Compressibility of Unconsolidated, Arkosic Oil Sands

Abstract

Abstract Presented am results of compaction of 11 unconsolidated, fine- to medium-grained, arkosic sand cores, 1-7/8 in. in diameter and 3 to 4 in. long. Direct measurements of the pore fluid pressure and bulk volume changes of each sample were made as the pore fluids were expelled. At a constant overburden (external) pressure of 3,000 psi and a temperature of 140 degrees F, the calculated bulk volume compressibilities ranged from 7.4 x 10 to 3 x 10 psi, whereas the pore volume compressibilities varied from 10 to 10 psi in the 0 to 3,000 psi effective pressure range. The void ratios in the same effective pressure range varied from 0.85 to 0.19. Compressibility increases with increasing feldspar and clay content. Compressibilities obtained when using hydrostatic loading equipment are 55 to 100 percent higher than those determined when using uniaxial compaction apparatus. Introduction Numerous investigators studied the compressibility of consolidated rocks and unconsolidated sands. The writer conducted compaction tests in a hydrostatic cell on unconsolidated producing oil sands of Pliocene age from the Los Angeles basin, Calif. There is a lack of experimental data on the compressibilities of unconsolidated sands; yet, such sands are the cause of many well completion and producing problems worldwide. The samples tested were taken from a massive sand interval, greater than 100 ft in thickness. This unit is of deep-water origin and was probably deposited by a combination of bottom currents and distant-from-the-source turbidity currents. Some streaks of very coarse-grained sand and gravel occur in the generally fine- to medium-grained massive sand interval. It is very difficult to duplicate actual reservoir conditions in the laboratory because of the various loading conditions that may exist in the reservoir. Possible loading conditions on a hypothetical Possible loading conditions on a hypothetical sediment cube are presented in Fig. 1. The first condition presented (Fig. 1A) is polyaxial loading, in which none of the three principal stresses are equal. Some investigators prefer to call this stress condition triaxial loading. Although this stress condition may represent the subsurface conditions, it is extremely difficult to duplicate in the laboratory. The second possible loading condition (Fig. 1B) is hydrostatic, in which the three principal stresses applied are equal. This type of loading probably exists during the initial stages of deposition and compaction. The third type of loading (Fig. 1C) is triaxial, in which two of the three principal stresses are equal. Although some investigators justifiably refer to it as biaxial stress, the term triaxial is strongly imbedded in the civil engineering and earth sciences literature. In the uniaxial loading condition (Fig. 1D), the applied force acts in one direction only and is perpendicular to one surface of the sample material. perpendicular to one surface of the sample material. The four faces of the cube parallel to the direction of the stress remain stationary. This arrangement can be achieved by placing the sample in a thick-walled, cylindrical chamber, the sides of which are stationary. The pressure can be applied with either one or two pistons, and the change in the volume of the sample is reflected by the change in the length of the sample. In the field of soil mechanics this method is sometimes referred to as triaxial testing. This type of loading is probably approached in an oil reservoir as the reservoir pressure is depleted as a result of production. It pressure is depleted as a result of production. It should be mentioned also that some investigators reserve the term uniaxial for cases when there is a vertical stress, but no lateral restraint and hence no lateral stress. In biaxial loading (Fig. 1E), the two principal stresses are equal, while two faces of the cube are held stationary.