Propagation of Hydraulically and Dynamically Induced Fractures in Naturally Fractured Reservoirs

Abstract

The purpose of this project has been to provide a basis for selecting fracturing treatments provide a basis for selecting fracturing treatments that will cross and connect as much of the natural fracture system to the wellbore as possible. Interactions between induced and existing fractures have been examined in scaled laboratory experiments under different angles of approach and states of stress. For hydraulic fractures, when the angle of approach was 30 degrees, existing fractures tended to open, diverting fracturing fluid and preventing the induced fracture from crossing. At angles of approach of 60 and 90 degrees, hydraulic fractures tended to cross existing fractures when the differential stress was high enough. Dynamic fracturing tests were run for conditions where hydraulic fractures opened existing fractures to determine if more constructive crossing interaction could be obtained, but triaxial stresses tended to suppress cross-cutting dynamic fractures, and no crossing behavior was observed. An elastic solution for the stresses in the interaction zone has been used as a basis for a hydraulic/natural fracture interaction criterion. The criterion compares favorably with the experimental results. It has been used to delineate areas where crossing and opening interactions could be expected in Devonian shales of the Appalachian Basin by combining it with existing data on the state of stress and fracture patterns. Under preliminary assumptions, the criterion predicts opening of natural fractures in the central and western parts of the basin. In the northern and southern parts of the basin, there are zones where crossing interaction can be expected. Tectonic models were used to extrapolate fracture patterns from site specific core data gathered from 33 EGSP wells. Fracture systems oriented for crossing appear to be associated with two tectonic features: a Precambrian failed rift system that has been Precambrian failed rift system that has been reactivated periodically and deformation associated with the Appalachian orogeny.

Introduction

Tapping the natural fracture system has been recognized as essential to obtaining significant well stimulation in Devonian shale reservoirs of the Appalachian Basin. The purpose of this project has been to provide a basis for selecting fracturing treatments that will cross and connect as much of the natural fracture system wellbore as possible. There have been three specific objectives. possible. There have been three specific objectives. The first has been to develop a criterion, based on scaled laboratory experiments, for predicting the type of interaction that can be expected between induced and natural fractures under various angles of approach and states of stress. The second objective has collect data on the current state of stress fracture patterns in the Appalachian Basin so that the interaction criterion can be applied in the field. The final objective has been to synthesize the interaction criterion and field data in that specific types of fracture interaction can be predicted in different parts of the basin.

FRACTURE INTERACTION EXPERIMENTS

Test Conditions

The configuration of the fracturing experiments (Figure 1) is intended to model a wellbore located between two regularly spaced vertical fractures. The two parameters varied in the experiments were differential stress, S1-S3, and the angle, a, between the existing fractures the maximum horizontal stress, referred to as the angle of approach. Differential stresses were varied from 100 to 1600 psi, with a 600-psi minimum stress in each test.

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