This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper SPE 222993, “Multilayer Reservoir Mapping in Low-Resistivity, Low-Contrast Clastic Reservoir Using Integration of Ultradeep Azimuthal Resistivity 1D and 3D Inversions,” by Ayman El-Khamry, SPE, Saudi Aramco, and Ahmed Taher, SPE, and Mohamed Fouda, SPE, Halliburton, et al. The paper has not been peer reviewed.
The use of modern ultradeep azimuthal resistivity (UDAR) tools, along with other logging-while-drilling (LWD) sensors, increases horizontal well‑placement efficiency, improving net‑to‑gross (NTG) and maximizing reservoir contact. The depth of investigation (DOI) of electromagnetic (EM) resistivity-based technologies depends on multiple factors. The performance of EM resistivity tools in very low-resistivity formations has remained a challenge, limiting the signal propagation and, thus, the DOI. The complete paper presents a novel workflow for using EM resistivity‑based reservoir-mapping LWD technologies for successful well placement and multilayer mapping in very‑low‑resistivity, low‑contrast, and thinly laminated clastic reservoirs.
One of the main challenges for successful well-placement operations is the ability to resolve boundaries that are further away from the horizontal wellbore or that feature a poor contrast in resistivity values from surrounding layers. During the past two decades, azimuthal resistivity tools designs and EM signal-inversion algorithms have evolved to address this challenge. Ultradeep resistivity tools have been developed and have achieved successful mapping of boundaries up to 225 ft from the wellbore in a high-resistivity, high-contrast environment. As with previous EM resistivity tool designs, transmitter antennae emit EM waves into the surrounding formations, which are then detected by multiple receivers placed at different spacings from the transmitter. While the wider transmitter/receiver spacings are used to acquire measurements deeper into the formation, the shorter and narrower spacings provide signals enabling closer boundary mapping at higher resolutions. The signals also are transmitted at multiple EM-wave frequencies; these different signals are then fed into advanced inversion algorithms to detect multiple resistivity boundaries at a distance from the wellbore. Fig. 1 shows a horizontal well using UDAR 3D inversion in a complex clastic reservoir.
EM resistivity measures the change in phase angle or signal attenuation, which is transformed to a resistivity measurement. In a high-resistivity formation, the change in the phase angle is minimal, which imposes a significant challenge for resolving changes in resistivity. For reservoir-mapping tools, this would result in difficulty in mapping different boundaries of high‑resistivity ranges. For deep resistivity tools, this can be resolved by using an additional high-frequency measurement to improve multilayer inversion.
