Abstract

It has long been known that an accurate understanding of borehole geometry is critical for the proper design of any primary cement job. This is particularly true of shallow, large hole sections drilled in the off-shelf, deepwater blocks of the Gulf of Mexico. This drilling environment is typically characterized by weak, unconsolidated formations; narrow tolerances between pore and fracture gradients; shallow water flow hazards; and abrupt variations in downhole pore pressures. Complex and expensive sub-sea wellheads and blowout preventers are required. The longstanding solution to the problem of identifying borehole geometry has been the wireline caliper. Large hole size and the deepwater drilling environment make this solution impractical for deepwater cementing. Traditional Logging-While-Drilling (LWD) approaches for acquiring borehole configuration data are also ineffective for these shallow hole sections due to borehole size. Recent developments in the applications for Resistivity At the Bit (RAB) tool show great promise for effectively addressing this problem. This paper will discuss RAB large hole caliper technology, how it can be used for proper cement design in deepwater well construction and the results of the field test.

Introduction

Determination of borehole geometry is a critical and often overlooked component to the proper design of a primary cement job (Ref. 2). Evaluating the actual geometry of the borehole allows cement designs to: match cement volumes pumped to actual volumes required; accurately place top of cement; accurately design the hydrostatic pressures exerted by the cement column to the conditions in the well; analyze potential for, and position of shallow water flows; evaluate friction pressures, flow regimes and the effectiveness of mud removal practices; and finally, evaluate the placement and effectiveness of centralization programs. Each of these applications for borehole geometry data will be discussed.

Successfully obtaining borehole geometry data in deepwater, large-diameter, hole sections has been technically difficult and impractical in the past. The longstanding solution has been to assign arbitrary excess factors to cement volumes pumped. Recently, a further application for the RAB tool has been discovered in large boreholes that may provide valuable insight into their actual geometry.

RAB-LWD caliper logging has proven successful in determining the geometry of large boreholes, up to 60 inches (Ref. 3). RAB has successfully overcome the limitations of some competitive techniques. The theory, operation and limitations of the RAB caliper tool will be briefly discussed below.

RAB large hole caliper tool has been successfully fieldtested in two deepwater development wells in the Gulf of Mexico (Ref. 3). In these wells, RAB borehole geometry data was used in conjunction with cementing design software to successfully design primary cementing jobs. This field test and the resultant data will be reviewed providing a case study for the application of the tool in deepwater, sub-sea cementing design.

Deepwater, Shallow Casing Cement Design and the Need for Accurate Borehole Geometry Data

Caliper data is often only gathered for production casings or liners where accurate cement placement and bonding is critical to the success of the well.

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