A Single-Shot Method for Capillary Pressure Curve Measurement Using Centrifuge and Quantitative Magnetic Resonance Imaging

Abstract

Abstract The centrifuge method for capillary pressure curve measurement involves increasing the centrifuge speed in steps and measuring the liquid expelled from a short core plug, at equilibrium, for each step. However, the traditional methods for deducing approximate solutions for the capillary pressure curve are based on the assumption that the capillary pressure is zero at the outflow end of the core. In addition, the traditional centrifuge methods for capillary pressure measurement are time consuming. A full capillary pressure curve requires approximately 10 different rotational speeds. We have observed for most sedimentary rocks that the experimental magnetic resonance free induction decay is single exponential and the effective transverse relaxation time (T2*) is largely insensitive to fluid saturation. These features ensure that Centric Scan SPRITE (single-point ramped imaging with T1 enhancement) is a quantitative magnetic resonance imaging (MRI) method, since its local image intensity is directly proportional to the local fluid content. We propose a single-shot method to measure the capillary pressure curve of a long rock core using a single-speed centrifuge experiment and one-dimensional Centric Scan SPRITE MRI to determine the fluid saturation distribution, S(r), along the length of the core. A full capillary pressure curve can be directly determined by the relation of S(r) and the capillary pressure distribution, Pc(r), along the length of the core. The single-shot method, employing a desktop centrifuge and a desktop permanent magnet based one-dimensional MRI instrument, has been applied to measure the primary drainage, imbibition, and secondary drainage capillary pressure curves for reservoir rocks. The proposed method for determining the capillary pressure curve is rapid, cheap, and precise. The capillary pressure curve can be obtained straightforwardly with about 40 data points. The duration of the experiment is approximately 10 times less than the traditional method. Since only a single moderate rotational speed is employed, the outflow boundary condition can be maintained, and the effect of gravity can be neglected. In addition, the long rock cores employed for the single-shot method result in a relatively small radial effect. INTRODUCTION Capillary pressure results from the interaction between a wetting fluid, and a non-wetting fluid, as well as their bounding solid matrix. Capillary pressure critically influences the initial reservoir fluid distribution and dynamic processes of oil recovery. Capillary pressure is the most fundamental rock-fluid property in multi-phase flows, just as porosity and permeability are the most fundamental properties in single-phase flow in oil and gas reservoirs [1]. In evaluating hydrocarbon reservoirs, laboratory capillary pressure curve measurements on reservoir cores are directly applied to determine many basic petrophysical properties, for example: pore size distribution, irreducible water saturation, residual oil saturation, and wettability of reservoir rocks. In addition, they are used to determine the initial water and oil saturation as a function of height above the free water level, approximate oil recovery efficiency, and to calculate the relative permeability [2–4]. Capillary pressure can also have a significant impact on water flood performance [5]. In the laboratory, the capillary pressure curve can be determined withmercury injection method,porous diaphragm or membrane, andcentrifugal methods, based on hydrostatic equilibrium [6]. The porous diaphragm method is a direct and accurate technique, but the measurement is extremely time-consuming, since the equilibration time can be weeks or months for each individual pressure point. The mercury injection method is rapid, but it is a destructive method. In addition, the mercury injection measurement does not provide information on reservoir wettability, and mercury is hazardous to human health and the environment.