Geostopping With Resistivity-Forward Modeling to Prevent Drilling Into The Loss-Circulation Zone of a Prolific Carbonate Reservoir

Abstract

Summary The field-development plan for a Sarawak Shell Bhd.-operated gas field, located in the South China Sea offshore Sarawak, Malaysia, specified drilling of horizontal wells into the Tertiary-Miocene carbonate reservoir. The wells were planned as high-capacity producers with a big-bore, long-casing flow design. The traditional well design dictated that before entering the reservoir a casing had to be installed to stabilize the hole in soft shale. The uncertainty of detecting the formation top resulted in a premature casing commitment of at least 30 ft true vertical depth (TVD) above the top of the reservoir and the need to use an expandable liner to cover 300 ft of exposed shale above the reservoir. To obviate this problem, the capability of one of the components in the logging-while-drilling (LWD) tool array, specifically the electromagnetic-wave-resistivity (EWR) forward-modeling technique, was used to detect the top of the carbonate formation (i.e., the top of reservoir), immediately before drilling into it. A standard LWD tool is configured to prioritize EWR forward-model response as the carbonate-formation top is approached. This configuration, together with an appropriately designed bottomhole assembly (BHA) and well trajectory, enabled the successful implementation of the plan to stop drilling approximately 1 ft TVD above the carbonate top. At this point, a conventional 9?-in. casing string was set at an optimum depth. This eliminated potential well-control problems, costly remedial actions associated with lost circulation, and inferior cementation of the9?-in. casing string. Thereafter, the wells were drilled horizontally in a conventional manner into the carbonate-gas reservoir. This paper compares predrilling EWR forward modeling of the proposed well trajectory with the actual well data while drilling. The predrilling- and post-drilling-modeled data are presented. The cost savings from employing this technique are variable, ranging fromsubstantial—in the event of a well-control situation and attendant high losses—to moderate if the need to set an expandable liner is eliminated. A minimum of U.S. $1 million per well was saved with this technique. Introduction Sarawak Shell Bhd. operates numerous gas fields in the central Luconia area located in the South China Sea, offshore Sarawak, Malaysia. The field-development plan for the M4 field specified drilling two horizontal wells into the Tertiary-Miocene carbonate reservoir. The wells were planned as high-capacity producers with a big-bore, long-casing flow design. In the central Luconia area, the drilling of development wells is challenging because frequent mud losses or total loss of circulation are encountered as a result of the karst phenomenon, which is the secondary erosion of limestone formation producing subterranean fissures, conduits, and caverns as well as fractures in the carbonate reservoir. The carbonate reservoir is overlaid with a thick layer of soft shale, which needs to be drilled with high mud-weight drilling fluids: 13.27 ppg. A 9?-in. casing string has to be set before drilling into the carbonate reservoir to avoid borehole collapse when drilling into the reservoir with lower mud weight to prevent severe mud losses. Because the shale section above the carbonate reservoir lacks any geological character or marker beds to help delineate the top carbonate, it was common practice to set the casing point at least 30 ft TVD above the top carbonate. Subsequent drilling left approximately 300 ft of exposed soft shale along the borehole before entering the carbonate reservoir, which necessitated the installation of an expandable liner to isolate the soft shale. Therefore, a new methodology was sought to minimize the length of the openhole shale section.