A Practical Approach To Obtain Primary Drainage Capillary Pressure Curves From Nmr Core And Log Data (2001)

The purpose of the current study was to find a way of constructing 1 drainage pressure curves from NMR T2 distributions with an accuracy which can satisfy most of practical applications. The application that initiated the study was to predict initial virgin reservoir saturations from NMR log data. However, an equally important application proved to be a fast, cheap and non-destructive estimation of pressure curves on core samples. Proposed method of relating NMR distributions to 1 drainage pressure curves can be seen as a development that enables better and faster integration of core pressure data and log interpretation. Namely, once a calibration of T2-Pc conversion against a few core samples has been established, the pressure information can easily be obtained continuously across the whole reservoir section logged with NMR tool. Advantage compared to other methods comes from the fact that NMR T2 distribution is much more directly related to the porestructure and, thus to pressure information A serious shortcoming of all conversion methods proposed so far is that they could be applied to NMR measurements on fully water-bearing rock only. The presence of hydrocarbons strongly dictates the shape of the T2 distributions and thus invalidates the predicted pressure curve. In this paper we present an approach which can greatly alleviate this problem. The presented technique can be applied to NMR T2 distributions obtained on rocs at any water saturation. INTRODUCTION Distribution of the pore necks is an important characteristics of a reservoir rock because it determines the magnitude of pressures in a rock and thus (among other things) the saturationheight profile. Formation evaluation methods requires measurements of both a pore-neck distribution (capillary pressure curves) and a pore-size distribution (NMR, image analysis) of a rock. Of those, at present only NMR measurement can be performed in a borehole by a wireline logging tool. The rest have to be conducted in laboratory on core samples, and have either a limited range, or are destructive. Developing a reliable method to convert NMR T2 distributions to 1 st drainage pressure curves lifts these limitations, and provides a continuous reading of pressure curves with depth. In this study we have been considering only 1 drainage pressure Hence, to facilitate the readability of this communication we will be further referring to 1 drainage pressure curves simply as capillary pressure curves. In the first part of the paper we describe how pressure curves are constructed and present the appropriate definition of the optimal conversion constant between NMR relaxation time T2 and pressure Pc. Then practical ways to determine this constant for a particular core dataset are discussed. To quantify uncertainties introduced by using the so-obtained NMR pressure curves we assessed the accuracy of saturation prediction with core calibration of the data. Also default sandstone parameters were determined which should be applied to log data if no core information is available. Also, attention is given to prediction of the Entry Pressure, another important pressure curve parameter. Further, we present a way to reconstruct NMR T2 distributions at Sw=1 from the measurements on partially saturated rocks. This step enables one to reliably construct pressure curves from NMR data even if the latter are measured when hydrocarbons are present in the porespace. The proposed technique is based on reconstruction of the spectra at Sw=1 from spectra of bound water and model spectra of moveable water. The position of the moveable water peak is determined from a correlation between geometrical mean T2 of spectra at Sw=1 and irreducible water saturation. The full pressure curves can then be calculated from the reconstructed spectra. This reconstruction is a key element in implementation of the pressure calculations to log data. EXPERIMENTAL Most of the study was performed on the data from “The NMR catalogue” compiled in 1996 by Applied Reservoir Technology Ltd., UK, as a result of a JIP (joint industry project). This core database comprises data from 19 contributing oil companies each submitting 20 samples from one oil or gas field. This resulted in a database of 380 core plugs. In this study we have used NMR measurements obtained on 100% brine-saturated samples and on the same samples after drainage with air in a centrifuge. The NMR measurements with interecho time of 0.35 ms were used. For pressure curves only data on Hg-Air injection were available, so in this work only this type of pressure curves will be discussed. Hg-Air injection measurements were performed only on 10 samples from each field, thus for this study a set of 186 samples (with both NMR and Hg-Air) was available. For this work all T2 distributions were obtained by processing the original NMR decay curves with Shell decay inversion software. The results of this study have been collaborated by verification against a large in-house dataset that has been acquired over the past years. The core samples for NMR experiments have been cleaned using the soxhlet extraction with toluene and methanol and afterwards dried at 80°C. Further, they were saturated with 50,000 ppm CaCl2 brine. Desaturation of samples in the centrifuge was performed at 5.5 bar pressure. A comprehensive treatment of Nuclear Magnetic Resonance (NMR) and its applications to Oil and Gas industry can be found in refs. (Kenyon, 1997, Kleinberg et al., 1996). Here only a brief summary of relevant facts is given. An NMR logging tool (or laboratory instrument) measures the strength and the decay with time of the signal induced by magnetisation of hydrogen nuclear spins. The strength of the signal is proportional to the amount of hydrogen atoms in the fluid (water or hydrocarbon) in the measurement volume and thus to the porosity of a rock. The decay of magnetisation represents a sum of exponentially decaying contribution which come from hydrogen spins experiencing different local surroundings. For a hydrogen in a water molecule in a water-wet rock, the relaxation time, T2i is proportional to the Volume-to Surface ratio of the water phase in a pore. 1 1 2 2 − − + = ,bulk T V S T