In general acid treatment can be applied for skin factor reducing, permeability increase of oil well borehole zone and reservoir hydraulic connectivity enhancement. A composition of HCL and HF, or mud acid, is the most efficient and well-known solvent for that purposes in terrigenous reservoirs. However, the efficiency of this agent can vary significantly. The processes of secondary precipitation and gelling of acidizing reaction products in tight reservoirs make goals listed above harder to achieve. Also, there can be pore volume colmatation by solid particles produced by clay minerals swelling and dispergation due to reaction with acid composition. As a result, the permeability of reservoir rock and well efficiency can be reduced, as it can be seen in practice.
This paper introduces the results of mud acid treatment performed on core samples of West Siberian tight shaly terrigenous reservoirs. It is shown that applying the HCl + HF acid composition with HF concentration more than 3 % on core samples with permeability less than 0.01 mkm2 leads to reduced permeability up to 19,7 – 82,9 %. Wherein the core samples with lowest permeability and acid solutions of a higher concentration provide the most negative effects. The fractured rock acid treatment provides the same effect with permeability reducing up to 40 %. However, if the same experiment is performed on a fractured matrix stabilized with a proppant agent, then positive results can be achieved. Therefore, using fractured and stabilized core samples with permeability of 13.7·10-3 mkm2 and acid solution with 6 % HCl concentration and 1.5 % HF concentration in laboratory tests shows increased permeability up to 0.021 mkm2, i.e. 1.5 times higher. The conclusion of the article declares that using a proppant agent while formation fracturing provides the most efficiency of the following acid treatment.
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