Integrated Approach for the Characterization of Low Resistivity and Low Contrast Pays – A Case Study from Mahanadi Offshore, India

Abstract

Abstract The deep water formations from Mahanadi Offshore is one of the major focus for hydrocarbon exploration in India. The reservoirs in this field primarily have low resistivity and low contrast character due to presence of thinly laminated shale-sand laminae with overlying shales. Conventional logs do not sufficiently distinguish between the overlying shales against the laminated reservoir intervals- owing to poor vertical resolution of the tools to characterize the thin beds properties. The operating company followed the standard acquisition of conventional logging data along with tensor resistivity measurement for a laminated-shaly sand analysis (LSSA) followed by formation testing. While multiple new pay sands were discovered through this method, the sand-shale-fluid model of LSSA is insufficient to provide fluid typing. Furthermore, fluid sampling using formation testers in these formations are risky due to unconsolidated nature of the formation, Plugging & SOBM invasion etc. Thus, the delineation of the formation fluid using methods other than just the formation testing is critical. The acquisition of magnetic resonance data and an integrated interpretation involving conventional logging, tensor resistivity and NMR data helped overcome these challenges, prior to formation testing. The electrical anisotropy and borehole image data helped identify thin beds. LSSA outputs based on conventional and tensor resistivity data indicated hydrocarbon bearing intervals. A continuous magnetic resonance measurement acquiring the longitudinal relaxation (T1), transverse relaxation (T2) and the diffusion coefficient (D), helped derive a lithology independent fluid typing in these wells to provide insights before formation testing. Hydrocarbon bearing sands were identified as per the results of the integrated analysis, and the formation testing confirmed the same. Conventional resistivity inversion, LSSA saturation and flushed zone saturation from magnetic resonance provided an understanding of the invasion. The hydrocarbon bearing intervals delineated from integrated formation evaluation using LSSA and 2D NMR analysis were successfully tested and proven using formation testing and sampling. Mineralogy information helped in accurate formation evaluation. Subsequently, the exploratory well logging programs have been modified to include the magnetic resonance log in future wells. This paper provides a framework for integrated data interpretation using multi-component induction, magnetic resonance, acoustics, mineralogy data and resistivity imager logs to quantify reservoir fluids and subsequently confirmation from the formation testing and provides a workflow for exploratory wells for Mahanadi Offshore, India.