A New Approach to Production Log Analysis
Authors: Nerby G. , Ytre-Eide Tor , Rudshaug Terje , Espedal A. 
Published: December 2007

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Existing models for production log analysis (PLA) are based on observations at ideal surface conditions an d explicit correlation charts with a variety of assumed parameters. This has led to unreliable description of the parameters. This has led to unreliable description of the complex multiphase well inflow behavior and consequently erroneous analysis.

Correct analysis is vital to the understanding of dynamic reservoir behavior such as depletion, cross-flow, breakthrough and coning. For recompletion requirements the economic impact of wrong analysis is obvious; loss of production and possible additional workovers or in worst production and possible additional workovers or in worst case, loss of the well.

A well approach to PLA has been developed which enables use of in-situ measurements of physical fluid flow behavior directly in an implicit computation of flow regime, phase velocities, pressure gradient, slip and heat transfer.

The main features are: - Transfer functions coupling the actual specific properties of the fluid element to the sensor response properties of the fluid element to the sensor response taking into consideration the constitutive physical relations and the physical rheology of the current flow pattern and fluid system in exposure to the sensor pattern and fluid system in exposure to the sensor environment. - Dynamic flow models are coupled in a continuity principle for mono-and multiphase flow. The models are principle for mono-and multiphase flow. The models are based on general continuity, constitutive relations and documented correlations. - Correlations for physical fluid properties such as density, viscosity and surface tension based on one or more defined thermodynamic reference states. - Statistical modules for filtering, variance estimates, curve fits and regression analysis between general properties and parameters in the multiphase flow hierarchy. - A line search optimizing technique base don golden selection one-dimensional search using a conjugate direction propagation principle. This line optimization splices together the log measurements and theoretical predictions. The search is made with respect to the predictions. The search is made with respect to the multiphase flow fluxes as free variables.

The paper gives an in-depth description of all the mathematics used and includes a PLA example from a North Sea well. The PLA includes breakthrough of both formation and injection water, and sever crossflow during shut-in.

Introduction

Production logging (PL) is one of the few tools available for direct monitoring of the dynamic reservoir and well inflow performance. Especially in the mature stages of field production, it has proved vital to understanding of reservoir depletion, cross-flow, breakthrough and coning problems or recompletion requirements. Traditionally, however, PL-data has been used for qualitative analysis, the reason most probably the complex multiphase well inflow behavior. Consequently a vast amount of information collected in expensive PL operations is rarely actively used for quantitative reservoir management purposes.

This has been the motivation for the development of a new approach to production log analysis. In the following, the background and the principles of the system are described together with an example that illustrate the performance of this concept.

REVIEW OF PRODUCTION LOG ANALYSIS (PLA)

Some of the most advanced current PLA models utilize flow regime maps that are based on observations at ideal surface conditions for small pipe diameters. In addition, the sensor responses requires explicit correlation charts based on a variety of assumed parameters. To de rive flutes an empirical slip and pressure gradient function is found from the maps and the explicit corrected measurements.

These principles have several drawbacks. (1) A flow map referred to laboratory conditions with definite fluid properties cannot be extrapolated directly to downhole properties cannot be extrapolated directly to downhole conditions i.e. any flow regime prediction needs to be related to downhole conditions. (2) Explicit chart corrections for the PL sensors are inaccurate and should be replaced by implicit computations based on in-situ physical and dynamical fluid behavior. physical and dynamical fluid behavior.