Approximately constant specific storage

at fixed value of 175 IVPa, despite the accumulation of 0.8% axial strain

/ General trend of 'normd

loafing path (TEN16)

Axial load reduced from 298 MPa (near peak

stress) to 225 fVPa

50 100 150

Differential Stress (MPa)

200 250 300

Figure 7.13: The ejfect on a) permeability and b) specific storage o f reducing the differential stress (or ‘offloading’) to fixed values (175 MPa and 225 MPa) during dilatancy (sample TEN 16). Both properties are plotted as a function o f differential stress in order to compare measurements made over a range o f axial strain (-0.8%) at a fixed value o f differential stress (sample TEN 16).

7.11.3 Results o f the Effect o f Reducing the Axial Load to the Differential Stress Corresponding to the Differential Stress on the Normal Loading Path at which the Previous Procedure o f Reducing the Axial Load was Undertaken and Permeability Measured 3 .5 -- 2.5 - 5 2 = 1.5 -- 0 .5 4

* N ormal axial loading (Ten 27) m offload from 229 M Pa to 213 M Pa

m offload from 246 M Pa to 22 9 M Pa

m offload from 265 M Pa to 239 M Pa X offload from 277 M Pa to 264 M Pa

Permeability measurements . V made at sequentially highdr

differential stresses

Approximate trend lof permeability

Onset of dilatant microcrack growth (D) and concommitant

permeability increase

Compaction and associated i

permeability reduction

/

V

♦ /

/ Increasing axial strain

50 100 150 D ifferential S t r e s s (M Pa) 200 250 300 B) 2.5 n 2 (O 1.5 0.5

* thermal axial loading (TEN27) # offload from 229 MPa to 213 MPa A ofload from 246 MPa to 229 MPa 1 offload from 265 MPa to 239 MPa

X offload from 277 MPa to 264 MPa

Approximate trend of specific storage

Specific storage measurements made at sequentially higher

differential stresses

A - *

100 150

Differential Stress (MPa)

200 300

Figure 11.14: The effect on a) permeability and b) specific storage o f reducing the differential stress from the normal loading path during dilatancy. In this

stresses (sample TEN27). Experiments conducted following this loading procedure are considered to be representative o f a ‘normal ’ loading path and

will be referred to as such throughout the thesis.

Figures 7.14a and b show the evolution of permeability and specific storage as a function of axial strain. All data are normalised to the values of permeability and specific storage initially measured under hydrostatic stress conditions at the start of the experiment, i.e. the initial starting permeability and specific storage (see Tables 10.1 and 11.1). Measurements are categorised into those made following the normal axial loading path and those made where the axial load was reduced to the differential stress at which the last permeability and specific storage measurements were made following the sample procedure of reducing the axial load (Figure 7.12b).

Both permeability and specific storage generally increase as each measurement is made at sequentially higher differential stresses, so that at each point that the permeability measurement is made, reducing the differential stress to the previous differential stress at which an offloading procedure was conducted, results in relatively less closure of axial microcracks than the former procedure (Figure 7.12a).

Both test types exploring the effect of reducing the axial load on the evolution of permeability and specific storage during dilatancy emphasise both the difficulty in making such measurements, and the complexity of changes in the permeability and pore structure as peak stress is approached. Both permeability and specific storage decrease as axial load is reduced due to the relative closure of axially aligned dilatant microcracks. However, in the second type of experiment, since permeability and specific storage are measured at sequentially higher levels of differential stress, both properties generally increase as peak stress is approached. These measurements may still underestimate both properties at a given value of axial strain as reducing the axial load results in the relative closure of axial microcracks. However, this second methodology is considered to be the more useful technique for exploring permeability and specific storage changes during dilatancy and is the methodology adopted throughout this experimental programme. Thus, in all subsequent analyses, measurements derived following this methodology will be described as measurements made following a ‘normal’ loading path.

Furthermore, these investigations have emphasised the significant role of the axial differential stress in controlling permeability and specific storage. The roles of the non-hydrostatic stresses in controlling permeability will be fully discussed in Chapter 11.

7.12 Chapter Summary

This chapter documents the principal technical problems encountered and considerations undertaken whilst establishing the single-ended transient pulse permeameter, and outlines a number of

practicalities in making transient pulse permeability measurements. A number of key factors which affect the measurement of permeability have been highlighted and discussed. Factors include effects resulting from the compressive storage of the pore fluid reservoir, the effect of a downstream reservoir, the effect of the magnitude of the instantaneous change in pore fluid pressure of the transient pulse measurement, the effects of temperature in terms of the adiabatic thermal change associated with generating a transient pressure pulse and ambient temperature effects, the possibility of leakage of pore fluid between the jacket and sample, and the effect of active dilatancy during the measurement of permeability.

All permeability and specific storage measurements are within an accuracy of +/-10%. However, the errors are potentially higher at higher levels of stress (i.e. as peak differential stress is approached) because the damaged state of the rock is changing most rapidly under these conditions. However, by implementing the method of unloading the axial stress, discussed in detail in Section 7.11, we are confident that this reduces the errors at the highest stress levels to similar levels to those at low levels of stress.