Fluid Distribution Inter-Well Mapping in Multiple Reservoirs by Innovative Borehole to Surface Electromagnetic: Survey Design and Field Acquisition
Authors: Marsala Alberto F. , Lyngra Stig , Widjaja Danang R. , Laota Abdalla S. , Al-Otaibi Nashi M. , Zhanxiang He , Zhao Guo , Jiahua Xu , Yang Cao 
Published: March 2015

Abstract
Borehole to Surface Electromagnetic (BSEM) technology was conceived in the former Soviet Union and fine-tuned by the Chinese Bureau of Geophysical Prospecting (BGP). Saudi Aramco recently deployed the first BSEM pilot test outside of China (Marsala et al., 2011). This paper describes a new world first innovative electromagnetic borehole to surface survey in a well completed with multiple casings. The objective was to deploy a single BSEM survey to map the oil-water distributions in two separate reservoirs. This BSEM survey was conducted in a mature Saudi Arabian oil field composed of two main naturally fractured carbonate reservoirs, separated by a thick nonpermeable zone. The Upper reservoir is prolific, while the Lower reservoir is relatively tight and highly fractured. The reservoir pressure data from the early production period confirmed communication between the two reservoirs through several large scale fractures crossing the nonpermeable zone. In the Lower reservoir, well log observations show a variable oil-water distribution. No direct measurements of fluid saturations are available in the inter-well areas. The BSEM survey was designed to fill this data gap. In June 2012, a very challenging BSEM field acquisition was successfully completed with zero downtime and no accidents, obtaining very good data quality. Electromagnetic (EM) signals were transmitted at multiple frequencies from four source locations placed in a single vertical transmitting well that cross through both reservoirs and received by more than 1,000 surface stations, located in a grid at distances up to 3.5 kilometers away from the transmitting well. Multidisciplinary teamwork and independent peer reviews are undertaken to guarantee the optimal benefit from this pioneering technology. The business impact is to increase recovery by maximizing sweep efficiency and optimize well placements.

Introduction
A state-of-the-art Borehole to Surface Electromagnetic (BSEM) survey has recently been acquired in a giant mature oil field located in the Eastern Province of Saudi Arabia. The production from this field has been primarily from two fractured carbonate reservoirs, Upper and Lower, which are separated by a 500 ft thick, non-reservoir limestone formation. The Upper reservoir is prolific throughout the whole field and its high rate producers have been responsible for the majority of the historic field production. The Upper reservoir performance, including waterflood fronts, has been very predictable, which have made it easy to identify well targets and plan successful new development wells, sidetracks and other well remedial action based solely on well data like production performance, inflow profiles and saturation logs. The Lower reservoir is oil bearing only in the southern part of the field. This reservoir has low matrix permeability with well productivities and inflow profiles controlled mainly by a complex fracture system. A comprehensive Lower reservoir development drilling program is currently ongoing to augment the reservoir production. Due to the complex fracture system, the Lower reservoir development drilling program has been prone to unpredictable well fluid saturation results. Another element of uncertainty is the presence of several large near vertical fractures creating communication with the significantly more mature Upper reservoir. In structural positions where the basal part of Upper reservoir has been swept by water, these vertical fractures create pathways for water gravity dumping from Upper to Lower reservoir. A further complication is that the actual locations of these fracture communication pathways are not always known, which at times results in well logs showing unexpected water bearing fractures and water imbibed matrix.