Novel Hybrid Degradable Plugs to Enable Acid Fracturing at High Temperature High Pressure Conditions

Abstract

Abstract Current commercial dissolvable plugs have been found unsuitable when we hydraulic fracture extremely HTHP wells, especially with acid. Dissolvable alloys, the most popular materials, react too intensively in acid environment, and nearly all types of commercial protective coatings have been tested unsatisfactory due to inevitable defects/holidays. Additionally, they suffer from recrystallization issues that prevent plugs from dissolving. Thermoplastic Polyglycolic Acid (PGA) may have a significantly low degradation rate in acid, but its temperature rating is too low. To address this issue, we developed a new kind of fiber-enhanced degradable polymer composite material based on thermosets, which offers high strength and temperature rating, as well as low dissolving rate in acid conditions. Thanks to synergic effect from degradable metal slip design, a novel hybrid dissolvable plugs has been developed afterwards, which can not only hold pressure in HTHP condition and dissolve readily afterword, but most importantly can be dissolvable readily with acid (mainly through dissolving of metallic slips) if plug is stuck or set prematurely during installation process, a requirement crucial to our clients to develop wells suffering from deformative casing issues. Advanced degradable thermoset developed in this work offers temperature rating above 150℃ due to its highly cross-linked molecular structure. It exhibits extremely high strength (20~30 ksi [138-207 MPa] in tensile) at elevated temperature, and its dissolving rate in acid (15% HCl) is marginally higher than that in brine water during 2 hrs. of acid fracturing operation. Novel hybrid plug, with cone made from this new material and slips from traditional dissolvable alloy, is rated to hold pressure above 9000 psi (62 MPa) at 120℃ in acid fracturing condition, and degrades totally within 2 weeks, successfully meeting field requirement for HTHP wells. In case of premature set, this hybrid plug can be uninstalled reliably within hours through dissolving its dissolvable metal slips via acid. To the best of our knowledge, it is the first report of degradable thermoset composite material in downhole plug application, and the first totally degradable hybrid plug working at HTHP condition in our industry. This study could not only benefit oil and gas industry to hydraulic frac HTHP reservoir inaccessible before, but also provides a new functional material that should be potentially used in development of other advanced tools in applications including drilling, completion, work over, etc.