The Research on Novel Fracturing Sliding Sleeve Using High-Strength Dissolvable Ball Seat

Massive hydraulic fracturing in horizontal wells plays significant role in high-efficient recovery of unconventional oil and gas reservoirs. Besides Plug & Perforation (PnP) process, Packer & Sleeve (PnS) multi-staged fracturing and Cemented Casing Sleeve (CCS) multi-staged fracturing technology were also widely used in the unconventional fields. Ball-activated sliding sleeve is one key tool frequently applied to perform selective completion and massive fracturing. However, down-hole intervention and milling-out job for ball seats are required which leads to drawbacks of high operation cost and low efficiency. In addition, CO2 fracturing gradually draws attention of operator for well productivity and recovery enhancement. Down-hole BHA is faced with corrosion issue. One innovative sliding sleeves with dissolvable ball seat was developed as the alternative to existing tools. Two different materials were proposed in this paper. The matrix body of ball seat using magnesium-aluminum-based dissolvable alloy or iron-based dissolvable alloy was evaluated in the lab according to mechanical properties, anti-corrosive and anti-erosive properties. Surface treatment was performed by plasma spraying process to form tungsten-carbide protective coating. Magnesium-aluminum-based dissolvable alloy was characterized with compressive yield strength of 640 MPa. Iron-based dissolvable alloy features with Vickers hardness of 300 HV, compressive yield strength of 760 MPa and slower dissolving rate of 4.3 mm per year. Sleeve prototypes were validated in the lab with maximum holding pressure of 74 MPa when engaging with balls. After ball seat dissolves completely, sleeve I.D. reaches 90 mm. As a slow-dissolvable material, modified iron-based alloy is considered as the optimal candidate. In field trial, one PnS multi-stage fracturing string with four new sleeves was tripped into one mature oil well. Ball engaging with seat, activating sleeve and proppant pumping were successfully performed in fracturing operation. Almost no ball-seat fragment was found in the flow-back fluid. It is proven that the novel technology exhibits advantage of corrosion resistance prior to fracturing, erosion resistance during fracturing and intervention less operation such as milling-out job of ball seats.