CSEM Method Research Articles

Time-Lapse CSEM Monitoring: Correlating the Anomalous Transverse Resistance with SoPhiH Maps

The CSEM method, which is frequently used as a risk-reduction tool in hydrocarbon exploration, is finally moving to a new frontier: reservoir monitoring and surveillance. In the present work, we present a CSEM time-lapse interpretation workflow. One essential aspect of our workflow is the demonstration of the linear relationship between the anomalous transverse resistance, an attribute extracted from CSEM data inversion, and the SoPhiH attribute, which is estimated from fluid-flow simulators. Consequently, it is possible to reliably estimate SoPhiH maps from CSEM time-lapse surveys using such a relationship. We demonstrate our workflow’s effectiveness in the mature Marlim oilfield by simulating the CSEM time-lapse response after 30 and 40 years of seawater injection and detecting the remaining sweet spots in the reservoir. The Marlim reservoirs are analogous to several turbidite reservoirs worldwide, which can also be appraised with the proposed workflow. The prediction of SoPhiH maps by using CSEM data inversion can significantly improve reservoir time-lapse characterization.

Vertical dipole CSEM: 3D acquisition and data impact from infrastructure

Abstract Marine CSEM methods for subsurface investigations were developed nearly 30 years ago (Cox, 1981), and have found extensive applications within the offshore hydrocarbon exploration industry over the past 15 years (e.g. Constable, 2010). These methods detect contrasts in electrical conductivity, exploiting the fact that the electrical conductivity of hydrocarbon-saturated reservoirs is significantly smaller than in the surrounding sediments (Ell-ingsrud et al., 2002). While marine CSEM has proven itself as a valuable tool in exploration and mapping of frontier areas, there is growing increased interest in applying CSEM for near-exploration and reservoir monitoring applications. A vertical-based time domain EM exploration method (Barsukov et al., 2007) has been developed by the Norwegian geophysical company PetroMarker, founded in 2005. In the past the method has primarily been used for exploration, but since the vertical method relies on a stationary acquisition mode with very high accuracy in transmitter positioning it allows both repeatability and freedom of operation close to existing infrastructure and installations, and in vulnerable environments. Over the past few years there have also been improvements in receiver technology, allowing efficient 3D acquisition. We will present improvements to the method and a 3D field data example. We will also present investigations of the impact of the method on field data from infrastructure.

Potential applications of time-lapse CSEM to reservoir monitoring

Monitoring of changes in brine chemistry (salinity and temperature), during water-flooding is important for injector optimisation, understanding efficiency, detecting early water breakthrough, locating bypassed hydrocarbons or detecting scaling in the heterogeneous reservoir. It is already known that water injection into the oil-leg of a hydrocarbon reservoir can be monitored by both seismic acquisition and also CSEM methods. The problem of an interfering pressure signal for seismic impacts quantitative evaluation for time-lapse analysis, while with EM there are the counterbalancing effects of salinity and temperature. CSEM becomes favoured when the pressure effects on seismic are dominant, or heavy oil is present with similar acoustic properties as the formation and injected waters. The quality of measurement for both methods is influenced by reservoir facies variations, acquisition repeatability and overburden heterogeneity. Time-lapse seismic is unlikely to detect brine distributions injected into the water-leg or aquifer, although it may detect associated pressure up effects. However, our calculations show that it may be possible for time-lapse CSEM to distinguish the inter-mixing of different brines in the subsurface hydrocarbon reservoir. Specifically, low salinity injection or injection into a highly saline formation can clearly be detected with this technique.

Detection and imaging sensitivity of the marine CSEM method

We have developed a formalism to systematically study the sensitivity of the marine controlled source electromagnetic method for hydrocarbon exploration, taking into account measurement errors. We utilize error propagation to estimate the data uncertainty, and find that contributions that scale by the transmitter dipole moment give qualitatively different behavior than noise contributions that are independent of the dipole moment. The uncertainty is used in quantitative criteria to determine if a hydrocarbon reservoir can be detected and whether it can be successfully imaged. Our formalism can be used to make detailed feasibility studies incorporating equipment accuracy limitations, and to identify which experimental component that must be improved to ensure overall improvement in sensitivity. The criteria can be applied to identify the limiting factors for detection and imaging of targets with increasing burial depth. We have defined experimental accuracy conditions for next generation CSEM equipment to successfully image a large hydrocarbon reservoir at 5 km burial depth.

Development and persistence of sandsheaths of Lyginia barbata (Restionaceae): relation to root structural development and longevity

Strongly coherent sandsheaths that envelop perennial roots of many monocotyledonous species of arid environments have been described for over a century. This study, for the first time, details the roles played by the structural development of the subtending roots in the formation and persistence of the sheaths. The structural development of root tissues associated with persistent sandsheaths was studied in Lyginia barbata, native to the Western Australian sand plains. Cryo-scanning electron microscopy CSEM, optical microscopy and specific staining methods were applied to fresh, field material. The role of root hairs was clarified by monitoring sheath development in roots separated from the sand profile by fine mesh. The formation of the sheaths depends entirely on the numerous living root hairs which extend into the sand and track closely around individual grains enmeshing, by approx. 12 cm from the root tip, a volume of sand more than 14 times that of the subtending root. The longevity of the perennial sheaths depends on the subsequent development of the root hairs and of the epidermis and cortex. Before dying, the root hairs develop cellulosic walls approx. 3 µm thick, incrusted with ferulic acid and lignin, which persist for the life of the sheath. The dead hairs remain in place fused to a persistent platform of sclerified epidermis and outer cortex. The mature cortex comprises this platform, a wide, sclerified inner rim and a lysigenous central region - all dead tissue. We propose that the sandsheath/root hair/epidermis/cortex complex is a structural unit facilitating water and nutrient uptake while the tissues are alive, recycling scarce phosphorus during senescence, and forming, when dead, a persistent essential structure for maintenance of a functional stele in the perennial Lyginia roots.

A relation between the CPA and CSEM methods

CSEM is a non-perturbative method for calculating the density of states in disordered systems. This method and similar, specially CPA-based, ones are compared.