Reducing Breakdown Pressure in Hydraulic Fracturing: A Novel Cyclic Injection Procedure

Abstract

ABSTRACT: In hydraulic fracturing, it is not unusual to encounter difficulty in breaking down rock formations. This is often the case when the well design or pumping equipment has a limitation in reaching a higher-than-expected breakdown pressure. Such high breakdown pressures could be attributed to greater rock strength, excessive near-wellbore stress concentration, and/or unusual near-wellbore completion complexity and tortuosity. Cyclic fracturing has been applied inconsistently in the fields with unpredictable, occasional success. Based on a comprehensive series of laboratory experiments, this paper introduces a novel cyclic injection procedure to effectively fracture rock at lower-than-normal breakdown pressure without incurring additional equipment or hardware costs. The experiments reveal that • With an optimized cyclic loading design, we can effectively and consistently fracture the studied rock below 85% of expected breakdown pressure. • The key to success is less dependent on magnitude or number of loading cycles, but more on the patterns of cycle loading and unloading process. • In both dry cyclic loading and fluid cyclic injection cases, fracture propagation was dominated by laminations. The novel approach provides an effective way to initiate fractures with reduced pumping pressure. This can eliminate the need for higher specification in well design and pumping equipment, and potentially avoids a larger operational footprint. 1. FORMATION BREAKDOWN AND CYCLIC FRACTURING In hydraulic fracturing operations, it is not unusual to encounter difficulty in breaking down rock formations to initiate and propagate fractures away from the wellbore. This is often the case when the well design or pumping equipment has a limitation below a higher-than-expected breakdown pressure. Such high breakdown pressures could be attributed to greater rock strength, excessive near-wellbore stress concentration, and/or unusual near-wellbore completion complexity and tortuosity. In regions outside of the United States, with challenging tectonic settings and deeper target formations, the breakdown pressures often exceed the pressure rating of pumping equipment. In Oman, fracture stimulation operation experimented with the use of heavy brine to help increase the bottom hole pressure to breakdown the formation (Briner et al, 2015). In China’s Sichuan basin, breakdown pressures exceeding the equipment rating have resulted in the inability to fracture several stages in horizontal wells, often requiring additional perforation or sand jetting of the near-wellbore region before the formation would breakdown. In such circumstances, it is common to see breakdown pressures higher than 15k psi, even though once breakdown is established, the fractures propagate readily at a pressure lower than 15k psi. Therefore, stepping up to the expensive 20k psi equipment just to initiate hydraulic fractures seems imprudent. A common, often impromptu remedial solution would be to cyclically load the well pressure just below the pressure limit, where occasionally formation breakdown is achieved.