porosity
1. from the reservoir-engineering standpoint, one of the most important rock properties is porosity, a measurement of the space available for storage of petroleum hydrocarbon. porosity is defined as the ratio of the void space in a rock to the bulk volume of that rock multiplied by 100 to express in per cent. porosity may be classified according to the mode of origins (1) original and (2) induced. original porosity is typified by the intergranular porosity of sandstones and the intercrystalline and oolitic porosity of some limestones. induced porosity is typified by fracture development as found in some shales and limestones and by the vugs or solution cavities commonly found in limestones. rocks having original porosity are more uniform in their characteristics than those rocks in which a large part of the porosity is induced. for direct quantitative measurement of porosity, reliance must be placed on formation samples obtained by coring.
2. early investigations of porosity were conducted to a large extent by investigators in the fields of ground-water geology, chemical engineering, and ceramics. therefore, much of the interest was centered on the investigation of porosity of unconsolidated materials. in an effort to determine approximate limits of porosity values, slichter and, later, graton and fraser computed the porosity of various packing arrangements of uniform spheres. the porosity for cubical packing (the least compact arrangement) is 47.6 per. cent, and that for rhombohedral (the most compact arrangement) is 25.96 per cent.
3. the investigators recognized that naturally occurring materials were composed of a variety of particle sizes and that not only the arrangement but the regularity and distribution of particle size would affect porosity. the configuration of the pore space is different from that which would be obtained from the packing of uniform spheres. furthermore, a portion of space is filled with clay and cementing material.
4. grain-size distribution may be characterized in part by skewness of the distribution. skewness is a statistical measure of the uniformity of the distribution of a group of measures. in general, smaller grain size and greater angularity tend to increase the porosity while an increase in range of particle size tends to decrease porosity.
5. in dealing with reservoir rocks, it is necessary, because the cementing materials may seal off a part of the pore volume, to define total porosity and effective porosity. total porosity is the ratio of the total void space in the rock to the bulk volume of the rock; effective porosity is the ratio of the interconnected void space in the rock to the bulk volume of the rock, each expressed in per cent. from the reservoir-engineering standpoint, effective porosity is the quantitative value desired, as this represents the space which is occupied by mobile fluids. for intergranular materials, poorly to moderately well cemented, the total porosity is approximately equal to the effective porosity. for more highly cemented materials and limestones, significant difference in total porosity and effective porosity values may occur.
6. materials having induced porosity such as the carbonate rocks have even more complex pore configurations. in fact, two or more systems of pore openings may occur in such rocks. the basic rock material is usually finely crystalline and is referred to as the matrix. the matrix contains uniformly small pore openings which comprise one system of pores. this system is the result of the crystalline structure of the rocks. one of or more systems of larger openings usually occur in carbonate rocks as a result of leaching or fracturing of the primar3; rock material. vugular pore openings are frequently as large as an ordinary lead pencil and are usually attributed to leaching of the rock subsequent to deposition. fractures also may be quite large and contribute substantially to the volume of pore openings in the rock. both fractures and vugs may be closed or partially closed by precipitated calcite or other similar material. vugs and fractures are highly variable in size and in distribution. therefore, even more than for intergranular materials, laboratory measurements are required for quantitative evaluation of porosity. void space in the rock to the bulk volume of the rock, each expressed in per cent. from the reservoir-engineering standpoint, effective porosity is the quantitative value desired, as this represents the space which is occupied by mobile fluids. for intergranular materials, poorly to moderately well cemented, the total porosity is approximately equal to the effective porosity. for more highly cemented materials and limestones, significant difference in total porosity and effective porosity values may occur.
6. materials having induced porosity such as the carbonate rocks have even more complex pore configurations. in fact, two or more systems of pore openings may occur in such rocks. the basic rock material is usually finely crystalline and is referred to as the matrix. the matrix contains uniformly small pore openings which comprise one system of pores. this system is the result of the crystalline structure of the rocks. one of or more systems of larger openings usually occur in carbonate rocks as a result of leaching or fracturing of the primary' rock material. vugular pore openings are frequently as large as an ordinary lead pencil and are usually attributed to leaching of the rock subsequent to deposition. fractures also may be quite large and contribute substantially to the volume of pore openings in the rock. both fractures and vugs may be closed or partially closed by precipitated calcite or other similar material. vugs and fractures are highly variable in size and in distribution. therefore, even more than for intergranular materials, laboratory measurements are required for quantitative evaluation of porosity.
参考译文：孔隙度
1．从油藏工程的观点来看，孔隙度是岩石最重要的特性之一，它用来量度 岩石中用以储存石油烃类物质的有效空间。孔隙度定义为岩石内的孔隙体积占岩石总体积的百分率。按其原始形态，孔隙度可分为原生孔隙度和次生孔 隙度。原生孔隙度以砂岩粒间孔隙度和某些石灰石的晶体粒间和鲕状孔隙度 为代表。次生孔隙度是以某些页岩和石灰岩的裂缝和石灰岩中的岩穴和溶洞 为代表。具有原生孔隙度的岩石比次生孔隙度占大部分的岩石在性质上更具 有一致性。孔隙度直接定量测量的可信度取决于取心所得地层的岩样。
2．早期对孔隙度的研究主要是由地下水地质学、化学工程和制陶术等领 域的研究人员进行的。因此研究的兴趣主要集中在对松散材料孔隙度的研究。为了求得孔隙度的近似界限，斯里西特以及后来的哥拉登和弗雷塞计算了均 一球粒在不同堆积形式下的孔隙度。立方形堆积(最不松散的堆积)的孔隙 度为47．6％，菱形堆积(最紧密堆积)的孔隙度为25．96％。
3．研究人员发现自然形成的物质是由不同尺寸的颗粒组成的，并且不仅 是颗粒的堆积形式，而且颗粒尺寸的均匀度和分布都会影响孔隙度。自然界物质孔隙结构与均匀球粒的堆积而形成的孔隙是不同的。而且，部分孔隙被 粘土和胶结物质填充。
4．粒径的分布具有偏态分布的特点。偏态是对一组分布均匀性测量的统 计度量。通常，颗粒越小曲率越大孔隙度越大，而当颗粒尺寸增大时孔隙度 会减小。
5．在研究储层岩石时，因为胶结物会堵住一部分孔隙的体积，所以对于 油藏流体有必要定义总孔隙度和有效孔隙度。总孔隙度是岩石中孔隙的总体 积与岩石总体积的比值。有效孔隙度是互相连接的孔隙的体积与岩石总体积 的比值。两者都以百分率表示。由于有效孔隙度表示了可流动流体所占有的 空间，因此从油藏工程的观点来看，有效孔隙度是所需要的定量值。对于由 晶体构成的物质，其晶粒间含泥浆少或较少，因此总孔隙度与有效孔隙度近 似相等。对于含泥浆较多的物质或石灰岩，其总孔隙度和有效孔隙度会有很 大差别。
6．类似碳酸盐岩这种含有次生孔隙结构的物质具有更为复杂的孔隙构 型。事实上，这种岩石具有两种或更多形式的裂口。基性岩石物质具有完美 的晶体结构，一般构成基质。基质含有均匀的小的裂口，它们构成一类体系。 这一体系是岩石晶体结构引起的。由于原始岩石的淋滤或压裂，导致碳酸盐 岩石中产生了一种或多种大的开孔。空孔穴开孔通常与一支普通的铅笔一样 粗，这是由于沉积后岩石的侵析作用造成的。岩石的裂缝也可能很大，形成 了基本岩石内孔开体积。裂缝和岩穴会被方解石沉淀物或其它类似的物质完 全或部分地封闭。岩穴与裂缝在孔的尺寸和孔分布上变化不一。因此，对于晶体物质，实验室对孔隙度的定量测量比测定晶体物质更有必要。