EM, when integrated with seismic, may provide resistivity information beneficial to field understanding.
At RSI, we believe that controlled source electromagnetic (CSEM) and magnetotelluric (MT) data can have a significant impact in reservoir characterization studies, as an integral component of an integrated interpretation strategy. When carefully combined with seismic and well log data, CSEM/MT data can be used to supply information that is not available, or is unreliable from a single data type.
By integrating complementary sources of information and exploiting the strengths of each, estimates of rock and fluid properties such as hydrocarbon saturation, lithology and porosity can be obtained with greater confidence than from any one data type alone.
CSEM data provides key information regarding the resistivity of the earth. CSEM is particularly sensitive to resistive features in the sub-surface, which may, under the correct geological scenarios, be indicative of hydrocarbon accumulations. While MT lacks the sensitivity required to resolve the details of resistive structures such as hydrocarbon reservoirs, it is extremely sensitive to background structure. As such it is a natural complement to CSEM in many situations.
RSI offers CSEM survey design and analysis services, which can be applied prior to acquisition or before a decision to invest in multi-client data is made. RSI uses all available seismic and well log information in our robust modeling workflows to assess sensitivity and effectively design or assess surveys to ensure that critical exploration, appraisal or development challenges can be successfully addressed. . Our workflow constructs a detailed and geologically consistent background model that is used to assess the sensitivity of CSEM or MT data to the structures of interest. The effect of lithological changes, fluid property changes, and anisotropy on the expected CSEM or MT response can be quantified so that appropriate acquisition parameters are chosen.
Once CSEM or MT data has been collected, data is quality controlled and conditioned. A staged modeling and inversion approach is applied, from 1D through 2D to 3D, to ensure all the complexities of the subsurface structure are accounted for and understood. Understanding and quantifying electrical anisotropy is critical. Integrated interpretation proceeds in stages during which well log and seismic information are incorporated to ensure that the final result is consistent with all available data.
Solid reservoir characterization begins with thorough data analysis. If you get the rocks and rock physics wrong, EM won’t help. We don’t. We’re Rock Solid.
Survey Design and Assessment
RSI can help design a CSEM or MT survey finely tuned to meet the exploration, appraisal or development challenges to ensure a successful campaign. RSI can also assess the quality and effectiveness of available surveys prior to purchase of multi-client data to ensure that exploration goals are met.
RSI gathers all available background information, including seismic and well log information, and uses this to construct a detailed and geologically consistent background model of the area. This is used to construct a detailed anisotropic resistivity structure representing the survey area and challenges to be addressed. Using RSI’s Rock Physics modeling software, the effect of lithological or fluid property changes on the expected CSEM or MT response can be quantified and appropriate acquisition parameters chosen. A process of 1D, 2D and 3D forward modeling and inversion approach can then be applied as required to assess how accurately a CSEM or MT survey is likely to recover the structures and properties of interest.
CSEM/MT Data Conditioning
Robust data is the key to robust interpretation. Prior to any data analysis, we ensure that the CSEM or MT data meets RSI’s quality standards. Any issues related with the data are flagged and, when possible, corrected. Careful QC and conditioning of the data allows us to have more confidence in the resulting resistivity interpretations.
CSEM sensitivity analysis is a critical step in understanding the geophysical sensitivity to changes in reservoir properties such as porosity, saturation, clay content, resistivity, depth and thickness. “What if” scenarios can also be built to assess the effect of electrical anisotropy or the effect of changes in electrical background structure.
CSEM sensitivity can be run alone or in conjunction with seismic sensitivity. By conducting both CSEM and seismic sensitivity analysis in parallel and based on consistent well log data and rock physics modeling, the relative sensitivity of the two geophysical datasets can be compared. This allows the optimum geophysical attribute or combination of attributes to be chosen so that the exploration problem of interest can be efficiently addressed.
CSEM/MT Modeling and Inversion
Data interpretation proceeds in stages, each gradually adding complexity to the structure, from 1D to 2D to 3D. At all points during the analysis interpretation, well log and seismic information are used as a guide to ensure that the final result is consistent with all the available data. Seismic horizons are used for example to provide a structurally consistent earth model.
The first stage uses simple 1D modeling of a subset of the data in order to understand background trends, evaluate electrical anisotropy within the area and enable appropriate 2.5D and 3D inversion starting models to be constructed.
The next step utilizes RSI’s proprietary 2.5D anisotropic inversion algorithm to build an understanding of the resistivity distribution within the area of interest. Sensitivity of the data to this structure is assessed initially using unconstrained inversion, and is then refined by applying constraints based on seismic structure. Inversion of synthetic data allows an understanding of the accuracy with which the data and the inversion algorithm can recover an input model based on known geology, which provides an invaluable aid to the interpretation of results.
The final stage builds on the results of 1D and 2.5D analysis, by inverting the EM data for an anisotropic 3D resistivity structure.
The result is a resistivity section or volume that can be integrated with seismic, gravity or other sources of data using our workflows to provide a geologically consistent earth model.