A uniquely designed water-based mud (WBM) was used in two shelf areas of the Gulf of Mexico to drill highly reactive gumbo sections. The drilling performance and inhibition in these shale intervals resembled that of synthetic-based muds (SBMs) previously used in offset wells. Owing to new environmental regulations, obtaining the drilling-performance characteristics of a synthetic-based fluid and the stability of a water-based fluid is a high priority for Gulf of Mexico operators.A special characteristic of the WBM fluid is its ability to deliver SBM-like shale inhibition without use of any salts, through an ideal combination of a polyamine shale inhibitor, a polymeric encapsulator, and an antiaccretion additive.This paper presents field data comparing the drilling performance and the inhibition in gumbo-shale sections of the new WBM and the previously used SBM. The combination of this inhibitive WBM and the rotary-steerable tool produced an 85% higher rate of penetration (ROP) than in similar wells in the area drilled with SBM and mud motors. No accretion or bit balling was noted on bottomhole assemblies (BHAs) during bit runs, and the cuttings drilled were well encapsulated and protected from hydration. Economic savings were realized by reducing the drilling time and by not requiring the essential waste-management equipment and engineering associated with SBM-cuttings discharge. Furthermore, the new high-performance WBM was easily prepared at the wellsite with seawater, which also translated into cost savings by reducing the volumes handled by the workboats.These field applications demonstrate that this uniquely designed WBM can effectively replace invert-emulsion fluids in many highly reactive shale sections without sacrificing drilling performance or increasing environmental risk.Shale-Dispersion Suppressant. This is a low-molecular-weight copolymer that is fully water soluble and exhibits good biodegradability and low marine toxicity. The polymeric additive is designed

TX 75083-3836, U.S.A., fax 01-972-952-9435. AbstractA uniquely designed water-based mud (WBM) was used in two shelf areas of the Gulf of Mexico to drill highly reactive gumbo sections. The drilling performance and inhibition in these shale intervals resembled that of synthetic-based muds (SBM) previously used in offset wells. Owing to new environmental regulations, obtaining the drilling performance characteristics of a synthetic-based fluid and the stability of a water-based fluid is a high priority for Gulf of Mexico operators.A special characteristic of this fluid is its ability to deliver SBM-like shale inhibition without using any salts, through an ideal combination of a polyamine shale inhibitor, a polymeric encapsulator and an anti-accretion additive.This paper presents field data comparing the drilling performance and the inhibition in gumbo shale sections between the new WBM and previously used SBM. With instantaneous rates of penetration up to 85% higher than synthetic-based fluids used in similar well sections, the average time required to drill identical intervals was reduced dramatically. No accretion or bit balling was noted on bottomhole assemblies during bit runs, and the cuttings drilled were well encapsulated and protected from hydration. Economic savings were realized by reducing the drilling time and by not requiring the essential waste management equipment and engineering associated with SBM cuttings discharge. Furthermore, the new high-performance WBM was easily prepared at the wellsite using seawater, which also translated into cost savings by reducing the volumes handled by the workboats.These field applications demonstrate that this uniquely designed water-based fluid can effectively replace invertemulsion fluids in many highly reactive shale sections without sacrificing drilling performance or increasing environmental risk.