Preparation and degradable mechanism of self-breaking gel valve for underbalanced drilling

As an effective drilling pressure control technology, the gel valve offers a potential alternative to casing valves by utilizing its own strength to isolate downhole pressure. This paper presents a self-breaking gel valve synthesized using acrylamide and degradable crosslinker, which addresses drawbacks of existing gel-breaking techniques, such as time consumption and incomplete breaking. Firstly, the effects of monomer concentration, initiator concentration, and degradable crosslinker concentration on the performance of the gel valves were investigated. The results indicate that the gelation time can be controlled within 1–3 h, while maintaining degradation time control within 5–20 days at 90 °C. Then the pressure-bearing capacity and the effect of temperature on the degradation properties of the self-breaking gel valve were evaluated. The average pressure-bearing gradient of the gel valve at 90 °C can reach to 0.66 MPa/m. More importantly, the mechanism of gel breaking was studied. The results show that the ester groups hydrolyze gradually during the degradation process, leading to fracture of the crosslinking point inside the gel valve, a reduction in binding degree of intermediate water within the gel valve, and a decrease in crosslinking density. Additionally, there is a change in microstructure from a regular dense three-dimensional network structure to a linear random structure. The self-breaking gel valve eliminates the need for subsequent gel-breaking operations, reducing costs and preventing pollution from chemical gel-breaking agents. The self-breaking gel valve is expected to be used in pressure control for whole process underbalanced drilling.