The Dynamic Tamar Reservoir: Insights From Five Years of Production

Abstract

The Tamar Field was discovered offshore Israel in early 2009, and proved the presence of a new gas play in the Miocene-aged sediments of the deep Levant Basin. Subsequent discoveries in this “Tamar Sands” Play (e.g. Leviathan, Aphrodite, Karish, and Tanin) total over 40 Tcf of recoverable resources. Tamar is an elongated anticline, trending NE-SW. The field consists of three vertically stacked gas bearing reservoir intervals (A, B, and C Sands), which are separated by shale-dominated zones (AB and BC Shales). Seismic and well data confirm the lateral continuity of the major sand and shale units. The high-quality reservoirs were deposited in a relatively unconfined environment as compensationally-stacked basin floor fans, and are juxtaposed across post-depositional NW-SE striking normal faults. The reservoir intervals have a very high net-to-gross (75% to 95%), in predominantly fine-grained sands. Average total porosity ranges from 21% to 23%, and gas core permeability values average 600 to 1200 mD. Thin shale beds are also present within the reservoir intervals, and have the potential to baffle fluid flow. Many of these thinner shale beds are debritic in nature, while others are laminated components of heterolithic packages. Prior to production, all reservoir penetrations encountered a single Gas-Water-Contact (GWC), strongly suggesting hydraulic-connectivity over geologic timescales. To date, six high-rate (~250 MMscf/d) production wells have been drilled and completed in the field. The wells are equipped with downhole pressure-temperature (DHPT) gauges located approximately 250 meters above the completed sand face, which provide high frequency production data for reservoir monitoring and performance analysis. The subsea wells are tied back to the Tamar Platform via a subsea manifold and two 150km gathering lines. Since Tamar is presently the sole supplier of natural gas to the Israeli market (excluding minor LNG imports and production from the nearly-depleted Mari-B Field), the production rates are directly driven by market demand. This dependency creates a cyclic pattern of production rates on a daily, weekly, and seasonal basis. These cycles challenge both operations and conventional methods of reservoir performance analysis. Well pressure data are collected continuously from the DHPT gauges, and provide a history of pressure/temperature drawdowns and buildups throughout the life of each well. Pressure Transient Analysis (PTA) of the buildup data is used to monitor completion efficiency as well as to constrain and forecast reservoir performance. PTA-derived permeability, well interference signature, and reservoir pressure decline are all used to study a reservoir’s dynamic properties. This study integrates these dynamic methods with the geological database, and particularly with data from the recent Tamar-8 well. Tamar-8 was drilled in late 2016, close to 4 years after first gas. Tamar-8 was drilled to a TD below the lowest reservoir and then plugged back, sidetracked, and completed as a high rate producer. The pilot hole allowed full evaluation of partially depleted sands and a water-encroached swept zone. Wireline logs, pressure tests, and fluid sampling from this well provided valuable insights into reservoir dynamics, and enabled an improved calibration of both geologic and reservoir engineering models. The PTA and Material Balance results indicate that the wells are draining large extents of the reservoir, and that all wells are in communication either through the gas and/or through the aquifer. These conclusions are supported by pressure data from Tamar-8. Furthermore, the datasets confirm cross-fault communication, some degree of stratigraphic baffling, and a combination of both volumetric depletion and aquifer support. The integration of “dynamic data” (continuous production parameters) with “static data” (seismic, well logs, cores) indicates that the reservoir is indeed continuous and exhibits a high degree of hydraulic connectivity. Additionally, small scale features that impact flow on production timescales are now better understood. These types of insights may inform development decisions such as the timing and location of future wells. At Tamar, geoscientists and engineers continuously reevaluate both static and dynamic reservoir models. This collaboration is enhancing the original concepts for reservoir connectivity and performance, and is expected to result in an optimized development plan for the management and production of the field.

AAPG Datapages/Search and Discovery Article #90341 ©2019 AAPG Geoscience Technology Workshop, Exploration and Development of Siliciclastic and Carbonate Reservoirs in the Eastern Mediterranean, Tel Aviv, Israel, February 26-27, 2019