Routine permeabilities of tight gas sands are shown to be greater than under reservoir conditions, often by more than a hundred-fold, because of the great relief of stress, absence of connate water, and increased gas slippage. Correlations are presented that can be used to estimate in-situ permeability from routine data. Introduction Yearly compilations of U.S. oil and gas reserves by the American Gas Assn.1 show that U.S. gas reserves reached a maximum in 1967 of nearly 290 Tcf (8×1012 m3). With the exception of the year 1970 when Prudhoe Bay reserves were added, gas reserves have declined at a near-constant rate of 10 Tcf (2.8×1011 m3) per year since then. To help moderate or reverse this trend, the industry is extending its exploration and development efforts to include horizons with permeabilities in about the same range as common cement - i.e., microdarcies. The design of stimulation treatments to achieve commercial rates of production and reliable assessment of potential reserves in such low-permeability rocks demands accurate knowledge of their permeability, porosity, and flow properties. Though meager, there is sufficient information already available in the literature to suggest that some of the flow properties of these rocks differ markedly from those of more permeable rocks and, thus, require closer study. Results of several different studies of the properties of low-permeability gas-producing horizons have been published previously. A study by Thomas and Ward2 showed that the permeability of cores from the Pictured Cliffs and Fort Union formations were affected significantly by confining pressure. Porosities, however, were not altered greatly. They also reported that the presence of a simulated connate water saturation (about 500(0) reduced gas permeabilities to only 10% to 20% of the specific gas permeability. Vairogs et al.3 concluded that very low-permeability rocks are affected by stress to a greater degree than those having higher levels of permeability. This agreed with results reported earlier by McLatchie et al.4 Tannich5 mathematically studied liquid removal from fractured gas wells in low-permeability horizons and concluded that in very low-permeability rocks, cleanup times could be extensive but that permanent formation damage was not likely. The study, however, provided no measured experimental data of the flow properties of low-permeability rocks.
JONES JR., FRANK O., PAN AMERICANPETROLEUM CORP., TULSA, OKLA. MEMBER AIME Abstract The capabilities of small proportions of divalent cations, such as calcium or magnesium, for controlling clay blocking are reported. Potentially sensitive formations can be exposed to fresh water if at least one-tenth of the salts dissolved in both the native water and invading fresh waterare calcium and magnesium salts. Often, even less suffices. The behavior evidently depends upon the cation exchange properties of the clays, which favor adsorption of calcium and magnesium over sodium. Clays having sufficient adsorbed calcium and magnesium resist dispersion by water and consequent blocking of permeability. An allied phenomenon, the dependency of clay blocking on salinity contrast, is also reported. Abrupt change from highly saline to fresh water can cause blocking which may not occur if salinity is lowered slowly. A process related to osmosis is thought responsible for this behavior. Applications of finding to drilling, fracturing and water flooding are discussed. Introduction Many formations, when exposed to fresh water, canlose permeability through clay effects. For example, drilling-mud filtrate can cause oil permeability decreases, which persist long enough to interfere with drill-stemtesting and well completion. Ultimate productivity may suffer when clay damage is severe, especially if drawdown pressures are low.' The injection pressures and the time required for water flooding can increase if clay blocking occurs. Earlier work indicates that clay blocking is caused by obstruction of flow channels by clays or other mineral fines dispersed by fresh water. It is well known that increasing salt concentrations in water tends to prevent clay blocking. Also it is well established, although perhaps not as widely known, that the nature of the dissolved solids is also important. If, for instance, formation clays are exposed to calcium chloride solution and then exposed to weaker solutions of calciumchloride or distilled water, considerably less permeability damage results than if the clays had been exposed instead to sodium chloride solutions. The reasons for this behavior are:clays are cation-exchange, or base-exchange, materials similar in this respect to zeolites or exchange resins, andclays in the calcium form do not disperse easily in fresh water, whereas clays in the sodium form do. The base-exchange form of a clay is easily altered by flowing a solution through it. For example, asodium clay can be changed to a calcium clay simply bypassing a calcium chloride solution through the clay bed. There is general agreement that the reason calcium and magnesium clays do not disperse easily in fresh water is because the calcium ions do not ionize easily from the clay surfaces. Sodium clays, however, are believed to allow the sodium ions to ionize from the clay particle surfaces. In fresh water, this allows the particle to accumulate a net negative electrical charge great enough for the particles to repel one another and, therefore, dispersemore easily. The foregoing suggests that, even in the presence of a large excess of sodium, a relatively small ratio of divalentcation, such as calcium or magnesium, might serve to prevent clay dispersion. The divalent ion probably adsorbs to a disproportionately large extent, because of being adsorbed more tightly, with the result that the proportion of divalent cation on the exchange positions of the clay might then be great enough for the clays to resist dispersion. Consequently, an investigation was conducted to find how mixtures of salts in solution influence clay blocking. APPARATUS AND PROCEDURE Water permeability tests were conducted using conventional techniques. The equipment is illustratedschematically in Fig. 1. Flow rates of solutions driven through small core samples at known temperatures and pressures were measured. Regulated compressed air, usually at 10 to15 psig, was used to drive the solutions. Core samples0.75 in. in diameter and I in. long were used for the most part. Cores were saturated by evacuating and then admittinga de-aerated solution. Tight core samples were first saturated with carbon dioxide gas and then evacuated and saturated with the test solution. After saturation, 140 psig pressure was imposed for several hours. JPT P. 441^
A laboratory study was made to develop means to predict the effects of increased net overburden pressures on fracture capacity. Both fabricated cores and reservoir core samples containing natural fractures were investigated. Response to confining pressure increase was found significant and similar for both types of fracture systems. A linear relationship was found between the cube root of permeability and the logarithm of confining pressure.
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