Geo-Stopping at the Top of Carbonate by Reducing the Depth Uncertainty using Seismic While Drilling, Near-Bit Gr, and Resistivity Real-Time Data, A Novel Carbonate Reservoir Entry Strategy in Deep Water Malaysia

Setting casing as close as possible to the top of the reservoir is critical in deep water carbonate exploration drilling strategy in Malaysia for several reasons, such as the pressure difference from the overburden and mud loss related to vugs, fractures, and karsts. Missing accurately setting this critical casing point may lead to severe rig NPT and related additional operational costs and, in extreme cases, may lead to abandoning the well. This challenge requires a novel and fail-safe strategy to ensure the identification of the Top of Carbonate (TOC) precisely during drilling. An expected pressure ramp at the top part of the carbonate reservoir in the subject exploration well located in deep water off Malaysia was a critical drilling challenge. Hence, accurately establishing the TOC depth, followed by placing a casing shoe close to it, and thus isolating the overburden shale was necessary before drilling the reservoir section. The seismic uncertainty of the TOC at the pre-drilling phase was large. Hence a comprehensive solution involving seismic while drilling, near-bit gamma ray, and resistivity measurement was deployed to progressively reduce the depth uncertainty while drilling. This solution allowed the client to accurately identify the TOC depth and eventually decide on the section TD to set the casing. Borehole Seismic (VSP) while drilling look-ahead technology was selected as the ideal solution to reduce the depth uncertainty of the TOC. To acquire VSP data a downhole receiver was deployed in the drilling Bottom Hole Assembly (BHA) and surface sources were used to provide the acoustic source as for normal wireline VSP. During drilling, the BHA was stopped when required; the surface sources were fired and the signal was recorded downhole by the receiver. In seismic while drilling technology, the shooting sequence triggers the tool to stack live data shots and create a data extract. Once drilling resumes this extract is transmitted to the surface by the mud pulse system for QC on the rig and further VSP processing. Subsequently, the data extract was processed to create a zero-phase multiple-free Corridor Stack. The result was available within 20 minutes allowing the team to decide on a further drilling strategy A Comparison of the VSP corridor stack and the Surface Seismic allowed the correlation of the TOC event wavelet. The wavelet picked from the Surface Seismic was used to identify the corresponding wavelet on the VSP data. As the time/depth of the VSP downhole receiver was a known point we could then calculate the time that the TOC wavelet was ahead of the current VSP receiver depth, creating an estimate of the depth of the top carbonate (Figure 1). As the VSP downhole receiver was much closer to the target than the surface seismic sources and receivers, the look-ahead estimate was more accurate, and as more VSP data was recorded the depth estimate was refined. The Near Bit GR and Resistivity services were also deployed to acquire those data as early as possible. The GR of the carbonate was substantially lower, and the resistivity was much higher than the overburden shale. The sudden deflection of those measurements indicated the TOC as soon as the drill bit penetrated the reservoir. These complementary measurements were planned as a backup to the seismic- based solution and eventually helped to identify the TOC with confidence. The lookahead TOC depth was predicted when the deepest VSP level was approximately 120 m above the Carbonate with uncertainty as low as 2m with the precise depth being confirmed by Near-bit measurement. This accurate while drilling novel approach saved rig time and money by eliminating the need for conventional wireline look ahead VSP and enabled drilling of the reservoir section safely by placing the casing shoe exactly at TOC