high pressure inside the specimen then expelled fluid to yield a final pore

fluid volume very close to the initial value. An alternatively conclusion concerns the high internal pore fluid pressure causing a breach of the copper jacket and a gradual inflow of Nj fluid from the vessel to the sample after point C.

Experiment 0185 (dashed line - 0 = 0 .9 % ), similarly displays initial compaction of « 10% of pore volume (lOmm^) up to SOMPa, and thereafter fluid intake up to 220MPa, resulting in an absolute intake value of 22mm®, at point B. This is similar to test 0166 and again is thought to be the consequence o f grain

rearrangement, associated cracking and pore volume development. Increasing the confining pressure further from 230MPa to 450MPa (B-»D) causes no further appreciable pore volume change. The confining pressure was held constant at 450MPa for somins, during w hich time no pore fluid flo w was detected confirming no compaction or dilation from point B to point D. The spike at point B is unexplained at this stage; representing a piston movement o f only

Upon unloading at a rate of 2.5MPa/min, sample 0185 behaved similarly to test 0166; fluid being drawn in as elastically closed cracks re-open. At point C, the confining pressure was held constant for a number of minutes and pore fluid was further drawn into the sample. This can be explained by low permeability impeding the intake of fluid initially, and the final pore fluid volume datum point representing actual porosity at the end of the experiment. This result reinforces the ruptured jacket theory for experiment 0166 which showed pore volume reduction during the same period.

In summary, the similarities of the tw o sets of results are clear. Ten percent compaction by «SO M Pa, (I5mm^ and 9mm^ in the tw o results of different porosity - 2.0% and 0 .9 % ), and fluid inflow after an increase in confining pressure to 220MPa. Compaction induced microcracking in the rock is thought to induce an increase in permeability sufficiently to allow the flo w of fluid into pre-existing rock pore volume. The final pore volume recorded reflects this permanent increase in permeability.

6.3.3.3.

Darley Dale Sandstone - Drained Experiments.

Fig. 6.3.3.3.a. and 6.3.3.3.b. display the hydrostats of four experiments. All tests were conducted at the same stressing rate of lOMPa/min, except test 0080 which was conducted at 2.5MPa/min.

The different stressing rate seemed to have little effect

on the shape of the

hydrostat, probably due to

the high porosity and

p e r m e a b i l i t y of t h e

sandstone which permits

adequate fluid flow even at

relatively high stressing

rates. 500 2 - 4 0 0 - 0080, 200m m 3 added to Pvol value*. 0 0 6 0 /

I

100

Fig. 6 .3 .3 .3 .a. Results of two drained hydrostatic compaction experiments (0060 & 0080), see text.

A smooth positively concave line displaying no inflection point within the range of 0-450MPa is found for each sample.

Acoustic emission results, although not displayed on the figures for clarity, were recorded fo r many of these tests and similar repeat tests. Fig 6 .3 .3 .3 .C . shows

the peaks in AE for different confining pressures for all the tests. The lack of definite AE peak from all AE

Q. 2 0 0

2 00m m 3 added to Pv values.

0067

t I I I I t * ' ’ 100 150 2 0 0 250

Pore Volume Loss (mm3).

10000

AE data for suite of tests.

Different symbols denote different test result,

Fig. 6 .3 .3 .3 .b. Results of two drained hydrostatic

data at any confining compaction experiments, (0065 & 0067), see text.

pressure, and the smooth

curved of the hydrostat verify the lack of a definite critical crushing pressure, and support the notion of continual crushing of different grain sizes at different critical pressures.

This is expected from grain radii data indicating various grain sizes, (further details in section 6 .3 .4 .). Upon unloading a portion of the p o r e v o l u m e l oss is r e c o v e r e d ( e l a s t i c deformation) and a portion is not recovered, (permanent deformation), this is clearly visible from the hydrostats.

For each experiment the

confining pressure was cycled to a different confining pressure in each test. Subsequently reloading the samples caused the hydrostat to retrace the unloading path until the previous maximum is exceeded, after which the original loading hydrostat curve is extended.

8000 - 2 6000 IXJ « 4000 - 2000 - D A V O O OO O I I I I - I ■ ! “ I "I 1 T r "1 I I I " I ' r ~ i ~ 0 100 200 300 400

Confining Pressure (Eff.Cp for Udrnd tests).

500

Fig. 6.3.3.3.C. Acoustic emission peaks for whole suite of hydrostatic compaction experiments. No clear peak is seen, see text.

--- -1 0 500 Pore Vol. 0080₀₀₆₀ c : 100 2 400 - 200 w 300 - 300 O) 2 0 0 - 400 100 û_ Confining Press. 500 240 300 120 180 60 Time (mins)

Fig. 6.3.3.3.d . Results of experiments 006 0 & 0 08 0 examined with respect to the changing Pvol/Cp gradient during stress cycling.

Figures 6.3.3.3.d. and 6.3.3.3.e. show the same four tests results as fig .'s 6.3.3.3.a. & b. in a different form. Each figure represents the results of tw o experiments.

The lower curve shows confining pressure against time. The middle curve shows pore fluid volume change against time. The uppermost curve is the gradient of the pore volume/time iPjCp) curve. The solid curves and the dashed curves represent different experiments. It can clearly be seen from fig 6.3.3.3.d. that the PjCp gradient varies w ith confining pressure during the unloading cycle (C-*D, and C'->D') - increasing as confining pressure decreases,

whereas the gradient is constant during loading (A-»B, and A'->B').

This is evident from the three cycles of test 0060, and for the one cycle of test 0080. This varying gradient is also visible from the curved unloading pore fluid volume/time curve for both tests. Fig. 6.3.3.3.e. shows results of tests 0065 & 0067. The Pv/Cp gradient curves in these figures show an inflection plateau on the unloading cycles, (A->B, A '-»B '). During unloading, the gradient initially increases, stabilising after «20M P a of unloading at » 4-4 (point A ), and after a further reduction in confining pressure of «lO O M P a the gradient then

Q .