A Method to Identify Net Pay Zones in Clastics and Carbonate Using Movable Water and Uncertainty Level Concepts rather than Fixed Cut-offs

摘要

A complete formation evaluation workflow plays akey role in resources and reserves quantification ofany play in the oil and gas industry, and it includestwo main steps:(1) Quantification of reservoir propertiesThe purpose of quantification of reservoirproperties (ex., porosity, saturation, permeabilityand lithology) is for1. Resources estimation,2. Input for pay zone determination.(2) Determination of pay zoneThe purpose of pay zone determination is for1. Determination of zones suitable forperforation, stimulation or completion toinduce production,2. Reserves estimation.Reservoir properties obtained in the first step enablethe resource estimation and provide the main part ofthe input for pay determination. However, the secondstep, determining pay zone, is a key step in convertingresources to reserves to production. Without a correctpay determination, the reservoir properties quantifiedmay not correctly reflect the reservoir’s ability toproduce the hydrocarbon it contains. In another word,reservoir properties quantification step can onlyindicate which zones contain hydrocarbon (HC) whilepay zone determination step can tell which zones willproduce HC.Determination of net pay zone can greatly help themost important aspects in various plays:perforation/stimulation interval selection, horizontalwell placement and resources/reserves booking.The oil and gas industry has traditionally determinedpay zones based on fixed reservoir properties cut-offvalues (PCT). The advantages of cut-off based paydetermination method include (a) the obvious pay zones arepicked in many cases, (b) it is very simple. Its disadvantagesinclude (a) non-obvious pay zones are missed, (b) “pay”zones determined by it may produce mainly water due tohigh movable water saturation.A method is developed for identifying pay zones based onmovable water and uncertainty level dual-concept (PDC).This new method not only identifies both the obvious andnon-obvious pay zones, but also avoids picking zones thatare HC-bearing but mainly produce water due to highmovable water saturation.The method has been successfully applied in multi fields. Afew field application examples in clastic and carbonateformations are discussed in this paper, and all the identifiedpay zones in the examples are confirmed by wirelineformation test (ex., MDT) or drill stem test (ex., DST).