Crosswell Technologies: New Solutions for Enhanced Reservoir Surveillance
Authors: Denney Dennis 
Published: February 2015

This article, written by Senior Technology Editor Dennis Denney, contains highlights of paper SPE 144271, ’Crosswell Technologies: New Solutions for Enhanced Reservoir Surveillance,’ by Bruce Marion, SPE, Muhammad Safdar, SPE, Michael Wilt, SPE, Ping Zhang, Fabian Loh, SPE, and Ajay Nalonnil, SPE, Schlumberger, prepared for the SPE Enhanced Oil Recovery Conference, Kuala Lumpur, 19-21 July 2011. The paper has not been peer reviewed.
Resolution and measurement diversity help characterize the subsurface and help optimize reservoir management. Crosswell measurements image the interwell space at reservoir scale. Crosswell-seismic data provide high-resolution reflection images of reservoir architecture. Crosswell electromagnetics (EM) expands the scale investigated by traditional resistivity logging to deliver a more extensive understanding of fluid distribution and movement at reservoir scale, away from the wells. These techniques complement each other, but integrating the measured data with conventional logs and surface data into the modeling workflow gives new insight into reservoir structure and fluid movement in the formation.
Introduction
Crosswell measurements enable greater understanding of reservoir heterogeneity and improve fluid-front monitoring. Geologic heterogeneity and the complexity of reservoir behavior when subjected to improved-recovery processes make crosswell surveillance useful in constraining scaling-up techniques during static modeling and in providing better predictability of injection performance. The two case studies detailed in the full-length paper describe early applications of crosswell surveys in the USA in the Chevron-operated Cymric field in California’s San Joaquin Valley.
Theory
Crosswell seismic produces images of the interwell space with a vertical resolution five to 10 times higher than that of surface-seismic data. Both direct- and reflected-wave fields are processed to provide a detailed image of the reservoir. By use of higher-frequency sources deployed alongside the reservoir interval, crosswell-seismic surveying generates high-vertical-resolution reflection images with resolution on the order of 1 to 3 m. This is the result of high bandwidth of the transmitters used during acquisition, with upper frequencies ranging from 800 Hz to 2 kHz.
Because seismic velocity is sensitive to pressure, temperature, and saturation changes caused by the injection of CO2, gas, and steam, crosswell seismic provides valuable information for reservoir characterization and for monitoring injection performance and efficiency. Crosswell seismic can detect compressional-wave velocity changes as small as 1 to 3% reliably. Also, because the migrated-reflection section provides a high-resolution image of the structure between wells, it helps determine the geologic causes of the observed performance and fluid flow.
Crosswell EM is a natural extension of the single-well induction-logging method whereby the transmitter and receiver are placed in separate wells. This geometry provides high-resolution resistivity distribution of the subsurface between the wells. When obtained at different timesteps during injection and production, this resistivity distribution provides valuable information that can be integrated with other data to interpret changes in the reservoir rock and the fluid distribution between wells. These data are valuable in updating the reservoir model with reduced uncertainty and enhanced predictability. Resistivity is especially responsive to changes in fluid composition in the reservoir and is an excellent indicator of temperature-related changes in the reservoir.