Acid choices and stimulation design improve efficiency
After prejob testing with operator-supplied field cores, hydrochloric acid (HCl), reservoir oil, and water, engineers selected 17% HCl (adapted hydrochloric acid) as the base fluid for an acid matrix treatment. Because the well was already producing, engineers recommended a prewash with a universal solvent to dissolve hydrocarbon film and minimize risk of creating unwanted emulsions. Dynamic acid dispersion, an emulsified acid-solvent system, was then pumped to solve the dual problem: to dissolve the heavy oil components and increase acid contact with the calcite rock.
To improve stimulation uniformity across the heterogeneous targeted intervals, nonpolymer VDA acid was used to divert fluids away from high-permeability areas. As the VDA acid reacts with the rock, its viscosity increases, temporarily blocking the permeability. Later, the system degrades in contact with a solvent or produced hydrocarbons.
For overall stimulation design, engineers recommended targeting intervals with the best reservoir properties to maximize productivity improvement and operational efficiency without wasting time and acid on low-potential rock. This targeting, in combination with the Discovery MLT system, was expected to considerably reduce treatment time and improve efficiency.
Production increases; operation time and cost decrease
Tatnefteotdacha supervised and organized the work on Well 2, which began with an âacid treatment on paperâ meeting where the diverse participants resolved issues of equipment compatibility, business culture, and personnel interactions.
After the targeted stimulation operation, Well 2 liquids production increased by 2.5 times over its unstimulated production. Productivity index of Wells 1 and 2 were calculated at 0.24 m3/d/bar and 0.27 m3/d/bar, respectively, or about 12% higher for Well 2.
Well 1 production began a steep decline 6 months after treatment but continued to produce at low levels for at least 12 months. In Well 2 the production improvement continued with no decline after more than 9 months.
In terms of costs, the targeted treatment in Well 2 saved 234 hours (58%) as compared with the treatment in Well 1. In addition, although the treatment in the Well 2 operation used a specialty acid system, the targeted design saved 24% on the overall job cost compared with the overall cost of the Well 1 treatment. This work proved the effectiveness of the Discovery MLT system and Schlumberger technology, enabling an extension of the stimulation campaign for another year. In 2017, Schlumberger stimulated 13 wells, 9 of them multilaterals. Tatnefteodacha estimated the total campaign cost savings exceeded USD 1.6 million.