Time-Lapse Production Logging and the Concept of Flowing Units

Abstract

Permeability heterogeneity is a well known feature that is present in most if not all reservoirs. Initially when the reservoir is in hydrostatic equilibrium the differences in permeability within the same reservoir unit do not manifest. However as soon as production starts and a pressure drawdown is introduced to the system fluids will start moving towards the wellbore following the easiest paths within the rock. Therefore a system of "flowing units" develops with time allowing the zones with relative high permeability to produce faster than the relatively low permeability zones. When a well is completed and perforated it is good practice to evaluate reservoir properties and well performance by carrying out transient well testing and multirate tests. Well test interpretation gives a good estimation of kh, skin and boundary effects. Multirate tests, particularly important in gas wells and high flowrate oil wells, allow evaluation of well deliverability and non-Darcy effects. It is therefore important to define the correct thickness that is actively contributing to flow. The effective thickness will be the sum of all the high permeability flowing units. In fact, and depending on the permeability contrast, the few most permeable units are responsible for most of production. Production Logging can verify and quantify flow contribution from these intervals. Differential depletion within this interbedded layered system occurs by stages. Initially the zones with the highest permeabilities will produce faster and drain their connected volume until a differential pressure threshold is reached and crossflow from the surrounding poorer-quality reservoir facies starts. A production logging survey is a snapshot of the flowing profile across the producing intervals at that particular point in time. In order to monitor this dynamic behaviour running production logging surveys at different times during the field's life is highly recommended especially in mature fields that produce from layered systems or interbedded units within the same formation. Time-lapse production logging provides a realistic visualization of flowing behaviour and helps pre-development of a production optimisation strategy focused in maximizing reserve recovery. The previous statements are supported by field examples where this pattern of permeability zonation has been observed over time. Introduction The term Flow Unit has been used originally to describe the correlateable units in reservoirs1.Hydraulic Flow Units were later introduced to cluster core plugs with similar petrophysical properties2. The Stratigraphically-Modified Lorenz Plot has been used more recently to identify Flow Units3 in the sense of ‘flowing units’ identified by the production log.The term Flow Unit means different things to different people so we use the term "flowing units" in this paper to define the units detected by production logs to be flowing. This paper investigates the evolution of these flowing units over production time in a well-layered system with crossflow. The paper shows the link between evolution of flowing units with time and initial evaluation measures. Laterally continuous, multilayered reservoirs with high permeability contrast between layers with crossflow behaviour have a distinctive long-term production behaviour. Preferential depletion develops over time and the most permeable and consequently the most productive intervals act like "fracture channel" systems collecting the produced fluids from the surrounding less permeable units and carrying them to the wellbore.These reservoirs are called double porosity, but as this terminology is often associated with naturally-fractured reservoirs we call these double matrix porosity.In well testing these reservoirs are also called dual permeability. The most common way of quantifying the production profile along the perforated section in a well is to run a production logging survey. Time-lapse production logging suggests a monitoring plan of evaluating well's production profile at different points in time, which is considered fundamental in understanding production dynamics and production optimisation.