ABSTRACT

Most thermal heavy oil recovery methods entails changes of pore fluid, pressure and temperature which in turn induce complex changes in the elastic properties of reservoirs that are in general unconsolidated or weakly consolidated porous rocks. In this paper, laboratory measurements of velocities and attenuations under different conditions of temperature and stress were performed on samples of a weakly consolidated reconstituted sandstone saturated with various fluids (air, water and glycerol). The sample investigated is representative of weakly cemented sandstone reservoirs with high porosity and permeability. The experimental results demonstrate the strong impact of the nature of the pore fluid on the compressional and shear wave velocities and attenuations. The influence of temperature and stress are discussed, together with the wave dispersion mechanisms.

1. INTRODUCTION

Seismic techniques such as 2D or 3D seismic reflection surveying, sonic well-logging, vertical seismic profiling (VSP), among others, are used in petroleum industry both for reservoir characterization and for production monitoring. The seismic parameters of interest are the intrinsic velocities and attenuations. Many studies have been conducted on changes in wave velocities associated with oil production [1, 2, 3, 4]. The experimental determination of attenuation is more difficult than the measurement of velocities [5, 6], resulting in a very limited amount of available data in this area. Moreover, there is a need of laboratory measurements of the wave velocities and attenuations under various conditions of temperature, pore pressure and fluid saturation so as to determine the changes in rock properties resulting from oil production. The purpose of this paper is to present some data on the dependency of both P-wave and S-wave velocities and attenuations on the pore fluid, the stress and temperature in poorly cemented porous sandstone. To do so, ultrasonic measurements have been carried out. The important effects of the pore fluid viscosity in glycerol-saturated samples and some dispersion mechanisms are discussed.

2. MATERIAL DESCRIPTIONS

Cylindrical reconstituted samples are prepared from Fontainebleau sand, composed of mono-crystalline quartz sub-spherical grains [7]. This sand is moderately well sorted with a mean grain size of 250 micrometers (coarse grain). In order to reconstitute the sample, sand is poured into a stainless steel mould and a silicate solution is then circulated through the specimen, precipitating silica at the contacts between grains. After several circulations and oven-drying, the samples exhibit a weak cementation. Samples have a porosity from 37 % to 40 % (deduced from the results of Tomography scanner, Micro Tomography scanner, Purcell tests and from weight measurements) and a high permeability of about 3 to 4.10-12 m² (3 to 4 D). The Micro Tomography scanner image of the reconstituted sample (Fig. 1) exhibits a poorly consolidated nature. Several clusters of particles are observed, as results of artificial cementation processes. The grains (in white color) are highly angular. In brief, this reconstituted sandstone can be considered as a representative model of sandstone reservoirs with weakly cemented, porous and highly permeable nature. Two types of fluids were used to saturate the samples: water and glycerol.

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