Managing mature fields effectively and efficiently requires monitoring changes in formation fluid saturations as well as production from individual wells. Reservoir saturation monitoring is usually performed using slim pulsed neutron logging (PNL) tools because they can be deployed through tubing and operate in different modes, thus providing a wealth of information. However, several environmental factors can complicate the analysis, including complex completions and unknown or variable borehole fluids (gas in particular), which affect the PNL raw measurements and computed outputs. Factors related to the nature of the reservoir, such as complex lithology and multiple fluid phases, further complicate the analysis, making accurate fluid saturation evaluation and reservoir fluid-front mapping very challenging. An innovative pulsed neutron technology, recently introduced in the UAE, can help in reducing the evaluation uncertainty. The new device is fitted with multiple detectors and is used with newly developed algorithms to provide self-compensated formation sigma and hydrogen index (HI) measurements, overcoming many of the limitations of previous devices in complex environments. Additionally, the new tool provides a new formation property sensitive to gas-filled porosity, called the fast neutron cross section (FNXS), which, in adequate conditions, can be used to complement the analysis or highlight gas in the absence of openhole logs. The new PNL tool was run for the first time in an offshore UAE mature field targeting Jurassic formations. The production in the field started in the 1960s, followed in the 1970s by down-flank injection of water with much lower salinity than the connate water, and in the 1990s by crestal gas injection. The Jurassic reservoir mineralogy is a complex mixture of calcite, dolomite, and anhydrite. Completions consist of multiple combinations of tubing, casing, and hole sizes along with packers and other hardware components; often the borehole is filled with gas across the zones of interest, which has proven an obstacle to PNL interpretation. The new PNL device was tested in several wells in which it operated in inelastic gas, sigma, and HI (GSH) mode and carbon/oxygen (C/O) mode. Integration of all the recorded information made possible to reliably track the three-phase fluid saturation changes even in the gas-filled wellbores with complex completions. An additional benefit with the new tool was that because the C/O data were recorded at a speed twice as fast as that of the previous-generation PNL tool, it was possible to acquire the logs in the limited allocated time to help resolve the oil saturation in reservoir zones with variable salinity. The saturation analysis was compared to production logs and well production data where available.
The Jurassic carbonate Reservoir-X, offshore Abu Dhabi started to produce hydrocarbons in the early sixties of the last century. The reservoir is generally formed of regressive sedimentary cycles and can be divided into four highly heterogeneous carbonate units with mixed compositions of limestone, dolomite separated by dense anhydrite layers. Down-flank water injection using relatively low-saline sea water had started in the early 1970s. As of the present day, this ongoing process has highly altered the original formation water super-salinity, in a way that the Archie water saturation computation for Reservoir-X is very challenging and not a straight forward practice anymore. The objective of the presented approach is to introduce a resistivity independent saturation across Reservoir-X, using the novel Pulse Neutron Log (PNL) Spectroscopy measurements.In order to meet the subject challenges; a comprehensive logging suite consisting of gamma ray, resistivity, density, neutron and spectroscopy is used. The particular interpretation methodology is chosen such that it becomes possible to solve for the hydrocarbon saturation using elastic and inelastic capture spectroscopy. A further observed added challenge is the presence of light hydrocarbon and gas in the porous Reservoir-X layers with the unknown mixed-saline formation water in addition to mud filtrate. Here we present a method for hydrocarbon typing across Reservoir-X.The precise and accurate acquisition of spectral elemental yields allows for a detailed lithological evaluation, hence allowing for an accurate porosity determination, also taking into account wellbore fluid properties that affect the logging sensors due to the presence of formation water and mud filtrate in the formation. This helps to obtain a resistivity independent hydrocarbon saturation that is independent of Archie parameters m, n and water formation salinity. The solution has also been verified by a subsequent, downhole wireline formation tester fluid identification analysis.
Pulsed neutron reservoir saturation monitoring logs are being acquired since the early stages of the subject offshore field life to monitor variations of saturation with time in addition to identifying pay zones for well intervention and reactivation operations. This is important given the increasing complexity of reservoir management due to water and miscible gas secondary and tertiary oil recovery schemes adopted by the operator. However, with the introduction of new pulsed neutron technologies and resource limitations, a benchmarking exercise was imperative to confirm data quality and identify any discrepancies in the recorded data. Two candidate wells were selected based on the maximum Jurassic reservoir units coverage, including a group of lithologies (limestone, dolomitic limestone, anhydrite etc.) and borehole environments (single to multiple casing/tubing, cement and borehole fluids) to establish different correction parameters. The study was conducted with back-to-back logs utilizing current and new pulsed neutron tools from different service providers. In addition to the comparing the changes of formation capture cross section (SIGMA), borehole salinity, sigma and porosity measurements against the logging environment and changing completion hardware, three-phase fluid interpretation products were compared. The results were validated via well performance and production logging data. The approach is trying to support the reservoir saturation monitoring end users within the asset units operating in mature carbonate fields to design their time-lapse analysis methodology and to minimize the fluid saturation interpretation uncertainties, and as a result improve confidence in fluid front mapping and infill well planning.
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