Studying Vugular Rocks By-Constant-Rate Mercury Injection

Abstract

American Institute of Mining, Metallurgical, and Petroleum Engineers, Inc. This paper was prepared for the 46th Annual Fall Meeting of the Society of Petroleum Engineers of AIME, to be held in New Orleans, La., Oct. 3–6, 1971. Permission to copy is restricted to an abstract of not more than 300 words. Illustrations may not be copied. The abstract should contain conspicuous acknowledgment of where and by whom the paper is presented. Publication elsewhere after publication in the JOURNAL OF PETROLEUM TECHNOLOGY or the SOCIETY OF PETROLEUM ENGINEERS JOURNAL is usually granted upon request to the Editor of the appropriate journal provided agreement to give proper credit is made. Discussion of this paper is invited. Three copies of any discussion should be sent to the Society of Petroleum Engineers office. Such discussion may be presented at the above meeting and, with the paper, may be considered for publication in one of the two SPE magazines. Abstract Constant-pressure stages are used for determining capillary pressures by mercury injection (Purcell's method). However, this does not always reveal the existing vugs. In practice, knowing the degree of vugular porosity practice, knowing the degree of vugular porosity is extremely useful when an immiscible drive recovery method is used. This paper describes an automatic device for measuring neck radius size and distribution as well as for determining vugular volumes. The method consists of injecting mercury at a constant rate into a rock sample. The pressure and injected volumes are recorded simultaneously and the data are numerically processed. The injection rate must be sufficiently slow for the pressure loss caused by flow inside the rock to be negligible in the face of capillary pressure variations occuring at the interfaces pressure variations occuring at the interfaces moving inside the rock. Before reaching a vug the pressure increases to a high enough value for the free surface of the mercury to cross through the vug neck. Then when the mercury arrives in a vug, the diameter of the meniscus gets larger and the pressure decreases according to Laplace's law. When the vug is filled up, pressure returns to its initial value. The difference between the volumes injected just before the pressure drop and the volumes after pressure has returned to its initial value, determines the volume injected into a vug or group of vugs controlled by a given neck radius. In this way a distinction can be made between vugular saturation and matrix saturation, and thus vugular saturation distribution can be determined versus both the total saturation and the neck radii. The device used for this is described. The relative importance of pressure levels (capillary pressure and pressure loss) used is discussed pressure and pressure loss) used is discussed The method of interpreting the curves is explained, and results obtained with different samples are presented. I - Introduction In all oil recovery methods, the texture of the rock matrix is certainly of great importance even though its knowledge is still limited. In particular, it is known that the displacement of one fluid by another is different in a homogeneous and in a heterogeneous rock. Nevertheless, the present lack of sufficiently sophisticated forecasting methods to account for heterogeneities should not prevent measurement techniques from being developed, for providing a better definition of porous-medium providing a better definition of porous-medium geometry.