Uncertainties in marine controlled source electromagnetic (CSEM) data consist of two independent parts: measurement noise and position uncertainties. Measurement noise can be readily determined using stacking statistics in the Fourier domain. The uncertainties due to errors in position can be estimated using perturbation analysis given estimates of the uncertainties in transmitter-receiver geometries. However, the various geometric parameters are not independent (e.g. change in antenna dip affects antenna altitude, etc.) so how uncertainties derived from perturbation analysis can be combined to derive error-bars on CSEM data is not obvious. In this study, we use data from the 2009 survey of the Scarborough gas field to demonstrate that (a) a repeat tow may be used to quantify uncertainties from geometry, (b) perturbation analysis also yields a good estimate of data uncertainties as a function of range and frequency so long as the components are added arithmetically rather than in quadrature, and (c) lack of a complex error structure in inversion yields model results which are unrealistic and leads to “over-selling” of the capabilities of CSEM at any particular prospect.
The ability of the marine controlled source electromagnetic method to resolve anisotropy in the sediment conductivity is not very well understood. In this study, we address the resolvability of anisotropy using a Bayesian approach. Two markedly different methods, slice sampling and reversible jump Markov Chain Monte Carlo have been used for the Bayesian inversion of a synthetic model of a resistive oil reservoir trapped beneath the seabed. We use this to identify which components of data can provide the strongest constraints on anisotropy in the overburden, reservoir and underlying sediments.
Advanced well placement technologies were utilized from the commencement of the development of the Vincent oil field located offshore, west of Australia. The first development well was drilled in June 2007. To date, there are a total of eight bi-lateral and five tri-lateral development wells with average horizontal well lengths in the reservoir exceeding 2000 m. Geological and reservoir complexities contributed to a challenging drilling and geosteering environment, which often resulted in a number of undesired sidetracks to achieve the optimum well placement and meet pre-drill objectives. Drilling challenges included very fast penetration rates requiring immediate well placement decisions to be made, drilling windows of less than 3 m true vertical depth (TVD) in sections of wells and loss of directional control due to encountering faults and dipping stratigraphic surfaces. Deep reading azimuthal resistivity logging-while-drilling measurements were used to position wells as high as possible to top structure and away from the oil-water contact. Interpretation of top structure from directional resistivity data is complicated by interference from dipping low resistivity intra-reservoir beds. Through detailed study of azimuthal resistivity responses from previous well campaigns and collaborative interpretation between the well placement and subsurface teams, the measurement signature between top structure and intra-reservoir surfaces was able to be distinguished. Accurate, high density well surveys are critical when steering within a thin oil column. In a few earlier wells, low stationary survey sampling frequency of approximately every 30 m at the end of each drilling stand did not capture the true well trajectory and resulted in incorrect well positioning and inadvertent penetration of the gas-oil contact in one well. This new rotary steerable system is a hybrid push- and point-the-bit technology and has potentially reduced the number of sidetracks required for optimal well placement.
In some of our sandstone reservoirs we have encountered apparent residual gas below the live gas column, and the quantification of the residual gas volume has been a challenge. The presence of such intervals of residual gas is related to the charge history of the field. If for any reason the trap was breached or tilted after gas charge, gas was replaced by water encroachment in the portion of the rock where the residual gas is now present, but the water displacement of gas was not complete. As water filled pores and pore throats, the free flow of gas stopped, allowing only water to pass through the pore system. This resulted in gas becoming trapped behind the encroaching waterfront as residual gas. The need to identify residual gas zones is important for several reasons, mainly, appropriate fluid corrections for porosity calculations and acknowledging that an absence of any well-defined water-leg will not cause an underestimation of water saturation by the incorrect use of Pickett plots. Appreciation of residual gas allows the use of imbibition saturation-height functions for both static and dynamic modelling. Quantification of residual gas saturations are complicated by sensitivity to porosity and cementation factor, ''m'', at low water saturations and to a lesser degree saturation exponent, n. A number of methods have been utilised in trying to quantify the range of in-situ residual gas saturations.
INTRODUCTION AND IMPORTANCE OFRESIDUAL GAS
We have now encountered many examples of residual gas. This paper represents the many learnings on how to identify, attempt to quantify and finally appropriately model the impacts of residual gas. The presence of residual gas is related to the charge history of the field. If for any reason the trap was breached or structure tilted after gas charge, gas will be replaced by water encroachment.