Equivalent formation strength as a proxy tool for exploring for the location and distribution of gas hydrates

Abstract Gas hydrate-bearing layers are normally identified by a seismic imaged bottom simulating reflectors (BSR) or by downhole log responses because of their high acoustic velocity and electric resistivity compared to surrounding formations. These gas hydrate characteristics can also result in contrasting in-situ formation compressive strengths. Here, we describe gas hydrate-bearing layers based on equivalent strength (EST), which relates to in-situ compressive strength, in five exploration boreholes drilled during the Indian National Gas Hydrate Program Expedition 02 (NGHP-02). For Site NGHP-02-23, a representative site for those that were established during NGHP-02, the EST evaluated from drilling parameters shows a constant trend of ∼2 MPa, with some strong peak values in the 0–271.4 m-below-seafloor (mbsf) interval, and a sudden increase up to 4 MPa above the BSR depth (271.4–290.0 mbsf). Below the BSR, the EST stays at ∼2 MPa to the bottom of the hole (378 mbsf). Comparing the EST with logging data and a core sample description suggests that the EST depth profiles reflect the formation lithology and gas hydrate content. The EST increases in sand-rich and gas hydrate-bearing zone. In the lower gas hydrate layers in particular, the EST curve shows the same approximate trend with that of P-wave velocity and resistivity measured during downhole logging. Similar relationships between EST, hydrate layer, and log responses are confirmed in other four sites drilled nearby in NGHP-02 Area B. These results suggest that the EST, as a proxy for in-situ formation strength, can indicate the location and extent of the gas hydrate as well as borehole logging. Although the EST was calculated after drilling, utilizing the recorded surface drilling parameters (weight on bit, top drive torque, RPM and rate of penetration) in this study, the EST can be acquired during drilling using real-time drilling parameters. In addition, the EST only requires drilling performance data without any additional tools or measurements, making it a simple and economical tool for the exploration of gas hydrates.