Abstract
A liquid permeameter for very tight rocks is described. High upstream pressures are achieved by a "pump" based on the thermal expansion of liquid. Confining pressures to 10,000 psi may be maintained with a modified Hassler sleeve. Pressure is measured with a low displacement, diaphragm-type transducer. Permeability is measured indirectly through pressure decline over a time period.
Introduction
Permeability is an important property in petroleum engineering, as well as in several branches of science. Ground water hydrology studies and some geological problems are concerned with permeability. Permeability measurement often is very difficult. In Permeability measurement often is very difficult. In this paper we describe an instrument designed and developed to measure liquid permeabilities of very tight Precambrian rocks. These are currently of importance in the study of the origin of life. Permeabilities in the submillidarcy range are also Permeabilities in the submillidarcy range are also of importance to the petroleum industry in the study both of cap rocks of oil and gas reservoirs and fluid flow and migration through source rocks.
DESIGN CONSIDERATIONS
From knowledge of these samples, we felt no other known permeameter would give reliable values. A liquid permeameter was necessary because gas might dehydrate the chert or other minerals, causing a shrinkage and an unnaturally high permeability. Thomas et al. reported air and water permeabilities of very tight rocks, with the air permeability value always being much higher than the water. Also, the expected low permeability would lead to low flow rates even at high pressure differentials. Ordinary pumps would seem to be unsuitable. Casual examination of the samples revealed fractures and without sign of a pore structure; hence, permeabilities would be strongly sensitive to external or overburden pressures. A method of varying this latter factor seemed desirable. Finally, since other physical measurements might be made on the sample, nonpermanent methods of sample mounting were desirable.
We decided on a novel approach to determine permeability. This involved a pump based on the permeability. This involved a pump based on the thermal expansion of liquid and the use of pressure decline to calculate permeability.
COMPONENTS
THE PUMP
Liquid flow rates of only a small fraction of a millilter/hour were anticipated, with permeability of about 1 microdarcy. Thus, we decided to develop a pump based on the principle of thermal expansion of a confined liquid. Liquid heated in a closed container cannot expand and will become compressed. When the system reaches a steady temperature, it also reaches a steady pressure that can be estimated from certain physical properties of the liquid and container. Liquid flowing from the container will cause a simultaneous pressure decline. The amount of liquid "pumped" from the container can be calculated from its volume and isothermal compressibility. A more accurate liquid volume can be obtained through direct measurement, since the volume of the container also decreases during the process.
The pump consists of a liquid-filled, steel tank similar to that used to store compressed gases. Its volume together with that of the flow lines upstream of the core was equal to 2,856 cc. This tank is immersed in a water thermostat having an adjustable, mercury-in-glass thermoregulator. The pump and the sample liquid were hydraulic oil (Pennzoil Medium, p = 0.871 gm/cc, = 70 cp at 75 degrees F). p = 0.871 gm/cc, = 70 cp at 75 degrees F). High-pressure tubing, fittings, and valves were used throughout the system (Fig. 1).
SPEJ
P. 206
Propagation of a plane-strain hydraulic fracture with a fluid lag: Early-time solution
