Fracture Diagnostics Using Low Frequency Electromagnetic Induction and Electrically Conductive Proppants
Last Reviewed Dated
The project goal is to develop a new Low Frequency Electromagnetic Induction method, which has the potential to estimate not only the propped length, height, and orientation of hydraulic fractures but also the vertical distribution of proppant within the fracture. The proposed low frequency electromagnetic induction tool can be used to detect far-field anomalies in the rock matrix from a single borehole.
The University of Texas at Austin, Austin, TX 78712
E-Spectrum Technologies, San Antonio, TX 78249
Hydraulic fracturing has become a major driver for unconventional oil and gas production in the United States. Knowledge of hydraulic fracture dimensions is of great importance when predicting production, validating reservoir and fracture models, and improving production economics. However, we still lack an inexpensive, direct, and repeatable post-fracturing diagnostic tool to measure the dimensions and orientation of propped hydraulic fractures. Project researchers will build and test a prototype for a downhole fracture diagnostic tool that can be used to estimate the orientation and length of the ‘propped’ fracture (not the created fractures), since this is the primary driver for well productivity.
The project team anticipates that the proposed technology will be a game changer in fracture diagnostics because it is inexpensive, repeatable, and fairly simple to operate. In addition to the key critical advantages mentioned above, the proposed technology can offer the following benefits consistent with DOE’s ongoing efforts:
Additional recovery: This tool can improve our understanding of true simulated rock volume because it tracks propped volume of hydraulic fractures and not shear slip events during a fracturing job. Therefore, it enables us to better model the reservoir and find effective re-fracturing candidates. Also, a true simulated rock volume map can help us design better simulations for subsequent wells.
Reduced costs: This tool can be operated at any time during the well’s life cycle and not necessarily during the hydraulic fracturing job (as is the case with microseismic monitoring). Therefore, the tool will reduce the equipment load during a fracturing job, thus reducing the environmental footprint. Because this technology, being a single wellbore application, doesn’t require a monitoring well, it can potentially be deployed in any hydraulically fractured well with or without a rig (can be deployed with a MAST truck). Due to the simplicity of deployment and ease of operation, the project team anticipates a much lower cost as compared to microseismic monitoring, along with more reliable results.
Environmental benefits: This technology tracks the location of conductive proppant using the proposed electromagnetic logging tool. Therefore, it can be used to track fractures that are hydraulically connected to natural aquifers. This tool can be operated alongside Cement Bond Logs in fractured reservoirs to ensure hydraulic isolation of oil and gas producing zones. Additionally, the inverted product of the measurement can be combined with other geophysical data (2-D and 3-D seismic and/or CSEM data) to find connections with natural fractures.
Accomplishments (most recent listed first)
A direct inversion model was used (this method analytically calculates derivatives by using introduced reasonable simplifying approximation). The conductance, area, and dip angle of a single fracture of symmetric shape can was recovered within a few minutes of computation time and with a satisfactory level of accuracy.
A new surface current approximation was introduced to the forward model to speed up the inversion analysis. This approximation allowed researchers to skip the computationally dominant step (only integrals are numerically calculated), and a satisfactory level of accuracy was obtained for the current operational frequency.
The near-surface test on the prototype tool was completed. The recorded signals were in very good agreement with the model results.
The near-surface test also shows that it is feasible to map fractures in the subsurface with the transmitters and receivers that have been built. Excellent signal to noise ratio was obtained.
These results clear the way for building the electronics and transmitters and receivers for a downhole tool. Currently the prototype tool is being driven by external electronics and hardware.
Construction of the near-surface tool testing facility was completed.
The air measurements were made in the lab with the coaxial, co-planar, and cross-polarized transmitters and receivers, and the measurements were in good agreement with the model results. When the same experiment was repeated within a piece of production casing, the signal levels were attenuated by a thousand times, showing that the investigation depth of the tool significantly decreased in cased wells. It limits the application of this tool in cased wells. An electrode-based tool is more suitable for cased-hole wells and is being studied separately.
The building of a set of coaxial, co-planar and cross-polarized transmitters and receivers was completed.
Work is now underway to improve the inversion algorithms for obtaining fracture dimensions from the tool measurements.
Preliminary sequential inversion algorithms were built to back out the fracture geometry from the coaxial, co-planar and cross-polarized measurements.
Experiments were conducted to measure the electrical conductivity and fracture conductivity of mixtures of petroleum coke and white sand.
Experiments were conducted to measure the electrical conductivity and fracture conductivity of mixtures of petroleum coke prepared in different ways (different calcining temperature and different starting material).
Data collected with the new transmitter and receivers were compared with the model results and excellent agreement was obtained. This verifies the model and provides confidence that it can be used to simulate tool performance over a wide range of conditions.
A new transmitter and receivers were built and tested at the E-Spectrum facilities.
The design of the induction tool was changed substantially based on the new simulations. The new improved design is much more compact than the original design.
The sub-contractor for construction of the tool was changed from Gearhart to E-Spectrum.
The EM numerical code was speeded up by almost an order of magnitude by solving surface electric field integral equations (S-EFIE) with impedance boundary condition. This makes the code much more general and robust, and allows researchers to make larger and more realistic runs in a reasonable period of time using a single computer core.
An EM transmitter and receivers were built and tested in the lab.
A design for an induction tool was finalized.
It was determined that based on this tool design it will not be possible to build a field deployable tool given the money available in the contract. Instead, the transmitters and receivers will be integrated into a lab prototype tool that will be tested in a simulated test site.
The subcontract to build the tool was moved from Gearhart to E-Spectrum Technologies.
A volumetric electric field integral equation (V-EFIE) for the induction logging problem was developed. The formulation takes into account a homogeneous formation matrix with/without a borehole. The electric permittivity and conductivity of both the borehole and the formation can be arbitrarily set. In particular, the properties of shale rock can be used. The integral operator used in the V-EFIE describes the field due to any material inhomogeneity with respect to the background matrix.
Project researchers formulated and implemented a complementary numerical solution scheme. As a first stage, researchers used a volumetric Method of Moments (MoM) discretization scheme, which translates the integral equation into a set of linear equations, with the unknowns being coefficients of finite support basis functions. Here, the SWG basis functions are being used, which can be thought of as roof-top basis functions defined on tetrahedron pairs. The obtained matrix equation is solved iteratively using an algebraic solver (e.g., the General Minimum Residual solver.
The MoM code for the solution of the V-EFIE was adjusted to the problem of inductive logging. This included the implementation in code of the magnetic dipole excitation of electric fields and the numerical computation of scattered magnetic (secondary) fields from the fracture. The results were computed for various sizes of fractures with varying conductivity.
As a part of the adaptations, the effect of the borehole’s modeling (for non-cased wells) was also examined and it was concluded that for non-cased wellbores it can be assumed that the borehole is filled with the same material properties as those of the background. This allowed us to reduce the number of unknowns by not defining unknowns on the borehole’s volume, this saving storage and computation time. Further acceleration of the solver was obtained by taking advantage of the solution’s smooth behavior with change in the transmitter’s location. By efficiently using the solution for previous source location as the initial guesses to the MoM iterative solution, the number of iterations was reduced by roughly an average factor of 2. As a result, we were capable of analyzing fractures of up to 30m in radius and conductivity of up to 30S/m (smaller fractures can be analyzed with greater conductivity values). These computations were performed on 512 cores on the Stampede supercomputer of the Texas Advanced Computations Center.
The performance (storage and computation time) of the MoM iterative solver was studied and it can be concluded that there is a need for further acceleration of the solver in order to solve for larger and more conductive fractures in reasonable times. This can be achieved by implementing a matrix-free fast integral equation method. The team is currently working on the implementation of such a technique called the Adaptive Integral Method, which should reduce the computational cost.
A new setup was installed for measuring the proppant conductivity in a fractured core.
The direct inversion algorithm is used to quickly invert the data and calculate the area, conductance, and dip-angle of a single fracture in a few minutes. The algorithm is based on the approximation used to calculate surface currents without solving the linear system of equations. The resolution of inverted parameters was shown to be very satisfactory for field deployment in open-hole wells.
Coaxial, co-planar and cross-polarized transmitters and receivers have been built and tested in the lab and in a near surface well. Both the lab results and the near-surface test on the prototype tool that were completed clearly show that the tool response is in very good agreement with the model results. The results also show that it is feasible to map fractures in the subsurface with the transmitters and receivers that have been built with excellent signal to noise ratio.
These results pave the way for building the electronics and transmitters and receivers for a downhole tool. Currently the prototype tool is being driven by external electronics and hardware. Modeling of the induction tool was completed and the results indicated that an EM tool would work well in open-hole completions. The design of the induction tool was changed substantially based on the new simulations. The new improved design is much more compact than the original design.
Lab measurements were made for the electrical conductivity of mixtures of proppant and sand. It was found that the electrical conductivity remained high even for a 50:50 mixture of sand and petroleum coke. The fracture conductivity of such mixtures was measured to be less stress sensitive. Values of the proppant conductivity were found to be 2,000 to 5,000 times larger than the shale conductivity. This means that mixtures of sand and proppant can be used as a proppant.
Current and future work is focused on four main remaining tasks:
Conduct the inverse analysis when multiple fractures are present in the wellbore. This is a considerably more complex computational task and will require the very fast forward model the researchers have recently built. The presence of multiple fractures complicates the inverse problem because several fractures influence the received signal and the convolution of the geometry of individual fractures is more challenging.
Improving the radial resolution of the tool. A multi-frequency analysis will be carried out.
Building the electronics and the transmitter and receivers for the downhole prototype tool.
Simulating real field scenarios; and testing the inversion algorithm for multi-stage and multi-cluster cases.