FEF into perforation friction and near-wellbore (NWB) friction.

Computer analyzed case studies are presented, which illustrate: 1) the correct procedure for the RST, 2) the methodology of Enhanced FEF Analysis 3) the high level of accuracy possible, and, 4) how to increase the percentage propped hydrofrac treatments that can be placed safely and effectively.