Criteria For Proppant Placement and Fracture Conductivity
Authors: van der Vlis A. C. , Haafkens R. , Schipper B.A. , Visser W. 
Published: December 2007

American Institute of Mining, Metallurgical, and Petroleum Engineers, Inc.

The design of a hydraulic fracturing treatment has to include the recipe of the treatment and an estimation of the expected productivity improvement. For such a frac-job design one has to decide which criteria should be applied for proppant admittance and placement. From the resulting propping-agent placement. From the resulting propping-agent concentration in the fracture a conductivity value has to be derived for predicting the productivity improvement.

For low-viscosity fracturing fluids and proppant concentrations up to 5 lb/gal, this paper recommends a value of 2.0 for the admittance criterion, which is defined as the ratio between fracture width and maximum proppant diameter. High-viscosity fracturing fluids require a ratio of 2.6 for proppant concentrations up to 8 lb/gal. Fluids carrying this concentration of proppant can be pumped without triggering internal bridging. The latter value is recommended as a maximum placement criterion.

Empirical relations have been developed which allow a sufficiently accurate estimate of the conductivity for fractures propped with sand, glass beads and walnut hulls. The Brinell hardness number is used as a practical parameter for rock-strength classification, which number can be established by a very simple measurement.

The conductivity relations and the placement and admittance criteria facilitate computerisation of hydraulic frac-job design.

Introduction

The purpose of a hydraulic-fracturing treatment design is–the development of a treatment recipe and–the estimation of productivity improvement.

A vast amount of well-completion and reservoir data is needed for design calculations. The data should preferably be derived from available drilling reports, preferably be derived from available drilling reports, core analyses, pressure build-up tests, etc. With the data gathered, a fracturing design should be made such that the propped length of the fracture, as well as its conductivity, are optimal. This means that injection of proppant should be programmed to arrive at the proppant should be programmed to arrive at the maximum allowable concentration in the fracture.

To decide whether injection of proppant can be started, one has to know the width of the fracture on proppant arrival and the minimum width required for proppant arrival and the minimum width required for proppant admittance. Various theories allow proppant admittance. Various theories allow estimation of the fracture width during the fracturing process. Since their basic assumptions differ, widths process. Since their basic assumptions differ, widths calculated from them will also differ. Allowance has to be made for some tolerance in fracturing design. To determine the minimum width required for admittance and placement of various proppants, experiments were carried out under simulated fracture conditions using low- and high-viscosity fluids.

Attempts to establish theoretical relationships describing fracture conductivity-in terms of proppant type, size and concentration, closing pressure proppant type, size and concentration, closing pressure and formation hardness - have been unsuccessful.