C
A D
B E
> Sandstone minerals and clays. Pore- lling and pore-lining minerals and clays in sandstones can decrease permeability. The minerals and clays have different morphologies, such as pore- lling kaolinite books (A), brous illite (B), carbonate overgrowth (C), feldspar overgrowth (D) and quartz cement (E).
nature of the damage is critical for designing a proper acid treatment. An improperly formulated acid treatment can precipitate reaction products in the formation, reducing rock permeability. A primary objective of designing an acid treatment in sandstones is optimizing damage removal, while minimizing formation of damag- ing precipitates. The first 3 ft [0.9 m] into a formation from a wellbore experiences the greatest pressure drop during drawdown, and is critical for flow. This region, sometimes called the critical matrix, is the volume that matrix acidizing treatments target for cleanup.
If HF comes in contact with calcium carbon-
ate [CaCO3] during a treatment, then it leads to calcium fluoride [CaF2] precipitation. For this reason, a matrix treatment usually includes a preflush stage with an acid such as HCl or an organic acid to dissolve most of the carbonate
1. A water block is a production impairment that may occur when the formation matrix in the near-well area becomes water-saturated, thereby decreasing the relative perme- ability to hydrocarbons. Water block may result from the invasion of water-base drilling or completion uids or from ngering or coning of formation water.
2. For more on matrix acidizing: Crowe C, Masmonteil J, Touboul E and Thomas R: “ Trends in Matrix Acidizing,” Oil eld Rev iew 4, no. 4 (October 1992): 24– 40.
3. Al-Anzi E, Al-Mutawa M, Al-Habib N, Al-Mumen A, Nasr-El-Din H, Alvarado O, Brady M, Davies S, Fredd C, Fu D, Lungwitz B, Chang F, Huidobro E, Jemmali M, Samuel M and Sandhu D “ Positive Reactions in Carbonate Reservoir Stimulation,” Oil eld Rev iew 15, no. 4 (Winter 2003/2004): 28– 45.
The main treatment that follows is often either a mud acid, a combination of HF and HCl, or a retarded formulation such as the ClayACID fi nes-control retarded acid, which is a combination of fluoboric acid [HBF4] and HCl. The HBF4 hydrolyzes slowly to form HF and also
minerals.5
4. The acid formulation used in any speci c instance is dependent on formation mineralogy.
5. Organic acids are blended with ammonium chloride [NH4Cl] brine to minimize clay swelling. For further infor- mation: Thomas RL, Nasr-El-Din HA, Mehta S, Hilab V and Lynn JD: “ The Impact of HCl to HF Ratio on Hydrated Silica Formation During the Acidizing of a High Temperature Sandstone Reservoir in Saudi Arabia,” paper SPE 77370, presented at the SPE Annual Technical Conference and Exhibition, San Antonio, Texas, USA, September 29– October 2, 2002.
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