Securing Depleted Sandstone Gas Reservoirs in Oman With a Novel Reverse Hybrid Fluid System (RHS) Technique

Abstract

Abstract High differential depletion in deep tight gas reservoir causes limitation on the treatment pressure that can be applied to initiate and place optimum hydraulic fracture treatments. This paper will outline how a technique, applying viscous fingering, was successfully deployed to place hydraulic fractures in very challenging highly differentially depleted gas sandstone reservoir wells in Oman. Most gas wells in PDO are completed with a 4 ½" 17ppf 13Cr cemented completion in stacked reservoirs. The differences in hydrocarbon mobility across the different target units will inevitably cause differential depletion. The challenge was to reduce surface treatment pressures while maintaining the ability to place a meaningful hydraulic fracture. The first Reverse Hybrid Fluid System (RHS) treatment in Oman was placed in PDO in 2019 and since then this technique has successfully managed to secure zones that would otherwise have been avoided. Long term monitoring of the wells with RHS was set up to observe the impact of this change in hydraulic fracture design on well productivity. When simulating the hydraulic fracture treatments, the anticipated pressures excide the maximum allowable surface pressure to maintain the completion integrity. Without the possibility to reach the required pressures, early screen outs or the inability to breakdown the formation was observed in the field. The RHS technique managed to significantly reduce the treatment pressure while still maintaining the ability to create geometry and compensate for the low fluid efficiency, observed during the data frac, by using a Crosslinked Pad (CP) followed by slurry stages with Linear Gel (LG) or High Viscous Friction Reducer (HVFR) to carry the proppant. The difference in viscosity would carry the proppant in a viscous fingering pattern rather than plunger sweep pattern. This viscous fingering effect is very well documented in the industry, specifically in Enhance Oil Recovery (EOR). Conventional hydraulic fracture models, on the other hand, lack the ability to simulate this and, hence, optimizing the design and finding the right mobility ratio between the fluids is an area for much improvement. The change in technique meant a change in the hydraulic fracture design as well as the type and proppant concentrations. A plan was put in place with production logs to monitor any impact of RHS on production, long term impact is yet to be assessed. RHS provides the ability to target reservoir units otherwise not feasible with combinational hydraulic fracture techniques, helping to secure the ultimate recovery from wells where differential depletion is observed.