Drilling Through Shales Below Depleted Sands: Case Study of a Niger Delta HPHT Gas Development Well

Abstract

To sustain gas supply to NLNG T1-T7, it has become imperative to access deeper, geologically complex HPHT reservoirs in the Niger Delta. These hydrocarbon targets typically lie beneath hydrostatic geological intervals and are overlain by depleted and/or producing reservoirs hence choosing the right casing depth is a key parameter in executing HPHT wells In the wells in Astra East Field, it was decided to set a casing shoe in a homogeneous shale of ~200ft vertical thickness, located between overlying heavily depleted sand and underlying over pressured sand. This poses an interesting question of the direct practical importance: at what depth should the casing shoe be set to minimize the risk of kicks and/or wellbore instability (because of too low mud weight) and losses (because of too high mud weight). To help answer this question, wireline log data and drilling observations in other wells in the same field, where such shales were found, were analyzed, and evaluated. The electrical resistivity in the shales consistently showed the following signature from top to bottom across the shale: Zone 1: Relatively high electric resistivity just below the depleted sand, near constant with depth. Zone 2: The signature gradually decreasing with increasing depth, and finally, at the lower section of the shale. Zone 3: Relatively low electric resistivity just above the over pressured sand, the signal again near-constant with depth. The drilling observations revealed that wells with the casing set in Zone 1 experienced severe mud losses and differential sticking, while wells where the casing set in zones 2 or Zone 3 did not. This observation could be explained as follows - The electric resistivity signature as a function of depth (in the three zones described above) may reflect the pore pressure in the bounding sands: at the top, the shale "feels" the sand depletion, transmitted over the years via pore fluid pressure diffusion, which compacts the shale, presses the grains contacts closer together, thus increasing electrical resistivity. In contrast, at the base, the shale "feels" the overpressure in the sand below, maintained over the millions of years of geologic diagenesis. This keeps the deeper part of the shale at relatively low effective stress (compared to the upper part), with relatively low grain contact pressure, thus reducing electrical resistivity. We postulate that there may be a mechanism-based explanation for the heavy losses and sticking when the casing is set in Zone 1. We also inferred from the drilling data that the tendency to set the casing shoe quite shallow (in Zone 1) in previous wells and in the well in case study was driven by concern of wellbore instability and severe losses experienced while drilling through the intra-reservoir shale. Closer inspection reveals that this concern is probably not justified, as the apparently high risk of wellbore instability at the top of the shale was caused by using a too-high pore fluid pressure (i.e., one unaffected by the depletion of the sand on top of it). For future planned wells in the field, new LWD data acquisition practices have been developed for early detection of the onset of overpressures (top of Zone 2). This will improve the accuracy of casing point selection and the chances of successfully drilling across these intervals without well control issues.