Abstract

Beginning in 2003 Petroleum Development Oman (PDO) began testing the crosswell electromagnetic method for waterflood monitoring in Oman. The tomographic method, which determines interwell resistivity from inductive EM signals, is well suited for tracking injected water volumes, especially when this fluid is a low resistivity, high salinity brine.

The method was applied in two Omani oil fields for tracking injected water. In the first field several vertical well pattern pilots were established to measure water flood sweep efficiency in two zones of a shallow, but relatively thick, multi layer reservoir. In the shallow zone a two year monitoring program revealed that much of the injected water had bypassed the reservoir, likely escaping through high permeability streaks. In the deeper pilot the EM results showed an excellent interwell sweep but some leakage to the overlying formation. For the second field the method was applied in existing widely-spaced barefoot injection wells. In this old line drive flood we wished to image the residual oil saturation and injected water volumes. In this case the resistivity anomalies in this mature water flood were fairly subtle. Interwell formation heterogeneity could only be revealed by calculating resistivity differences between the initial or background model and the image after inversion. One of these difference images revealed that a significant oil bank remained adjacent to the central producer.

The overall experience with the crosswell EM tomography revealed that this technique can have great value in imaging water flooding. Drawbacks include the costs of installing special monitoring wells that are required under some conditions and the low resolution that is obtained in cases where the length of the survey interval is short relative to the well spacing. A good reservoir model is required in order to use constraints that are required for steering the crosswell EM inversion.

Introduction

As their oil fields mature Petroleum Development Oman (PDO) has begun to look for new methods to monitor the distribution of oil and water during production and injection. This is particularly important in shallow low-pressure fields where fracture flow and premature breakthrough can occur.

A major issue is that most monitoring technologies tend to be well based and these interrogate only a small reservoir volume away from the wells. One of the emerging technologies with the capability of interrogating between wells is the crosswell EM Induction method (Wilt et al., 2008). This technology can map the interwell resistivity at well separations up to several hundreds of meters and thereby offer an idea of the fluid saturation deep into reservoir away from the wells. Beginning in 2003 Petroleum Development Oman (PDO) and Shell began testing the crosswell electromagnetic method for waterflood monitoring in Oman.

A field trial of this technology was commenced in 2003 in two Omani oil fields for tracking injected water. In this paper we provide a general overview of the technology and then describe the results achieved for each of these cases.

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