Detecting Shallow Drilling Hazards in Large Boreholes Using LWD Acoustic Measurements

Abstract

Shallow drilling hazards present a significant risk to the health, safety, and environment of upstream oil and gas operations. Shallow marine formations are often near the critical porosity limit, where the sediment is very poorly sorted, mechanically weak and often unconsolidated. As the sediment begins to compact, well-construction challenges are created as formation stresses and fluid pressure changes, usually resulting in the operator having to set extra casing strings near the surface. The United States Department of the Interior, Minerals Management Service (MMS) requires the use of a diverter and a conductor string of casing if there is any possibility of a shallow hazard in the area of a proposed well. Proving shallow hazards do not exist in the area requires resistivity and porosity logs approved by the MMS. This operator requested these logs be run in 22 and 17 ½ in. boreholes using Logging While Drilling (LWD) technology as no wireline logging was planned. In large hole sizes, the porosity log is generally the limiting factor for most wireline and LWD technologies, with nuclear measurements from most vendors designed for hole sizes no bigger than 12 ¼ in. LWD Acoustic measurements in this well provided acceptable determination of porosity in borehole sizes up to 22 inches in extremely slow formations.The objective of this application was to acquire an LWD acoustic compressional slowness log below the drive pipe. Modeling shows and supports the ability to measure in this demanding environment where the compressional slowness often approaches that of the drilling mud. A 9 ½ in. LWD acoustic device was optimized for shallow data acquisition for the large borehole size and very slow acoustic velocities anticipated. While drilling, real-time LWD logs showed conclusive evidence of the absence of hazards and were immediately accepted by the MMS for the waiver. This paper validates the feasibility of using LWD logs to gain a conductor waiver from the MMS. Benefits include reduced operating costs associated with a wireline program and the savings involved in removing the conductor string from the casing program. On this multi-well batch drilling project operational savings of about 2.15 million USD were realized.