2.2.2. Instrumentation
The flow rate was measured using am Optiflux 4000 electromagnetic flow meter (Krohne, Duisburg, Germany) that was mounted downstream of the main pump Q2 as shown in
Figure 2. To measure the pressure losses along the annulus, three differential pressure (DP) cells of type Fuji were used, see
Table 3. These measured the differential pressure across three sequential 2 m sections of the test section. The pressure cell DP1 was used for measuring the gauge hole annular pressure losses. The other two DP cells had one or two pressure outlets mounted near the washout section and the corresponding pressure measurements were affected by the diameter changes at the ends of the washout section. It turned out that DP3 was faulty and no data are available from this sensor.
The location of the pressure ports were azimuthally between probes 6 and 7, and axially at the same position as the conductivity probes, with two of the ports shared between neighboring DP cells.
The propagation of the displacing fluid was monitored using conductivity probes, each consisting of a pair of electrodes. For each probe the conductivity between the electrodes was measured. 24 conductivity probes were located at four different axial stations along the test section (stations A, B, C and D in
Figure 2). Six of eight slots for conductivity probes were used at each station; slots 2 and 4 were installed but not used in measurements (
Figure 6). The number of actual probes was limited by the number of available channels of the amplifier. Note that no measurement, except visual, could be conducted in the wash out section. Here the measurement probes were positioned immediately upstream and downstream this section.
Each conductivity probe consisted of a pair of 3 mm diameter steel screws which were directed radially into the annulus and positioned 8 mm apart (center-center distance). Thus, the gap between the electrodes was 5 mm. In order to reduce any effects of the flow disturbance caused by the electrodes on the measurements, the electrodes in each pair were mounted at the same axial position, i.e., separated in the azimuthal direction. The penetration depth was 5 mm, while the width of the concentric annulus was ca 19 mm. Thus, the electrodes covered about one quarter of the radial distance in concentric annulus configuration.
The conductivity probes were connected to a signal amplifier which was specially designed for use with conductivity probes for measuring water wave heights (Wave Amplifier type 108, DHI, Hørsholm, Denmark). This amplifier gives an electrical AC signal around 2750 Hz. The resistance
λ per unit length between a pair of infinitely long parallel cylindrical electrodes of common diameter
δ, is given as shown in Equation (1) [
20]:
where
ψ is the resistivity of the fluid around the electrodes and
Δ is their center-center distance. The geometrical factor of Equation (1) thus changes by 8% with an increase in distance
Δ from 7.5 mm to 8.5 mm. Since the electrodes do not span the entire annular gap, a key question is to what extent they will detect conducting fluid which is not in direct contact with the electrodes. The measurement principle is illustrated in
Figure 7. As the displacement process progresses, the contact with the high conductivity fluid increases and a general increase in conductivity is measured. In this case, which is representative for the displacement experiments with concentric annulus, the displaced fluid is stratified above the displacing fluid due to lower density. The interface is here strongly inclined with respect to the cross-section plane, and almost parallel to the wellbore axis. Thus the length of electrode contacting the conducting fluid increases with time as the thickness of the displaced fluid layer diminished. This gives a fairly slow transition from minimum to maximum conductivity. When the interface is nearly normal to the wellbore axis, and thus parallel to the electrode axes, the transition from minimum to maximum conductivity will be much more abrupt, unless there is strong mixing between the fluids.
Except for an offset value, the measured voltage signal,
Vo, is proportional to the average conductance
G between the electrodes. Assuming that boundary effects can be ignored, the conductance
G is proportional to the length
Lp of the electrodes of a probe, i.e.:
where
σ ≡ 1/
ψ is the fluid conductivity. The conductivity increases with increasing length of probe being immersed in the high conductive fluid.
The wave amplifier ensures that the voltage across the electrodes is constant; being independent of the conductance between the electrodes. Instead, a change in the conductance will result in a change in the amplification in the wave amplifier, and thus a change in the output voltage. Thus, accounting for any voltage offset we may write the measured voltage signal as given in Equation (3):
where
K is a gain factor of the measurement system.
Both the displaced and the displacing fluids have some conductivity. However, the conductivity of the displaced fluid is an order of magnitude smaller than that of the displacing fluid, and we may approximate it as being non-conducting. The conductivity probe signal response will partly be due to changes in the local conductivity, σ, due to fluid mixing, and partly due to the position of the interface between low and high conductivity fluid relative to the probes.
The measured voltage for each pair of electrodes will vary between a minimum value
Vo,min corresponding to the displaced fluid with conductivity
Gmin and a maximum value
Vo,max corresponding to the displacing fluid with conductivity
Gmax. Assuming linearity between conductivity and volume fraction of displacing (high-conductivity) fluid we have:
Using Equation (3) we then obtain:
Equation (5) holds true for a homogeneous mixture of the two fluids, and is also expected to hold true in some average sense for a heterogeneous system where there is a macroscopic interface between the two fluids.
Although a given value of ϕ can be obtained from this expression both from a homogeneous mixture of two fluids and from a given configuration of a sharp interface between the same fluids, we may distinguish between these two cases by considering the time derivative of the signal Vo(t). A section with mixing will yield a lesser pronounced change of the signal with time than a sharp displacement front.
Let S be the total fluid domain around a pair of electrodes. Let further ϕ(r) be the local volume fraction of displacing fluid and define an interface ∂Si between displaced and displacing fluid by ϕ(r) = 0.5. Although the fluids in the experiments were miscible (both were water-based), we assume that the interface was fairly well defined through each displacement experiment, since the duration of each experiment (on the order of one minute) is short relative to the time for molecular diffusion over a typical length scale (annular gap width). The fluid-fluid interface may become convoluted due to convective processes driven by gravity or hydrodynamic instabilities, but microscopic mixing of the fluids before the washout is expected to be primarily due to turbulence in the entrance region to the test section.
Since Maxwell’s equations are linear, we postulate that the normalized signal response Vr will not depend on the absolute values of the conductivities. We also postulate that it will depend only weakly on the shape of the wall interface ∂Sw of ∂S = ∂Sw + ∂Si, where ∂S is the total boundary of the fluid domain. We here ignore the inlets and outlets of the test section, which are both far removed from the conductivity probe electrodes. Thus, we conclude that Vr almost exclusively depends on the distribution ϕ(r) and, assuming there is little mixing between the fluids, mainly on the interface ∂Si. Note that this interface is not necessarily simply connected, a well-defined fluid front. For example, there may be isolated pockets of bypassed displaced fluid. If, however, the interface ∂Si is a well-defined fluid front, separating the displaced and displacing fluids axially, we expect that the signal response Vr will be a monotonous function of the axial position of this displacement front.
An additional conductivity probe, type CTI-500 (Jumo, Fulda, Germany, hereafter called C0), was mounted on a spool section in the fluid outlet downstream of the test section. The spool was connected to the test section with a 2.5” flexible hose of length approximately 9.5 m. The probe is a combined inductive conductivity and temperature transmitter and measured the electrical conductivity and the temperature of the fluid which had exited the test section. From the measured conductivity the volume fraction of the conducting fluid and thus the overall efficiency of the displacement process can be determined. There was no special mixing element upstream of the probe C0. However, it is assumed that the two fluids were well mixed in the transversal plane at the location of the C0, both because of the length of the connecting flexible hose, the higher fluid velocity in the hose (1.38 m/s), and because of the change in flow direction at the outlet of the test section.
The objectives of the measurements with the conductivity probes were three-fold:
- (1)
detect the arrival of the displacing fluid at a particular probe location at the corresponding time where the signal Vr changes from low to high
- (2)
assess qualitatively the degree of mixing of the two fluids at the probe location by measuring the duration of the transition from low to high signal level
- (3)
measure and quantify the concentration of displacing fluid at the probe location
For the two first objectives it is only necessary to assume that the measured signal is a monotonously increasing function of the relative fraction ϕ of displacing fluid and that the two fluids have sufficiently large conductivity contrast. These were the main objectives of the distributed conductivity probes. The third objective requires a known relationship between the measured signal and the volume concentration of the displacing fluid and that the concentration is uniform in the sampling volume of the probe. In the present experiments the concentration of electrolyte, NaCl, in the displacing fluid was so low that a linear relationship can be assumed, as discussed below.
Since the scope of the measurements is to determine the displacement fluid volume fraction ϕ rather than the effective conductance G between the electrodes, we will in the following work only with the normalized probe response Vr. Since the absolute value V0 of each probe was not critical, the probes were not calibrated to give the same value for the same fluid (displaced and displacing). The offset and gain factors for each channel of the signal amplifier were adjusted prior to conducting the experiments to avoid saturation of the signal; meaning being out of range. Still this has happened in a few cases.
The linearity of
ϕ(
G) (see Equation (5)) was checked before the experiments by measuring the conductivity of a mixture of a non-conductive and a conductive water-based fluid. Thus, two prototype fluids A0 (water with 1.4% by weight of the (synthetic) clay laponite) and B0 (water with 0.8% by weight xanthan biopolymer and 1.0% by weight NaCl) were mixed in different ratios and the conductivity was measured using a handheld conductivity probe (Hanna Instruments HI933000). The results are shown in
Figure 8. The measurement points indicate a linear relationship between conductivity and volume fraction, as shown by the stippled line. For such a mixture the conductivity is proportional to the volume concentration of the electrolytic ions when the concentration is sufficiently low. Note that the fluids A0 and B0 were not used in the actual displacement experiments, but represent fluid system 1, except for the omission of barite, soda ash and biocide in the displacing fluid. This omission is not expected to have any impact on the linearity of the conductivity versus composition.
While conducting the experiments, the following data were logged 50 times per second:
Flow rate (FM)
Differential pressure (DP1, DP2, DP3)
Inner pipe rotation speed (RPM)
Mixed fluid conductivity downstream test section (probe C0)
Mixed fluid temperature downstream test section (probe T0)
Tank temperature of displacing fluid (probe T1)
Local conductivity at stations A, B, C and D (probes Aj, Bj, Cj, Dj with j = 1, 3, 5, 6, 7, 8)
The displacement process was also set up to be monitored visually using six low-cost web-cameras mounted along the test section as indicated in
Table 4.
Fluids were also characterized in terms of electrical conductivity using a HI933000 handheld conductivity probe (Hanna Instruments, Woonsocket, RI, USA). In addition, the pH was frequently measured. To keep the volume of test fluids at a minimum, as much as possible was reused. Therefore, there would always be a little degree of contamination of the fluids with the other fluid. As seen in
Figure 9, the conductivity of the displaced fluid increased over time, while the conductivity of the displacing fluid was more stable. Still the contrast is sufficiently large to be able to measure the displacement properly.