Summary
This paper describes the results of a crosswell electromagnetic (known also as DeepLook EM) induction survey between two widely spaced horizontal wells with associated 3D inversion results. The project was designed to map oil and water saturations in a naturally fractured reservoir located in Saudi Arabia. The survey was conducted between a ~ 1 kilometer (km) long horizontal water injector and a ~1 km long horizontal producer. The two wellbores are drilled at an average distance of 1.3 km apart in the reservoir. The coiled-tubing (CT) deployed system operated at a very low noise resulting in an extremely high-quality data set. 3D inversion yielded a resistivity distribution consistent with large volumes of swept reservoir within fracture corridors. The inversion also revealed significant unswept reservoir volume in between the wells. The described survey is the first of its kind in the industry.
Introduction
Crosswell electromagnetic (EM) was initially developed in the 1990s. The initial technology application involves applying inductive physics and vertical well 2D inversion to interrogate the inter-well resistivity distribution. The method has since developed into a mature oil and gas technology, especially for enhanced oil recovery (EOR) projects and time-lapse studies (Wilt et al. 1995). Applying the technology to horizontal wells involves an entirely new set of challenges. When the wells are oriented horizontally, the sensors are sensitive to both vertical and horizontal variations, transposing the inversion into a 3D problem. As the measurements are confined to a limited number of vertical levels, the problem is extremely undetermined; i.e., there are far fewer observations than required to determine the resistivity of all the individual voxels. An additional complication is that standard wireline deployment of tools is not possible in horizontal wells. Since coiled tubing (CT) or tractor tool deployment is required, the survey time requirement and cost are significantly higher than for the vertical EM application.
The study is part of a larger Saudi Aramco Research and Development (R&D) effort to understand the saturation distribution in fracture controlled reservoirs and to help devise strategies for improved production and recovery. The reported project follows on a previous crosswell EM study in which a similar technology was applied to vertical wells to study matrix saturation using 2D workflows (Marsala et al. 2008 and Malalla et al. 2009).
