Geo-Stopping With Deep-Directional-Resistivity Logging-While-Drilling: A New Method for Wellbore Placement With Below-the-Bit Resistivity Mapping (2016)

Summary The Wheatstone liquefied-natural-gas (LNG) Project in Western Australia uses subsea big-bore gas wells for producing the Wheatstone and Iago gas accumulations. These wells use a 9⅝-in. production conduit from the top of the gas-pay zone to the ocean floor to maximize their productive capacity. A critical design requirement is that the 9⅝-in. production conduit be set as close as possible to the target zone containing gas-pay intervals (i.e., preferably within 3 m) without actually penetrating into the productive core of a gas interval. In many instances, the top of the target zone and gas pay are one and the same. The wells must also intersect the horizontal top of the target zone at inclinations (inc) of 45–60°. These design requirements impose a significant challenge to the efficient execution of a Wheatstone well. Traditional “ahead-of-the-bit” logging-while-drilling (LWD) solutions are capable of detecting resistivity fluctuations up to 1 m in front of the drill bit. This, however, was considered an insufficient look-ahead distance to prevent premature penetration of a gas reservoir. Alternative solutions, such as pilot holes and biostratigraphic analysis of drilled cuttings, were also considered but found to be too expensive and/or operationally impractical. During a search for a potential solution to this problem, the Wheatstone Project team reviewed a new reservoir mapping-while-drilling technology (Beer et al. 2010; Jenkins et al. 2012; Leveque et al. 2012; Netto et al. 2012; Peppard et al. 2012; Constable et al. 2012; Viandante et al. 2013; Pontarelli et al. 2014; Seydoux et al. 2014; Dupuis and Mendoza-Barrón 2014) that was being field-tested for in-zone steering of horizontal wells and/or landing such wells at shallow angles of incidence relative to formation bedding (i.e., ≤ 13°). The technology, on the basis of deep-directional-resistivity (DDR) measurements, was recognized as a potential solution by the Wheatstone Project team, considering that it showed some promise for being able to predict/infer resistivity changes farther below the bit than previous systems. Deploying the technology at Wheatstone, however, would impose operating conditions for which it was neither designed nor intended (i.e., angles of incidence as high as 30–45°). Ultimately, the technology was successfully tested at Wheatstone and became the Project's default method for landing all big-bore gas wells to the required level of accuracy described above. This paper details (1) the safety and well-design drivers that precipitated the need for using DDR technology, (2) the physics and logic of this LWD system, (3) the new concept for applying the system in nonhorizontal applications, and (4) the observed results from all wells (seven in total) in which the technology was used. To fully evaluate the results, the below-the-bit resistivity and formation depths that were predicted by the DDR system while drilling above the gas-bearing target zone are compared with the actual formation depths and resistivities that were directly measured later when drilling through the target zone. Directly comparing the real-time DDR estimates to actual in-zone measurements, taken in the same strata and geographic locations, has allowed us to clearly determine both the system's geometric limits for detecting resistivity variations below the bit and its ability to accurately resolve the depth and resistivity of formations before penetrating them.