Factors Controlling Elastic Properties in Carbonate Sediments and Rocks - Coursework Example

Name Professor Course Date of submission Factors Controlling Elastic Properties In Carbonate Sediments And Rocks Introduction While characterizing the carbonate reservoirs, it is important to understand various factors responsible for influencing the behaviour of the saturated rocks (Baechle et al., 125). In most cases, researchers use seismic data in mapping the elastic properties, which they derive from the inversion of seismic data through the velocities to form vital inputs for the simulations and reservoir engineering calculations. Variation in the type of saturant and its dependability on the heterogeneity of the rock, pressure, and temperature control the elastic properties of the rocks. Moreover, it is important to characterize heterogeneities to acquire physical parameters and understand the mechanic's flow within the carbon reservoirs. With rapid and pervasive diagnetic modification, the carbonate sediments are susceptible to changes in the mineralogy and structures of the pore within the rocks. The major processes responsible for modification are cementation and dissolution, which creates or destroys the porosity. Under extreme conditions, such modifications could result in changes in the mineral contents from either aragonite/calcite to dolomite or contributing to the reversal of the pore distribution, which involve the dissolution of the original grains for the production of pores (Eberli et al., 2016). As a result, the process results in filling the original pores to form the rocks. Factors influencing the elastic properties in the carbonate sediments Modifications in the composition of the rocks play a vital role in altering their elastic properties, hence the sonic velocity. In addition, the modifications often result in varied connections among related elements: sonic velocity, pore type, diagenesis, and porosity. In carbonates, such alteration includes an array of sonic velocity with the compress wave velocity (VP) ranging from 1,700m/s to 1,600m/s and the shear wave velocity (VS) ranging from 600m/s to 3,500m/s (Assefa, et al., 12). The major factor that controls sonic velocity in rocks is porosity; however, in carbonate sediments, the pore type plays a vital role in the determination of elastic behaviour and the resultant sonic velocity within the rocks. The problem with the current theoretical equations is that they do not or to some extent insufficiently account for the modification of the elastic behaviour of rocks through pore type. Besides, most researchers currently use seismic inversions, calculation of the pore volumes, and the AVO analysis equations, which are susceptible to large uncertainty forms in the carbonates. Sonic Velocity The acoustic velocity presented within the unconsolidated carbonate rocks and sediments are influenced by various factors: shape and size of the grain, sorting, and the ratio that occurs between the matrix and grain. 60% and 1,700m/s are the average porosity and VP of carbonate mud considered pure. Whenever there is a compression of 170 MPa, then there is a decline in porosity to 29% but VP rises to 2,250m/s and VS of 900-1,200m/s. with such features, the samples are considered to have low shear modulus; as a result, they present behaviours similar to materials that lack rigidity. On the other hand, the carbonate sand tends to show relative consistency in the values of VP that range from 2,100m/s to 2,400m/s when confined in a 10 MPa and increment to 3,700 at 80 Mpa. The mix of mud and grain tend to produce a wide variety of the carbonate sediments with their relative amounts use in the classification of various carbonate rocks: grainstone has no mud, packstone supports grain, wackestone (>10% grain), and mudstone ( Read More

In most studies, the researchers undertook studies through a series of experiments with the aim of assessing effects of matrix/grain ratio on the velocities of the carbonate rocks under different pressure conditions (Zhi‐hua and Xing‐Yao 45). Moreover, commonly used grains are platy coralgal or skeletal grains and round ooid grains with 0.500mm and 0.375mm while fine fraction used are lagoonal mud with less than 64 microns in diameter. Figure 1: Graphical representation of matrix against velocity and porosity of the mixture of mud/grain.

From the graph, Porosity declines at the initial stage with the addition of matrix but rises with addition of 20% mud. Addition of 10% in weight often results in the production of samples from pure carbonate sand and mud. Initially, the porosity tends to decrease with addition of the mud to some point considered critical before reversal of the trend. With the void grain mixture, the critical point occurs at 10%; however, in the skeletal grain mixture, the point occurs at 20%. With the addition of the matrix, there is an increment in the porosity, which leads to the convergence to the natural porosity within the matrix material (Eberli et al, 658).

At the initial rise in the matrix, that is from 0 to 20%, the acoustic pressure also rises under confined pressures which concurs with the declining porosity. However, the trend could reverse if there is continuous addition of the fine-grained matrix. On the other hand, the velocity of ooid grain mixtures begins to decline at 40% matrix while for skeletal grain mixtures is 50% under confined pressures. With such results, it is clear that the shape of the grain and diameter of grain-to-mixture are vital for the change of pore elastic properties.

Moreover, the latter also influence changes in the critical porosity within the carbonate sediment. Figure 2: Illustrations on the limited influence of compaction of the sonic velocity within the carbonate rocks. Even the graphs show general increment of the lowest velocities with depths, there is occurrence of large velocity fluctuations at similar depths Compact and early cementation influence on velocity In the sediments considered siliciclastic, too much pressure plays important role in reducing the level of porosity and increasing velocity.

However, the carbonates are very prone to various diagenetic alterations, which in most cases occur faster than compaction. Therefore, the depth of burial and age are less significant for the velocity in carbonate compaction. Integration effect of the depositional lithology and different processes associated with post deposition could result in special distribution of velocity within the carbonates (Chen et al, 215). Initial consolidation, reduction the pore space, dewatering, and compaction of the sediment usually result in reduction of the initial values by about 60% porosity and a VP of 1,600m/s to the values closer to 50% and 2,000m/s.

Before the occurrence of compaction, the processes that occur include cementation and dissolution within the carbonate sediments. Cementation has the ability of transforming the sediments of carbonate into rock, which occur in shallow subsurface (Scotellaro and Mavko, 158). As a result, there is addition of stiffness to the young rocks that in turn increase significantly the velocity due to the compaction. Even though, in most cases there is a general trend that the velocity increases with depth, the common results are velocity inversions with the depth and variation in the compression wave values up to 4,000m/s.

Moreover, within the shallow burial depths, there is more than 4,000m/s especially in areas of shallow water. In such areas, there is frequent exposure and cementation of the sediments before burial. Below the sea floor (180-250m), the velocities tend to decline to about 2,000m/s before the value reaches about 5,000m/s. The diagenesis is not rapid within the deeper water with compaction recognized easily within the core of the slope (Baechle et al, 1014).

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