Fracturing treatments are often challenging in high-pressure/high-temperature, tectonically stressed areas with heterogeneous and complex lithology. This study presents case histories of two multistage fracturing campaigns executed on a tight gas formation in a deep volcanic reservoir onshore Japan. This work begins by highlighting the technical difficulties experienced during the first campaign, reviews the countermeasures developed over the course of the decade between campaigns, and finishes lessons learned from execution and evaluation of the second campaign.
A root-cause analysis was undertaken to understand the poor treatment results from the first campaign where stages were defined by no formation breakdown, poor injectivity or early screen-out. It included re-evaluation of core/petrophysical interpretation, stress model and net pressure history matching, and development of injectivity index diagnostic plots. The findings were used to identify updated technologies and workflows for the second campaign with consideration of limitations in the target well drilled +10 years before and uncompleted. Finally, details of field execution and post-job logging results are presented to verify effectiveness of proposed techniques and extract lessons learned for future operations.
The breakdown and injectivity issues of the first campaign appear to be tied to the initiation interval location and facies, where initiating in a massive lava facies was most problematic due to high stress and extreme tortuosity. Uncertainty in the propped height from the net pressure history matches showed room for optimization in treatment design. In the second campaign, with mitigation plans for breakdown issues, premature screen-outs and detection of propped height in place, nine fracture stages were attempted. Eight stages achieved successful breakdown with careful target selection and weighted brine. Two conventional treatments with crosslinked gel were placed in the intervals with high injectivity and, as a field trial, two slickwater treatments with high viscosity friction reducer were placed in intervals to deal with low injectivity. Issues with high apparent net pressure due to tortuosity continued, comparable to the first well, and efforts to further reduce treating pressure for future campaigns continues. Logging of the non-radioactive traceable proppant pumped revealed thin propped heights while production logging showed contribution from the zones treated with slickwater indicating it may be a viable solution for this type of challenging reservoir.
This work highlights a series of technical issues and possible solutions of multistage fracturing in a volcanic reservoir, validated through field execution. Proposed solutions partially solved the challenges, but at the same time they open further questions for future campaigns. This study can serve as a reference for fracturing operations in challenging analogue reservoirs.
