Drilling fluid technology for horizontal wells to protect the formations in unconsolidated sandstone heavy oil reservoirs

Major factors for damages in drilling the unconsolidated sandstone heavy oil reservoirs include: invasion of solids in drilling fluid, incompatibility between liquid phase of drilling fluid and crude oil, and hydration and expansion of reservoir clay minerals. Therefore, a solid-free weak gel drilling fluid system for horizontal wells to protect the formations was developed, i.e. seawater + 0.1%-0.2%NaOH + 0.2%Na2CO3+ 0.7%VIS + 2.0%FLO + 2.0%JLX, weighed with KCl or formate. The drilling fluid system has unique rheological property, temporally independent gel strength, and excellent lubricating and inhibition performance. It is compatible with formation fluids, it not noly meets the needs of horizontal well drilling, but also effectively protects reservoir. The technique is well performed in tens of horizontal wells in offshore oilfields, such as WC13-1, BZ34-1, NP35-2, and BZ25-1 oilfields.