Acid treatments for dolomite formation with low permeability differ from that applied for calcite rock. Unlike limestone acidizing where HCl reacts with limestone in rapid reaction rate to generate reaction products soluble in water. In Some cases, injectivity well can not effectively improve in dolomite formation with low permeability using conventional 15 wt% HCl acid. In addition, formation damage due to drilling or workover operations can not be removed. Therefore, the process of treating dolomite for injectivitiy improvement may consider a serious issue for acid stimulation. The challenges of dolomite acidizing are to optimize damage removal while maximizing rock permeability with wormhole occurrences.
Well DW-4 drilled and completed as horizontal open hole wastewater disposal wells for excess water production from offshore oil wells in Al-Khafji Field. This well is used to dispose the un-wanted co-produced water which shows injectivity decline with time due to sand plugging and oil content droplet.
Several acid stimulation treatments were conducted on subject well with low rate of success due to not considering the dolomite reaction rate and the chemical volume to be used. A simple previous treatment review was conducted to select the best chemical recipe and treatment volume for injectivity improvement. As a result of conducting new chemicals recipe with optimum treatment volume, a major improvement in well injectivity with formation damage removal was obtained. This paper demonstrated new design methodology with extensive field study to address the challenges and the best future operation practices for acid stimulation. It included a pre-flush of mutual solvents, then bullheading stage of 20 wt% HCl with intensifier to accelerate the chemical reaction with dolomite rock.
A carefully designed train of treatment fluids was applied to remove formation damage induced by drilling and injection fluids. Injectivity tests before and after each step of the treatment was recorded and evaluated. Proper design and execution of the stimulation treatment almost doubled the well injectivity index. Challenges, fluid selection, design criteria, field treatment, lessons learned, and results of the acid treatments were discussed in this paper.