Continuous Recording of Viscoelastic Relaxation Processes at a Constant Ultrasonic Frequency Due To Wave‐Induced Fluid Flow in a Microporous Carbonate Rock

Abstract

In a set of water injection, imbibition and drying tests on the Obourg Chalk, relaxation processes induced by variations in water saturation were continuously recorded at a constant ultrasonic frequency, with a peak in attenuation concomitant to a velocity increase from a lower to an upper bound. This behavior is well described by standard viscoelastic models, considering a continuously changing relaxation frequency which at water saturation above 80% matches the frequency of our ultrasonic pulse generator. Using a patchy saturation model, we show that the heterogeneity in fluid distribution is responsible for this relaxation phenomenon which could be observed at the ultrasonic frequency, thanks to the very fine scale of the rock microstructure combining low permeability and high porosity. We believe that our detailed data set can feed new models or help improve existing ones for a better understanding of wave‐induced fluid flow processes in reservoir rocks.

An Integrated Study of Water Weakening and Fluid Rock Interaction Processes in Porous Rocks: Linking Mechanical Behavior to Surface Properties

“…The P wave velocity increased because of the transition from a relaxed to an unrelaxed regime; this was also manifested by a peak-like shape of amplitude, where the minimum amplitude corresponds with the inflection point on the velocity plot. It was shown [13] that this behavior can be successfully modelled using the patchy saturation model proposed by Johnson [24]. Note that near the velocity inflection point on the bottom and middle planes, After some time, a different kind of evolution was observed, referred to as stage #2 in Figure 13, which was interpreted as a relaxation transition induced by an increase in water saturation once the water front had reached the top surface of the sample.…”

“…When conducting triaxial tests or injection tests in our pressure vessel, three pairs of ultrasonic P wave sensors were fixed at different heights in three cross-sectional planes at 20 mm (bottom plane), 40 mm (middle plane), and 60 mm (top plane) from the bottom surface of the rock sample [13]. Such an array allowed us to measure the P wave velocity across three horizontal wave paths in order, for example, to follow the rising of the injected water in the injection tests.…”

“…During the injection tests, the P wave velocity and amplitude were recorded at three different heights using three pairs of ultrasonic sensors. For Obourg Chalk, such data were analyzed and thoroughly interpreted in a previous publication [13]; here we want to highlight the differences with a new data set obtained on the Ciply Chalk, and link these differences to possible water weakening effects. In Figure 13, we plotted the evolution of the P wave velocity and amplitude normalized to the amplitude at the start of the experiment vs. time, with time t = 0 corresponding to the beginning of the water injection into the dry samples.…”

“…In Figure 13, we plotted the evolution of the P wave velocity and amplitude normalized to the amplitude at the start of the experiment vs. time, with time t = 0 corresponding to the beginning of the water injection into the dry samples. The Obourg Chalk sample, referenced as OB15-73 in our previous works [9,13], was tested at 1.5 MPa confining pressure and 7.3 MPa differential stress. The Ciply Chalk sample was also tested at 1.5 MPa confining pressure and at 4.0 MPa differential stress.…”

“…OB15-68, which was analyzed by Geremia and David [13]. Two different stages can be distinguished: in stage #1, the P wave velocity and amplitude are impacted by the rising of the water injected from bottom to top into the dry sample.…”