Fracture Statistics from Horizontal Wellbores
Authors: Belfield W.C. , Sovich J.P. 
Published: March 2010

With the advent of horizontal wells and the use of wellbore microscanners, the ability to analyse naturally fractured reservoirs has increased markedly. The microscanner provides an estimate of aperture size, fracture orientation, and the nominal depth for each intersected fracture. Depending on the length of the well, up to several thousand fractures can be identified and measured in a single well. We report on the statistics of over 13,000 fractures from several horizontal wells in an Austin Chalk reservoir. Of particular interest are the scaling characteristics of the fractures.

The fractures have a single preferred strike orientation (N74E) with a standard deviation of less than 10 °. In all wells there is a high probability of finding a fracture within any 1–5 m (5 ft.) interval. This demonstrates a homogeneous fracture distribution along the wellbore and implies similar homogeneity in the reservoir. However, the probability decreases when considering different fracture subsets characterized by an aperture cutoff.

Aperture size appears to have a log-normal distribution, but the smaller values approach the tools' resolution. Resolution also affects average fracture spacing. By including increasingly smaller aperture fractures, average fracture spacing decreases with power law behaviour. Spacing variance also decays in a hyperbolic manner.

One objective in analysing these statistics is to create a basis for generating stochastic simulations of fracture networks. The intent is to provide probabilistic answers to questions that arise during the evaluation of naturally fractured reservoirs. One possible approach is discussed.

Introduction

One objective of drilling horizontal well bores in naturally fractured reservoirs is to attempt to intersect a greater number of fractures than could otherwise be intersected by a vertical well. Commonly. this leads to higher production rates than for a vertical well. Often overlooked in this idealistic approach is the necessity of characterizing Fracture distribution in the reservoir- Differences in fracture density, aperture, and connectivity translate into varying production rates drainage radius and recovery.

In development of Fractured reservoirs with horizontal wells, proper well placement is critical to achieving maximum benefit. One approach to optimizing well placement is by stochastic simulation of fracture distribution and analysis of the multiple realizations. This will allow judgments to be made based on a probabilistic evaluation of these alternatives. Any stochastic model must rely on statistical inputs, in this case the statistics acquired from microscanner logs in six horizontal wells.

The wells are directionally drilled in an East Texas Austin Chalk reservoir, that produces solely from fractures. Individual wells have lateral extents ranging from 600 m (2,000 ft.) to approximately 1,200 m (4,000 ft.) The single reservoir unit is approximately 2,600 m (8,500 ft.) deep and is 26 m (85 ft.) thick. [n studying the statistics we focus on the scaling properties of the fracture system. This approach is possible because of the large amount of quantitative data gathered in the horizontal wells with the microscanner logs.

Fracture Orientation and Characteristics of Scale

Over 13,000 Fracture images from microscanner logs were analysed for orientation and aperture size.