Flow Visualization Technique for Investigation of Hydraulic Fracturing
Authors: Hejjo Hazem , Fagan John E. , Shah Subhash N. 
Published: December 2007

This paper presents a methodology for the determination of dynamic proppant concentration mapping in the High Pressure Simulator (HPS). It also presents a model that is being employed to real time display:the sand front propagating through the slotthe proppant concentration at the HPS facings, andthe dune build up as it occurs.

The paper will present the overall software and hardware schemes used to accomplish this visualization task, snapshots of the real time videos produced by the technique, and how the technique is used.

Introduction

In the pursuit of characterizing fracturing fluids, a vision system has been developed to profile the proppant transport in fractures. This system will not only provide slot flow observations such as dynamic proppant settling and dune formation but also determine, for the first time, the proppant concentration as a function of time and position. The vision system is installed on the HPS in the Fracturing Fluid Characterization Facility (FFCF) at the University of Oklahoma. Optical fibers embedded in the HPS are used to access the inside of the simulator. Light Emitting Diodes (LEDs) embedded in the HPS against the optical fibers are used to transmit light across the slot. Slurries flowing in the slot affect the light transmitted by the LEDs resulting in an image whose light intensity levels (gray levels) are related to the quantity of proppant suspended in the fracturing fluid. The effect the slurry has on light intensity is manifested in the increase or decrease of light intensities collected by the fibers.

A methodology presented in this paper involves real time processing (digitizing, quantifying, mapping, displaying and storing) of video data during slurry flow in the HPS. The method measures the increase and decrease of light intensity due to sand flowing through the slot and maps those intensities by using a calibrated standard to proppant concentration. Image processing tools such as black and white Charge Coupled Device (CCD) cameras, Digital Signal Processing (DSP) frame grabber, Enhanced Display Board (EDB), and frame accurate video recorders are used to achieve the task.

Furthermore, due to the hostile imaging environment in the HPS, the raw images acquired during a proppant test are noisy and not uniform. Therefore, a display model has been developed to recognize useful signs and generate images based on those signs eliminating the noise and nonuniformity.

Slot Flow Model

The slot used in the calibration tests is the FFCF High Pressure Simulator. The slot internal dimensions are 7 ft high and 9.3 ft long and the fluid enters and exits the slot through perforation manifolds representative of a wellbore The slot width can be adjusted dynamically over the range of 0 to 1.25 in. by a system of 12 hydraulically actuated platens. Each platen is 28 in. by 28 in. square and the platens are laid out in (3×4) matrix to form one face of the simulated fracture, see Fig. 1. Each platen surface can be covered with a replaceable simulated rock facing (1 in. thick). 9 of the 12 platens are equipped with instrumentation. Those nine platens (platen number 1, 2, 3, 5, 6, 7, 9, 10, and 11) are arranged in a (3×3) matrix and oriented toward the outlet. The inlet and exit manifolds 2.75 inches in diameter are outfitted with 22 perforations whose configuration and size can be easily changed using a series of blank and sized inserts.

20–40 mesh sand were used in all calibration flow tests. Sand-laden fluid was prepared in a 200 gal ribbon blender. Various quantities of dry sand were mixed with fluid in the ribbon blender producing calibrated sand densities. The sand-fluid mixture was pumped into the slot using a Moyno pump (6P10). The upper limit of operation of the Moyno pump is approximately 140 gpm at 600 psi. The fluid flow rate was controlled by the Moyno pump setting. The average sand concentration in the sand-laden fluid was determined by the various combination of these rates.

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