Acid treatments in deep wells drilled in carbonate reservoirs represent a challenge to the oil industry. Emulsified acid can be used in matrix acidizing and acid fracturing treatments. The delayed nature of emulsified acid is useful in creating deep wormholes and etched fracture surfaces. The high temperature of deep wells requires a special formulation of emulsified acid that is stable and effective at such high temperatures. The main objective of the present work is to study the performance of emulsified acid in stimulating high temperature deep wells drilled in dolomitic carbonate reservoirs.
A coreflood study was conducted using dolomite cores, which had dimensions of 1.5 in. diameter and 6 in. length. The initial permeability of the dolomite cores used in the current study ranged from 1.6 to 20.9 md. Emulsified acid systems were formulated at 0.7 acid volume fraction, 1.0 vol% emulsifier, and 15 wt% HCl concentration. Coreflood experiments were performed at a temperature of 300°F, at injection rates range from 0.5 to 10 cm3/min until acid breakthrough the core. Samples of the effluent fluids were collected and analyzed using ICP to measure the calcium and magnesium concentrations. All cores were CAT scanned before acid treatment to study the presence of vugs, and after acid injection to study the characteristics of the generated wormhole.
The results show that the emulsified acid created wormholes in dolomite cores, resulting in better treatment outcome. For emulsified acid injection rates in the range from 0.5 to 10 cm3/min, the volume of emulsified acid injected to achieve breakthrough increased with the increase in the emulsified acid injection rate. Also, there was no optimum injection rate for the injection rate range studied (0.5 to 10 cm3/min). The number and size of the created wormholes was function of the injection rate of emulsified acid. This paper summarizes the results obtained and recommends the use of emulsified acid systems for field applications with dolomite reservoirs.