1
r
Upstream Intensifier
Downstream Intensifier
Time
Figure 6.4: Steady state permeability measurement in the hydrostatic permeameter. Steady state flow is achieved when the rate o f inflow and outflow o f the pore fluid is the same.
For the steady state flow measurement the manual shut-off valves located in the high pressure piping between the intensifiers and the sample are opened. Once the desired effective pressure is achieved, a small pore pressure gradient is established along the sample length and maintained constant. For a test conducted at an effective pressure of 30 MPa, the pore pressure in
the upstream intensifier is set typically to 1 MPa greater than the pore pressure in the downstream reservoir. The intensifiers are controlled under pressure control, so that fluid is injected into the sample at both sample ends, observed as an increase in piston displacement in both intensifiers (Figure 6.4) until full sample saturation is achieved. At this point the intensifier piston displacement curves diverge (Figure 6.4) as the piston in the intensifier generating the relatively low pore pressure begins to back off and the upstream intensifier piston continues to advance. Once steady state flow is achieved, observed as an equal rate of fluid flow into and out of the sample, the steady state flow measurement is made. The flow rate is determined from the displacement of the pistons in both intensifiers, measured by the internal LVDT’s in the intensifiers. Fluid flow rates can be established in either direction along the sample length. With a fixed pressure gradient, measuring the flow rates allows us to calculate permeability using Darcy’s law (Equation 3.1).
6.3.2.2 Transient pulse permeability measurements in the hydrostatic permeameter
Transient pulse permeability measurements can be conducted using three different configurations of the permeameter. Following the nomenclature introduced in Chapter 4, the three modes of set-up are:
• double-ended transient pulse permeameter; upstream and downstream reservoirs present (y = 1)
• double-ended transient pulse permeameter; downstream reservoir has infinite compressive
storage (y = oo)
• single-ended transient pulse permeameter; upstream reservoir only present (y = 0).
In order to conduct transient pulse permeability measurements, the two manual shut-off valves located between the intensifiers and the rock sample are closed. The transient pulse method requires known volumes of fluid reservoir. The upstream fluid reservoir is defined as the volume of high pressure piping between the upper end of the rock sample and the shut-off valve located between the sample and the upstream intensifier, and similarly, the downstream reservoir is defined as the volume of high pressure piping between the lower end of the sample and the shut-off valve located
between the sample and the downstream intensifier. The volume of the fluid reservoirs is
approximately 6000 mm^.
6.3.2.S Double-ended transient pulse permeability measurements (y = 1)
This experimental set-up is similar to conventional transient pulse permeameters where both upstream and downstream reservoirs exist (Brace et al. 1968; Trimmer 1981; Trimmer et al.
1980).
The desired effective pressure is achieved as described in Section 6.3.1. In the case where y = 1, an equal pore pressure is generated in both the upstream and downstream intensifiers. When
intensifiers and the sample (Figure 6.3) are closed, creating an upstream reservoir volume and a downstream reservoir volume of known volume. In this way, the pore pressure in the sample and the fluid reservoirs is no longer controlled by the intensifiers.
A small pore pressure increase is generated in the upstream intensifier and the upstream manual shut-off valve opened momentarily to allow a small volume of fluid into the upstream reservoir, creating a pressure gradient between the upstream reservoir and the rock sample. Pore pressure changes are monitored continuously in both the upstream and downstream reservoirs, through the two monitoring pressure transducers, until the pore pressure reaches a non-changing or equilibration value in the sample and the two reservoirs.
6.3.2A Double-ended transient pulse permeability measurements (y = oo)
In this case, only one intensifier is used to maintain fixed pore pressure at one (upstream) end of the sample, the other sample end (downstream) being open to atmosphere. When a steady state rate of fluid flow into the sample is achieved a small increase in pore pressure is generated at the upstream end of the sample as described above and the manual shut-off valve closed immediately. Pore pressure changes in the upstream reservoir are monitored continuously until the pore pressure in the sample decays to the same magnitude as the instantaneous increase in pore pressure used to generate the transient pressure pulse (i.e. when P„^ / AP^,, = 0).
6.3.2.5 Single-ended transient pulse permeability measurements (
7
= 0)To isolate the downstream reservoir from the sample a piece of rubber cut to the same sample diameter (15 mm or 38 mm) is positioned between the sample end cap and the rock sample at the downstream interface, creating an impermeable lower sample boundary, corresponding to 7 = 0
(Hsieh et al. 1981). The rubber is sufficiently pliable and robust that it extruded laterally when confining pressure is applied thereby ensuring no pore fluid leakage between the heat shrink jacketing and the rubber disk. A transient pore pressure increase is generated in the upstream reservoir only, as described above in Section 6.3.2.3, and the pore pressure decays into the sample itself.
6.3.2.6 Data reduction fo r the transient pulse measurements
This double-ended experimental set-up is similar to classic transient pulse permeameters where both upstream and downstream reservoirs exist (Brace et al. 1968; Bernabe 1987b; Trimmer et al.
1980). For the three variations of the transient pulse permeability measurement, permeability and specific storage were calculated by comparing experimentally derived normalised pressure versus time curves with theoretical type curves (Hsieh et al. 1981) appropriate to the particular experimental configuration (Figure 4.1) following the procedure described in full in Chapter 4, Section 4.4.
6.4 Heat Treatm ent Equipment
In order to create a suite of rock samples with varying amounts of crack damage, heat treatment experiments were conducted using a commercial Carbolite furnace. Thermal cracking is monitored through recording the acoustic emission (AE) events. Due to the high temperatures (<900 °C) obtained, it is necessary to locate the PZT AE transducer at room temperature outside of the furnace. A stainless steel waveguide is built to transmit the AE signal from the sample to the PZT transducer. The waveguide is attached to one flat end of the sample via a stainless steel cone and assembled in a sample clamp (Figure 6.5). The use of a waveguide results in the attenuation of the AE signal, although since all amplitudes are equally attenuated this does not affect the event frequency/amplitude plot from which the b-value is derived (Meredith and Atkinson 1983). The furnace temperature is controlled through the in-built Eurotherm temperature controller. The sample temperature is monitored independently using a K-type thermocouple attached directly onto the sample surface. The transducer is logged by a MISTRAS acoustic emission analyser, and an AE threshold of 28 dB is set so that background noise did not swamp the AE signal from thermally- induced cracking in the sample.