A Study of Hydraulic Fracture Orientation by X-Ray Computed Tomography (CT)
Authors: Krilov Z. , Goricnik B. 
Published: December 2007

Presented in this paper is a novel technique of hydraulic fracture azimuth determination. Full size oriented cores, retrieved after microfracturing, were scanned using X-ray computed tomography (CT) to evaluate the fractures. The SEM/EDAX analysis was applied to confirm the presence of barite particles from drilling mud inside hydraulically induced fractures. The study, performed on core samples from the same sandstone formation of two adjacent fields, showed the good agreement of fracture azimuth data, obtained from CT analysis. Postfrac well production history indicates a significant hydrocarbon productivity increase, without interference to surrounding wells.

Introduction

Orientation of hydraulically induced fractures has a significant impact on final results of hydraulic fracturing operations.

Knowledge of hydraulic fracture orientation can be useful in many reservoir applications. Hydraulic fracture azimuth prediction becomes important in terms of improving recovery efficiency in case of producing wells, or optimizing the areal sweep efficiency in case of water flooding or EOR applications using injection wells. When designing the injection pattern and selecting optimum well locations, fracture azimuth should not be ignored. For waterflooding producing wells located perpendicular to the fracture direction will experience better areal sweep efficiency, than wells, situated parallel to the fracture direction. On the other hand, if fracture orientation can not be predicted, and spacing of wells is less than the designed propped fracture length - the wing of fracture can aim to the neighboring well, causing the failure of both wells. Also, if geological condition cause favorable fracture direction, the wing of fracture can reach the hydraulically isolated part of the reservoir, making it recoverable.

A number of techniques and methods for mapping or predicting fracture orientation can be found in literature. These can be summarized as:active fracturing techniques (tiltmeter arrays, triaxial borehole seismic),openhole logging techniques (borehole elongation orientation, television camera, sonic televiewers, impression packers), andpredictive oriented core techniques (strain relaxation, compressional-wave velocity, thermal expansion, differential strain curve, fracture point load test or residual stress measurement).

Although each of them under proper conditions can give more or less accurate and reliable results, each has limitations.

In this paper, a new approach to indirect fracture azimuth measurement, based on oriented core analysis is described. The method involves microfracturing technique (Fig. 1) and X-ray CT scanning of oriented cores.

During drilling, just after entering a zone of interest for future stimulation by hydraulic fracturing, the drilling process is temporarily interrupted and microfrac job, using relatively small volume of water base - barite weighted mud is performed. This is followed by coring operation and 3 to 10 m of full diameter oriented core is taken from the bottom of the well. The drilling procedure is then continued.

Conventional core analysis is performed by visual inspection or by the use of goniometer to characterize fractures, if found on the core surface. X-ray computed tomography is used for visualization and investigation of fractures inside the rock body.

Consecutive CT scanning of oriented core (Fig. 2), is made by taking axial cross sections subsequently reconstructed as tomogram images. Since the core orientation during scanning is known and fixed, fracture azimuth is easily determined (see Fig. 3).

Analysis of tomogram series furnishes data on fracture growth and position in the core. Hydraulically made fractures can be filled with mud used for microfrac operation. The presence of solid particles, particularly high density barite, can be easily detected in CT tomograms, to distinguish hydraulically initiated fractures from naturally generated ones. Also, the traces of microfrac fluid can be analyzed after location and detection in the fracture by CT scanning, using the other analytical methods, such as SEM/EDAX or chemical analysis of selected rock specimens.

Using the data from tomograms, 3-D reconstruction of hydraulically initiated fracture was made.

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