効率的な国内資源開発の追求～酸処理技術の新たな可能性

抄録

Creating a wormhole network in tight reservoirs is a long-explored concept in acid stimulation to extract oil/gas from low permeable rocks efficiently. Conventional acid stimulation technique, such as high-rate acid injection in limited entry intervals, are for focusing acid contact on limited parts of a reservoir and for suppressing rapid acid consumption near wellbore. These techniques facilitate creation of high-velocity flow channels called “wormhole” within dissolvable host rock, but for the cost of extra pumping power and acid volume.

However, if minerals existing along vein network can be dissolved using a fit-for-purpose acid recipe, stimulation should be done effectively without necessitating such costly techniques. This is because rapid acid consumption will no longer be an issue as the mineral filling veins are usually limited in its weight-ratio of less than 10 percent, compared to the host rock. In addition, spatial extent of vein network reaching tens of meters is not rare. Extremely deep penetration of acid can be achieved by targeting such vein network.

The new acid stimulation technique based on dissolution of mineral vein network in tight reservoir rocks is proposed to promote efficient developments for three domestic oil/gas fields. One of the three fields is Katakai gas field, which has been formed in a tight volcanic reservoir rock seating over 4000m depth and having an ultra-high temperature of 180 degree Celsius. Due to such high temperature, a special acid recipe composed of chelating agents is selected by long-term batch reaction tests. The recipe shows not only a high and stable acid capacity but also a low corrosive characteristics. A field test indicated an effectiveness of the new acid technique by achieving negative skin at minus 1.27 through the acid volume of 94kL at the maximum injection rate of 2.5 barrel per minute（BPM）.