We introduce an approach linking the downstream response directly to the rock permeability and compressibility for oscillating-pressure measurements in ultra-low permeability media with time-varying properties. A simpler solution for pressure oscillation propagation in tight rocks has been developed, and a frequency sweep is used to analyze the time-frequency feature of the response by introducing wavelet transform technique. Software incorporating test control, data processing, and parametrical analysis and inversion, based on this method, has been written.
Pour les mèsures de vitesse de transport dans les materiaux à très faible permeabilite (argiles), nous avons construit un appareil de chargement oscillatoire. Le principe d'analyse consiste d'une balayage sur les frequences characteristiques des donnees, et utilisant la methode de transformation en ondelettes pour examiner la reponse dans la domaine temps-frequence. Cela permet une optimisatin pour choisir la frequence optimale pour I'essais en temps reel, et nous donne une inversion directe pour les paramètres mentionees.
Wir stellen eine Methode vor, welche das flußabwartige Verhalten direkt mit der Gesteinspermeabilitat und Kompressibilitat fuer oszillierende Druckmessungen in extrem niedrig-permeablen Medien Verbindet. Eine vereinfachte Lösung fuer oszillierende Druckausbreitung in nicht-porösern Festgestein wurde entwickdlt. Dabei wird eine Frequenzanalyse durehgefuehrt, welche das Frequenzverhalten der Reaktion mittels einer Transformationstechnik untersucht. Eine Software wurde erstellt, welche die Experimentueberwachung, die Datenverarbeitung, sowie die Parameteranalyse und-inversion gestattet.
Measurements of transport properties in ultra-low permeability rocks (shales, intact igneous rocks, salt, low porosity carbonates, anhydrite, etc.) have long been a challenge for geologists and engineers working in geology, hydrogeology, oil and gas development, civil engineering, and environmental engineering. For low permeability rocks, steady-state techniques are excessively time-consuming; pressure pulse decay methods are more rapid, but equipment calibration remains a problem, and for both methods there are additional difficulties if properties change with time. Time-dependent variations in permeability may arise from swelling in shales, internal dissolved mass transport in salt, saturation effects in micro-fissured igneous rocks, chemical effects related to ionic transport in evaporites and shales, and even micro-fissure propagation over time in low permeability rocks. The technique of oscillating pressure permeability (OPP) measurement is based on propagation of a pore pressure oscillation. A variable pressure is applied at the upstream end of a sealed cylindrical specimen under radial confinement, and the downstream pressure is monitored in a low compliance system. With OPP, data can be obtained in a shorter time than for other methods, and more important, the frequency-dependent features of rock behaviour can be explored (Charlaix et al 1988, Kranz et aI 1990, Fisher 1992, Fisher and Paterson 1992, Rigord et al 1993). The excitation frequency can be selected explicitly to address the material transport properties most efficiently, given equipment resolution, sample parameters, and available time. For changing material transport properties, this optimum frequency range may shift during testing. Though published articles provide equations and means of estimating the permeability indirectly using a graphical approach, a frequency optimisation procedure in a software environment is needed for testing. We will introduce a more direct approach linking the response directly to the permeability and the rock compressibility. For material with ultra-low permeability, it is reasonable to assume that the stress wave propagates as in an semi-infinite domain, thus a solution linking the response directly to the permeability and the rock compressibility is obtained. Also, we will assume that these two parameters remain sensibly constant within one pressure cycle, a reasonable assumption providing that the oscillation half-amplitude is modest compared to the confining pressure. This approach appears to be somewhat simpler for parametric analysis, data processing, test control and also for permeability and storage capacity inversion to obtain compressibility. The downstream pressure data collected is tile sum of the signal, noise, and any distortion or linear drift arising from time-dependent changes in permeability, porosity, or compressibility. Data are commonly analyzed in time or frequency domains. We introduce the wavelet transformation to explicitly link time and frequency, and analyze data in the time-frequency plane. Now, by using a frequency sweep, the OPP method can provide information on time-varying frequency shifting of tile downstream pore pressure response, vital for cases where the permeability is changing with time.