A Novel Approach to Propagate MHz EM Signals and Map Reservoir Saturation Hundreds of Meters away from the Wellbore

Abstract

Introduction Obtaining accurate reservoir saturation information at the interwell level is a long-standing challenge that would pave the way for improved reservoir management strategies. At present, logs can provide high resolution information, but it comes at the cost of the depth of investigation (DOI). Typical logging tools are limited to at most tens of meters into the formation. The most common methods to monitor reservoir saturation at the field scale include seismic and electromagnetic (EM) surveys. 4D seismic has been successfully applied, mostly in clastic reservoirs to monitor fluid movement1-4, but it has shown mixed results when applied in carbonate reservoirs due to the fact that acoustic velocities of carbonates are relatively insensitive to changes of pressure and saturation5, 6. EM surveys, including crosswell EM7, surface-to-borehole EM8, and borehole-to-surface induced polarization9-11, have recently been tested in carbonate reservoirs and show promising results. And while these approaches enable penetration in the kilometer range and have the potential to provide actionable information, their resolution is relatively low due to the use of low frequencies — typically between 1 Hz to 100 Hz. Therefore, alternative and complementary approaches with higher intrinsic resolution are desired. Proximity sensing is a potential new approach to address the challenge of long range EM propagation in the reservoir, in the presence of conductive media. The proposed method relies on the use of naturally occurring waveguides in the form of a layer of sealing evaporite — such as anhydrite — bounded by conductive reservoirs. The hypothesis is that EM pulses traveling through a relatively nonconductive layer — such as evaporite seals — bounded by conductive layers — such as hydrocarbon reservoirs — will propagate with reduced attenuation and 3 The Aramco Journal of Technology Winter 2019 the pulse’s traveltime and attenuation will depend on the EM properties of the layers above and below. In other words, the resistive layer acts as a low-loss channel that enables the propagation of the EM signals far beyond typical ranges for the same signal traveling through the reservoir. At the same time, the speed of propagation and signal amplitude, when measured using multiple pairs of wells, can be inverted to provide a velocity map that can be interpreted into reservoir oil and water saturation. The concept has been so far demonstrated through qualitative 2D and 3D numerical simulations12-14 as well as laboratory experiments15. And while the sensitivity of the method to changes in saturation of the bounding fluids has been shown, the propagation range and the parameters controlling it had not been studied up until now. This study strives to estimate the propagation range under multiple reservoir saturation conditions as well as to determine which parameters should be considered toward the development and field testing of this approach. Maximum Transmission Loss (MTL) The applicability of proximity sensing to map reservoir saturation significantly depends on how far the signal can propagate before it is too weak to be detected for a desired traveltime measurement accuracy. To evaluate the propagation range, we have defined the maximum transmission loss (MTL) before the signal it is too weak to be detected as follows: Saudi Aramco: Company General Use TL(dB) = Pxmit(dB) – Pnoise(dB) + 10 * log10 T + 16 + 20 * log10 f + 20 * log10 std[t] (1) Att(dB) = 20 log!#$ %$ & (2) Saudi Aramco: Company General Use TL(dB) = Pxmit(dB) – Pnoise(dB) + 10 * log10 T + 16 + 20 * log10 f + 20 * log10 std[t] (1) Att(dB) = 20 log!#$ %$ & (2) 1 where TL is the transmission loss in dB, Pxmit is the transmitter power in dB, T is the averaging time in seconds, f is the frequency in Hz, and std[t] is the standard deviation of the measured traveltime. A system capable of providing 200 W (400 V and 500 mA) was considered for this study. It was estimated that after cable and circuit losses, half of this power (100 W) will be delivered to the transmitter (Tx) antenna. It was also assumed that the limiting source of noise is the circuit noise in the antenna amplifier is due to the Johnson noise, the voltage noise in the amplifier, and the current noise through the antenna impedance. Together, the estimated noise level is no lower than 1 × 10-19 W/Hz. Assuming 10,000 seconds of averaging time, the MTL for 10 nanoseconds of accuracy in traveltime measurements at operating frequencies ranging from 6.7 MHz to 53.7 MHz was estimated to be between 243 dB and 261 dB, Table 1. It is worth noting that higher frequencies result in greater MTL. This is expected because for a fixed averaging time, higher frequencies translate into a greater number of waveforms being averaged, and therefore, a higher signal level. The difference between the highest and lowest frequency considered in this study is no more than 18 dB. Numerical Simulations Quantitative 3D numerical simulations were performed in the frequency domain to estimate the maximum DOI for multiple configurations and reservoir properties. The maximum DOI was defined as the point where the signal attenuation reached the MTL defined in the previous section. The simulations were performed using a commercially available multiphysics software running on a cluster node with 20 processing cores and 196 GB of RAM. The numerical models consisted of three layers, representing a hydr carbon reservoir A (A), resistive seal (RS) and hydrocarbon reservoir B (B), respectively. The models included two fluid filled cased wellbores. The Tx and receiver (Rx) were located inside the wellbores in a crosswell configuration, Fig. 1. Both the Tx and Rx were simulated as half-wave dipole antennas composed of two hollow arms separated by a gap, Fig. 2. The Tx power source was not explicitly modeled. Instead, the lumped port condition was used to apply a uniform voltage across the faces of the dipole Wavelength (m) Frequency (MHz) MTL (dB)