Abstract

The problem of detecting entry points of oil in case of high water cut wells is extremely difficult, particularly so for high inclination wells. The complex flow behaviour of deviated wells, with water phase at the low side of the pipe and dispersed oil phase at the high side of the pipe makes this even more complicated.

The density tool is generally used in production logging for holdup measurements. However, due to the low contrast between the density of oil and water, it cannot accurately provide oil holdup in high water cut wells. Realistically, the density tool can only detect water or oil entries if the respective holdup change by 3-4% or more.

A digital entry fluid imaging tool is now available in the market which provides a digital water holdup measurement and can clearly identify the first point of oil entry. This aids in identifying the type of fluid being produced by each zone.

The fluid imaging tool has been run in a few wells of Bombay Offshore fields in both single and dual mode, along with the standard production logging sensors. This paper basically deals with how the integration of the two sensors have helped in determining the flow profile of these deviated, high water cut wells, and have led to more reliable solutions. Examples of a few wells from Bombay Offshore have been included.

Geological Setting

The Bombay High Field is a N-NW to S-SW trending doubly plunging anticline with a faulted eastern limb. Several oil and gas zones have been identified in this field. Out of these, the L-III reservoir is the most prolific in terms of accumulation of hydrocarbon. Deposited in Mid-Miocene period in a marine environment, this reservoir is essentially composed of limestone layers separated by thin shale sections. The shale seams divide the L-III reservoir into a number of zones which are termed (from top to bottom) as A1, A2, B, C, D & E. Several thin shale sections further subdivide A2 into seven sub layers (A2-I through A2-VII). The lateral continuity of these shale sections is not uniform. As a result, vertical interlayer communication within the reservoir shows a varying pattern.

The Neelam Field is one of the major oil fields of Bombay Off-shore basin. Mainly two hydrocarbon bearing zones have been identified in this field. Zone-A, which is the upper zone, is known as Mukta formation and zone-B as Bassein formation. The period of deposition belongs to Middle Eocene to Early Oligocene. The western flank of south culmination is steeper. Zone-A is gas bearing in the limited area in the Northern part of the field. Zone-B is also having a gas cap in the crest of southern section. This is essentially a layered limestone reservoir.

In both the fields, the limestone layers are characterized by extremely varying permeability which affects the production and injection process. Early break-through of injection water through different layers is posing a problem of high water cut in a number of wells. Production logging jobs are regularly being carried out in these fields for the diagnostic & monitoring purpose. FlowView* tool has recently been introduced in production logging services in these fields.

FlowView*1

The FlowView*, earlier known as DEFT*, is a digital entry and fluid imaging tool. It has four low frequency probes in a 4-arm centralizer (Fig 1). Each probe is located on the inner side of a centralizer blade. This configuration of probes gives a good coverage of the well bore in vertical, deviated & horizontal wells. Each probe provides a local measurements of resistivity of the fluid. High and low binary signals measured by the probes allow to discriminate between hydrocarbon (i.e. oil & gas) and water phases. A high signal results when the probe is in water and a low signal when it is in oil/gas (Fig 2).

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