Alexander Gribenko

Rigorous 3D inversion of marine CSEM data based on the integral equation method

Marine controlled-source electromagnetic (MCSEM) surveys have become an important part of offshore petroleum exploration. However, due to enormous computational difficulties with full 3D inversion, practical interpretation of MCSEM data is still a very challenging problem. We present a new approach to 3D inversion of MCSEM data based on rigorous integral-equation (IE) forward modeling and a new IE representation of the sensitivity (Frechet derivative matrix) of observed data to variations in sea-bottom conductivity. We develop a new form of the quasi-analytical approximation for models with variable background conductivity (QAVB) and apply this form for more efficient Frechet derivative calculations. This approach requires just one forward modeling on every iteration of the regularized gradient-type inversion algorithm, which speeds up the computations significantly. We also use a regularized focusing inversion method, which provides a sharp boundary image of the petroleum reservoir. The methodology is tested on a 3D inversion of the synthetic EM data representing a typical MCSEM survey conducted for offshore petroleum exploration.

Large-scale 3D inversion of marine magnetotelluric data: Case study from the Gemini prospect, Gulf of Mexico

Three-dimensional magnetotelluric MT inversion is an emerging technique for offshore hydrocarbon exploration. We have developed a new approach to the 3D inversion of MT data, based on the integral equation method. The Tikhonov regularization and physical constraint have been used to obtain a stable and reasonable solution of the inverse problem.Themethodisimplementedinafullyparallelcomputer code. We have applied the developed method and software for the inversion of marine MT data collected by the Scripps Institution of Oceanography SIO in the Gemini prospect, Gulf of Mexico. The inversion domain was discretized into 1.6 million cells. It took nine hours to complete 51 iterations on the 832-processor cluster with a final misfit between the observed and predicted data of 6.2%.The inversion results reveal a resistive salt structure, which is confirmed by a comparison with the seismic data. These inversion results demonstrate that resistive geoelectrical structures like salt domes can be mapped with reasonable accuracyusingthe3DinversionofmarineMTdata.

Large-scale three-dimensional inversion of EarthScope MT data using the integral equation method

In this paper we apply 3D inversion to MT data collected in the Northwestern United States as a part of the EarthScope project. By the end of 2009 MT data had been collected from 262 stations located throughout Oregon, Washington, Idaho, and most of Montana and Wyoming. We used data from 139 MT stations in this analysis. We developed fully parallelized rigorous 3D MT inversion software based on the integral equation method with variable background conductivity. We also implemented a receiver footprint approach which considerably reduced the computational resources needed to invert the large volumes of data covering vast areas. The data set used in the inversion was obtained through the Incorporated Research Institutions for Seismology (IRIS). The inversion domain was divided into 2.7 M cells. The inverted electrical conductivity distribution agrees reasonably well with geological features of the region.

Regularized Focusing Inversion of Marine CSEM Data Using Minimum Vertical Support Stabilizer

Marine controlled-source electromagnetic (MCSEM) surveys have become an important part of off-shore petroleum exploration. In this paper we discuss new advances in the development of 3D inversion methods for the interpretation of MCSEM data. Our method is based on rigorous integral equation (IE) forward modeling and a new IE representation of the sensitivity (Frechet derivative matrix) of observed data to variations in sea-bottom conductivity. We use quasi-analytical approximation for models with variable background conductivity (QAVB) for more efficient Frechet derivative calculations. In our regularized focusing inversion algorithm we introduce a new stabilizing functional, a minimum vertical support stabilizer. This stabilizer helps generate a focused image of the relatively thin and flat resistive structure of a hydrocarbon (HC) reservoir. The methodology is tested on a 3D inversion of the synthetic EM data and the interpretation of an MCSEM survey conducted in the Troll West Gas Province (TWGP).

Three-dimensional inversion of towed streamer electromagnetic data

ABSTRACT A towed streamer electromagnetic system capable of simultaneous seismic and electromagnetic data acquisition has recently been developed and tested in the North Sea. We introduce a 3D inversion methodology for towed streamer electromagnetic data that includes a moving sensitivity domain. Our implementation is based on the 3D integral equation method for computing responses and Frechet derivatives and uses the re‐weighted regularized conjugate gradient method for minimizing the objective functional with focusing regularization. We present two model studies relevant to hydrocarbon exploration in the North Sea. First, we demonstrate the ability of a towed electromagnetic system to detect and characterize the Harding field, a medium‐sized North Sea hydrocarbon target. We compare our 3D inversion of towed streamer electromagnetic data with 3D inversion of conventional marine controlled‐source electromagnetic data and observe few differences between the recovered models. Second, we demonstrate the ability of a towed streamer electromagnetic system to detect and characterize the Peon discovery, which is representative of an infrastructure‐led shallow gas play in the North Sea. We also present an actual case study for the 3D inversion of towed streamer electromagnetic data from the Troll field in the North Sea and demonstrate our ability to image all the Troll West Oil and Gas Provinces and the Troll East Gas Province. We conclude that 3D inversion of data from the current generation of towed streamer electromagnetic systems can adequately recover hydrocarbon‐bearing formations to depths of approximately 2 km. We note that by obviating the need for ocean‐bottom receivers, the towed streamer electromagnetic system enables electromagnetic data to be acquired over very large areas in frontier and mature basins for higher acquisition rates and relatively lower cost than conventional marine controlled‐source electromagnetic methods.

3D Inversion of Time-lapse CSEM Data For Reservoir Surveillance

Recent studies have inferred the feasibility of time-lapse controlled-source electromagnetic (CSEM) methods for the surveillance of offshore oil and gas fields. However, quantitative interpretations have not been shown to ascertain what information about the reservoirs that may be recovered. We present a 3D inversion study of synthetic time-lapse CSEM data for the lateral water flooding of a reservoir unit where the hydrocarbon accumulation is trapped by a thin dome structure. We demonstrate that even with few constraints on the model, the flooding front can be recovered from 3D inversion. In this paper, synthetic time-lapse CSEM responses are simulated with the threshold about the noise floor and subject to multiple 3D inversion scenarios. The time-lapse CSEM inverse problem is highly constrained though inherently 3D since the geometry of the reservoir is established prior to production from high resolution seismic surveys; rock and fluid properties are measured from well logs; and multiple history matched production scenarios are contained in dynamic reservoir models.

Integral Electric Current Method in 3-D Electromagnetic Modeling for Large Conductivity Contrast

We introduce a new approach to 3-D electromagnetic (EM) modeling for models with large conductivity contrast. It is based on the equations for integral current within the cells of the discretization grid, instead of the electric field or electric current themselves, which are used in the conventional integral-equation method. We obtain these integral currents by integrating the current density over each cell. The integral currents can be found accurately for the bodies with any conductivity. As a result, the method can be applied, in principle, for the models with high-conductivity contrast. At the same time, knowing the integral currents inside the anomalous domain allows us to compute the EM field components in the receivers using the standard integral representations of the Maxwells equations. We call this technique an integral-electric-current method. The method is carefully tested by comparison with an analytical solution for a model of a sphere with large conductivity embedded in the homogenous whole space

Advances in experimental research of induced polarization effect in reservoir rocks

We have studied the IP response of the multiphase porous systems by conducting complex resistivity (CR) frequency domain IP measurements for two different groups of samples: sands and sandstones containing salt water in pores and those whose unsaturated pores are filled with synthetic oil. We have observed the IP behavior in the imaginary parts of the analyzed complex resistivity curves. We have studied statistical aspects of CR measurements in HC-saturated samples using sand-cartridge-oil (SCO) and sandstone-oil (SSO) samples. A comparison of the complex resistivity of SCO and SSO samples with different saltwater pH values demonstrates a known shift of the IP peak to lower frequency with a decrease in pH. We used a GEMTIP model to analyze the IP parameters of the measured responses.

Geoelectrical Structure of the Lithosphere and Asthenosphere beneath the Northwestern United States

We have inverted magneto telluric (MT) data collected in nine states of the northwestern United States as a part of the EarthScope project for 3D imaging of electrical resistivity to a depth of 500 km using recent advances in extremely largescale electromagnetic modeling and inversion. The results of our mega-cell 3D inversion reveal multi-scale geo electrical in homogeneities in the upper mantle, which are closely related to major known tectonic features. Our geoelectrical model clearly shows a resistive structure associated with the Juan de Fuca slab subducting beneath the northwestern United States, and the conductive zone of partially melted material above the subducting slab due to the release of fluids from the down going slab. We observe extensive areas of moderate-to-high conductive asthenosphere below 100 to 200 km. The geoelectrical model also shows a prominent conductive feature associated with the partially melted mantle plumelike layer of the Yellowstone hotspot. These results correlate reasonably well with P-wave and S-wave velocity models independently obtained from seismic tomography.

3D inversion of SPECTREM and ZTEM airborne electromagnetic data from the Pebble Cu–Au–Mo porphyry deposit, Alaska

Geological, geochemical, and geophysical surveys have been conducted in the area of the Pebble Cu–Au–Mo porphyry deposit in south-west Alaska since 1985. This case study compares three-dimensional (3D) inversion results from Anglo American’s proprietary SPECTREM 2000 fixed-wing time-domain airborne electromagnetic (AEM) and Geotech’s ZTEM airborne audio-frequency magnetics (AFMAG) systems flown over the Pebble deposit. Within the commonality of their physics, 3D inversions of both SPECTREM and ZTEM recover conductivity models consistent with each other and the known geology. Both 3D inversions recover conductors coincident with alteration associated with both Pebble East and Pebble West. The high grade CuEqn 0.6% ore shell is not consistently following the high conductive trend, suggesting that the SPECTREM and ZTEM responses correspond in part to the sulphide distribution, but not directly with the ore mineralization. As in any exploration project, interpretation of both surveys has yielded an improved understanding of the geology, alteration and mineralization of the Pebble system and this will serve well for on-going exploration activities. There are distinct practical advantages to the use of both SPECTREM and ZTEM, so we draw no recommendation for either system. We can conclude however, that 3D inversion of both AEM and ZTEM surveys is now a practical consideration and that it has added value to exploration at Pebble. This case study compares 3D inversion results from SPECTREM 2000 fixed-wing time-domain airborne electromagnetic (AEM) and ZTEM airborne audio-frequency magnetics (AFMAG) systems flown over the Pebble Cu–Au–Mo porphyry deposit in south-western Alaska. Both 3D inversions recover conductors coincident with alteration associated with both Pebble East and Pebble West.