Drilling through and cementing in naturally fractured carbonate reservoirs always pose challenges due to complete losses and risk of losing well control, In such scenario, it can take multiple days to restore the circulation and well control. The historical attempts and current conventional loss-cure strategy is loading of loss control material [LCM]in the active drilling fluid system or pumping assorted pills with loss circulation materials that can effectively cure seepage to partial losses. But curing complete losses can take extra days resulting in non-productive time [NPT] and significant carbon dioxide emissions and loss of fresh water. With the recent drive for sustainability and time-effective drilling to reduce carbon footprint, the current need is to design a loss-cure strategy that can effectively eliminate the time required for cement plug placement, waiting on cement, and trips as all these operations to cure losses account for ~12% of total NPT during drilling operations.
A sustainable loss-cure strategy has been formulated by placing and squeezing an engineered pill against the natural fractures. The engineered pill is optimized after thorough fracture width analysis, laboratory design, and software simulation for particle size management. The pill is composed of engineered solid volume fraction and particle size distribution, a viscosifier, and unique dual fiber technology, which provide resistance against the differential pressure across the formation face eventually providing a sustainable barrier and seal to drill through the entire section or casing cementing later at higher equivalent circulating density [ECD]. The pill placement replaces the lengthy and time-consuming process of placing multiple assorted pills and a cement plug and thus reducing rig time and minimizing carbon emissions.
This paper explains the case histories of pill placement as part of sustainable loss cure strategy where complete loss was observed while drilling naturally fractured reservoir with mud weight of 8.7 to 8.9 ppg. Multiple conventional and advanced assorted pills were unable to control the losses or resume the drilling fluid circulation. A careful analysis of fracture width information and simulation of this data in software provided detailed input for the fluid design and lab test analysis. The pill was placed against naturally fractured zone and was squeezed at pre-determined pressure for effective sealing. After squeeze, the well was successfully drilled through the fractured zone till TD and desired cement job ECD (up to 15.8-lb/galppg) was achieved for zonal isolation.
The sustainable loss cure strategy has been developed based upon four steps plugging mechanism namely disperse, bridge, plug and sustain. The unique combination of stiff and soft fibers combined with viscosifier, and managed solids distribution enables the sustainable seal for fracture widths upto 5-mm. The pill can also be pumped through qualified downhole units for coarse particles.