*2.3.4 Acidisation—Tight reservoirs*

Acidisation of carbonate rocks (e.g., limestone and dolostone) to improve permeability and porosity can be successfully achieved with hydrochloric acid (HCL). Shales or sandstones containing significant proportions of carbonates can also be treated with HCL. The use of HCL becomes problematic when applied to other kinds of reservoir rocks; for instance, sandstones (chiefly composed of quartz and aluminosilicates), which are insoluble in HCL. The following are some of the problems associated with HCL [83]:


However, sandstones react favourably with hydrofluoric acid (HF). The fine particles of quartz and aluminosilicates which block the pores, especially at the near-wellbore region, are soluble in HF. HF can be introduced directly into the reservoir or produced through other chemicals like ammonium bifluoride (NH4HF2). Normally, mud acid (a blend of aqueous HCL-HF) is used to repair damages around the vicinity of the wellbore [83, 97]. Fluoride ion (F− ) is the only one of its kind that reacts with quartz in a way that repairs the damage near the wellbore [97]. Mud acid can be prepared by mixing a fluoride ion-releasing chemical, ammonium bi-fluoride salt (NH4HF2), with HCL. The reactions are expressed in Eq. (2) [98]:

$$\rm{HCl} \rightarrow \rm{H}^\* + \rm{Cl}^- \tag{2}$$

$$\rm H^{+} + \rm NH\_{4}HF\_{2} \rightarrow \rm NH\_{4}Cl + 2HF \tag{3}$$

For sandstone reservoirs, acidisation is performed in three steps: *preflush*, *main flush* and *after flush* [83]. In practice, at the preflush stage, HCL has been used for the dissolution of carbonates and positive ions (e.g., [83, 99–101]); nonetheless, its effectiveness is inconsistent and there are reported incidences of damages [102]. To circumvent this, it is possible to blend HCL with other chemicals to neutralise its adverse effects. This is demonstrated in Shafiq *et al*. [97], where HCL is combined with acetic acid (CH3COOH) to improve dissolution of carbonates and positive ions (sodium, calcium and magnesium), whilst eluding the damage that would have been triggered by pure HCL.

The second (main) stage of the acidisation process is the use of a fluoride ion (F<sup>−</sup> ) containing acid to dissolve the minerals (e.g., SiO2). HF is a commonly used acid but the fluoride ion is very reactive leading to a premature expending of the acid near the wellbore region. To decelerate the reaction rate, HF must be combined with other mineral acids. These are buffer acids, which may be, for instance, HCL or formic acid (HCOOH). The buffer acid retards the reaction rate of HF with the formation and preserves the pH of products of the reaction, which in turn prevents the precipitation of silica [97]. Other acid blends (mud acids) proposed by Shafiq et al., [97] for the main stage of acidisation include Hydrofluoric-phosphoric acid (HF-H3PO4) and fluoboric-formic acid (H3OBF4- HCOOH). In the former, H3PO4 is a substitute for HCL, while H3OBF4 replaces HF in the latter. The product of the reactions between HF and silica mineral is fluosilicic acid (H2SiF6). The reaction process is presented in Eqs. (4) and (5) [98, 103]:

$$\text{SiO}\_2 + 4\text{HF} \rightarrow \text{SiF}\_4 + 2\text{H}\_2\text{O} \tag{4}$$

*Production from Unconventional Petroleum Reservoirs: Précis of Stimulation Techniques… DOI: http://dx.doi.org/10.5772/intechopen.106318*

$$2\text{SiF}\_4 + 2HF \to H\_2\text{SiF}\_6 \tag{5}$$

Compounds such as silica gelatinous precipitate (Si(OH)4) are eventually formed when fluosilicic acid is decomposed to silicon tetrafluoride (SiF4) (Eq. (6)), which is then hydrolysed (Eq. (7)).

$$H\_2\text{SiF}\_6 \rightarrow 2HF + \text{SiF}\_4 \tag{6}$$

$$\text{SiF}\_4 + 4\text{H}\_2\text{O} \rightarrow \text{Si(OH)}\_4 + 4\text{HF} \tag{7}$$

There is also a tendency for other precipitates to form, which can be avoided by the circulation of HCL at the *preflush* stage to remove ions [97, 100]. The *after-flush* stage restores the wettability of the formation and removes the expended acids. Mutual solvents, HCL, acetic acid and other suitable chemicals are candidates for finalising the treatment process [104].

**Table 1** is a summary illustration and cross-section of approaches for stimulating the production of unconventional reservoirs.


**Table 1.**

*Techniques for stimulating unconventional reservoirs.*
