Crucial effect of system compliance on the maximum stress estimation in the hydrofracturing method: Theoretical considerations and field-test verification

Takatoshi Ito,Osam Sano,Hisao Ito,Harumi Kato,Akira Igarashi

doi:10.1186/bf03352601

Abstract

Abstract Why do the currently available data sets from field hydrofracturing tests indicate that the measured re-opening pressure lies close to the shut-in pressure far more often than can reasonably be expected? In order to explain such a strange phenomenon, it is necessary to take into consideration two additional factors, those of (1) a residual aperture of fracture and (2) hydraulic compliance of the test equipment, both of which are ignored in the conventional theory of hydraulic fracturing. The residual aperture causes pressure penetration into the fracture prior to opening, and its effect is to reduce the re-opening pressure by a factor of two from the value expected when the conventional theory is used. The conventional theory implies that the fracture always begins to open at a borehole pressure less than the shut-in pressure. However, due to the effect of large hydraulic compliance, the reopening pressure measured in the conventional manner becomes larger than the true reopening pressure and approaches the shut-in pressure. Contrary to this, the reopening pressure measured using the test equipment with sufficiently small compliance represents a good estimate of the true reopening pressure. This pressure is related to the maximum horizontal stress S H, and its measured value allows us to estimate the value of S H.

Highlights

It goes without saying that a knowledge of stress magnitudes and orientation is essential for understanding crustal dynamics
After the borehole pressure is beyond Pr0, the fracture begins to open
The results show that, despite assuming the small residual aperture of 2 μm, residual aperture causes significant pressure penetration into the fracture

Summary

Introduction

It goes without saying that a knowledge of stress magnitudes and orientation is essential for understanding crustal dynamics. This result shows that, as long as the borehole pressure P is smaller than the value of Sh (=10 MPa), the fracture surfaces are compressed by the stress SN with a value larger than P except in the vicinity of borehole wall. This means that the open portion of fracture is limited in the vicinity of the borehole wall, even if fluid pressure equal to the borehole pressure distributes uniformly inside the fracture and it pushes the fracture surfaces against SN Following this reasoning, the fracture aperture hardly increases as long as P is less than the value of Sh. with increasing P, the stress concentration in compression is weakened gradually due to the effect of the third term on the right side of Eq (6) and, the open portion of fracture becomes larger. This result shows that Q is not the key factor underlying the discrepancy between Pr(a) and Pr0, while the discrepancy will become larger than in the case of Fig. 2 with increasing Q, as has been shown in Ito et al (1999)

Variation with system compliance

Findings

Conclusions