Sandstone acidizing function

Acid reaction in sandstones is fast, and only formation damage that occurs very near the wellbore can be treated effectively. This is a function of the very high surface-area-to-volume ratios of siliceous minerals such as clays, feldspars, and zeolites—clays having the highest ratio. Large quartz grains have a very low surface-area-to-volume ratio. 

The point is that acidizing in sandstones realistically addresses skin only, which can have a drastic effect on well productivity. As shown in equation (3.1), well production rate, q, can be defined by Darcy s law for steady-state liquid flow in a radial reservoir as a function of permeability, k, and skin, s, among other factors. Again, skin, s, is a multicomponent term representing total skin s), composed of different skin contributors, as given in equation (3.3). 

Fig. 7. This figure explores the relationship between the relative thickness of source rock needed to generate sufficient organic acid to create 3% by volume of secondary porosity in a sandstone from feldspar dissolution as a function of the source rocks total organic C content. An acid-generating capacity of 0.417 mmol/g C. has been used for the basic calculation and has been taken from Lundegard and Kharaka (1990) and is derived from hydrous pyrolysis experiments. An average of 3% by volume of secondary porosity is reasonable for that present in sandstone sequences of most sedimentary basins. Consequently, only unrealistic high proportions of source rock to sandstone are necessary to account for 3% secondary porosity even if the acid-generating capacity is too low by a 100 times...

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