Rock Physics Modelling in Oil and Gas Field Development, A Methodology for Reservoir Characterisation below Shallow Gas

Abstract

Abstract A large number of oil and gas fields in the Peninsular Malaysia basin are screened (masked) by shallow gas, resulting in wipe out zones of the underlying seismic amplitudes as well as push down and time delay (sag) of the structural closure of economical oil or gas fields. Such effects lead to significant uncertainties in resolving and mapping of the objective reservoir units below the shallow gas. On these premises, 3D 4C Ocean Bottom Cable (OBC) seismic survey has been acquired and processed in one of the gas and oil fields in the Malay basin with the objective of resolving the multi-layered reservoir intervals through mode converted shear component data. This paper will discuss the rock physics and properties model that has been developed as part of a 3D 4C PP-PS Joint Seismic Inversion project with the primary objective of delineating and characterizing the various reservoir intervals and depositional environments of the oil and gas accumulation, a first in Petronas Carigali experience in reservoir characterization. This paper focuses on synthesizing missing logs, log conditioning, seismic petrophysical evaluation, rock physics modeling, invasion correction and fluid substitution to the fullwaveform (P- and S-wave) and density logs to make ready for seismic inversion. A total of 10 wells with 8 measured P- and S-wave logs are available for this study. The overall qualities of well logs were adequate for the purpose of this inversion project. These data have been used in the generation of rock physics models. The methodology applied was to construct a rock physics model that is consistent with the petrophysical analysis and the elastic properties as given by the conditioned well log data. The model was then applied everywhere to generate synthetic elastic logs and full offset synthetic seismograms that were then compared and matched to the recorded data respectively. Introduction Well log data are used to understand the relationship between the reservoir properties and subsurface seismic data. Since well log data provide constraints and calibration in the inversion workflow, their quality directly impact the confidence and the accuracy of the inversion results for quantitative interpretation and follow-on reservoir modeling. Well log quality and consistency are subjected to various factors like variation in tool measurements, borehole rugosity, borehole fluids, elapse time between drilling and logging, invasion of borehole fluids into the formations, alteration of the formation properties due to the presence of the borehole and/or borehole fluids, casing points, and missing data. Therefore, the primary goal in processing well log data is to minimize measurement related errors, generate and fill in the missing data, and to obtain consistent and accurate logs between all the wells.