Well and Reservoir Management in Natih Field: A Case History

Abstract

TX 75083-3836, U.S.A., fax 01-972-952-9435. AbstractThis paper presents the case history of a focused Well and Reservoir Management (WRM) effort in a giant densely fractured carbonate reservoir in Oman. The field is currently producing under the gas oil gravity drainage reservoir process. The fracture network in the reservoir is largely gas-filled and has a thin oil rim underlain by active aquifer. Horizontal and multilateral producers are positioned in the oil rim while aquifer influx is limited to the optimum rate by water withdrawal from flank water producers. Delicate balance of crestal gas injection, aquifer water withdrawal and oil production is required to keep the oil rim at its optimal position. As the fracture oil rim has been lowered over a period of several years, a large number of wells have gassed out and started to circulate injected gas. Initial analysis of the production trends indicated that additional compression capacity was required in the field.An integrated WRM effort was undertaken in 2003 to manage this complex field. Active reservoir surveillance planning and execution was carried out to map current fracture fluid contacts across the field. Strategic well optimizations and recompletions were performed in selected wells to capture production from oil rim and avoid short-circuiting of the injected gas. Also, several gassed out intermittent producers and very high GOR continuous producers were shut-in. An eightwell production logging campaign provided better understanding of well and reservoir behaviour. Well integrity tests in over 110 wells further assisted in problem diagnosis and workover planning.These focused WRM efforts have resulted in sustained 10% production gain over a one-year period. A major capex benefit was also realized when a better understanding of the reservoir and improved GOR performance of the field eliminated the previously perceived need for additional compression.

Field Scale Production Decline Characteristics During Gravity Drainage

“…Several other papers, including Ali et al [5] discuss field production history of gravity drainage fields. However the fields have production facility constraints or multiple recovery mechanisms, which make it difficult to generalize gravity drainage production characteristics.…”

“…An expanding gas cap or steam chest because of reservoir depletion would be an example of free fall gravity drainage. Published field observations of free fall gravity drainage production and decline analysis are limited [2,3,4,5], and the emphasis is more on ultimate recovery than production decline characteristics.…”