A New Approach for Evaluating Layer Productivity before Well Completion
Authors: Da Prat G. , Colo C. , Martinez R. , Cardinali G. , Conforto G. 
Published: December 2007

This work presents a new methodology using formation testing at openhole conditions for layer fluid identification and reservoir parameter's estimation. Real-time pressure and fluid identification data are obtained from the new wireline MDT Modular Formation Dynamics Tester. The tool's dual packer module makes zone isolation possible. Field cases from wells located in the San Jorge Gulf basin illustrate the testing methodology. The reservoir and well parameter's of permeability (vertical and horizontal) and formation damage are obtained from pressure transient analysis of buildup and interference pressure data taken in several wells. The evaluation method is consistent with results obtained from testing after completion (cased hole DSTs). Evaluation results can be used to decide whether to complete or abandon the zone of interest. Fluid type is identified in real time using the OFA Optical Fluid Analyzer module. Evaluation of anisotropy on a productive zone scale from the vertical interference test is shown.

Introduction

A main objective of formation evaluation at openhole conditions is the identification and description of hydrocarbon reserves with the best degree of resolution possible to assist in deciding whether to abandon or complete the interval of interest. Equally important is obtaining the reservoir pressure and reservoir parameter's for permeability and transmissibility. These are usually measured using logging and testing techniques both at open- and cased hole conditions. However, fluid identification success rarely exceeds 60% for reservoirs in the San Jorge Gulf basin, located in the central Patagonia region in southern Argentina, although in many cases a complete set of logs is used. This leads to completing, perforating and testing all prospective intervals, which has proved to be an expensive evaluation and completion process.

The main reason for these poor results is the multilayer nature of the gross prospective producing interval. The interval of interest in a typical well is between 800 and 1200 m thick, with approximately 40 lenticular reservoirs ranging from 1 to 10 in thick. As shown in the San Jorge Gulf basin stratigraphic sequence in Fig. 1, the upper intervals are laminated sands with a high clay content. The bottom layer's are tuffaceous sands of a variable, complex lithology. The sands are highly laminated, with a variable, high water saturation. In addition, the layer's are laterally discontinuous (1 to 3 km wide) and heterogeneous. The initial oil production rate is about 30 m3/d (usually obtained by fracturing) and most of the wells are produced by rod pumping.

The major challenges in the San Jorge Gulf basin for the past 60 years have been to identify the potential oil layers in a multilayer system and to determine the expected production rate and reservoir parameters of permeability and formation damage (mainly for fracture design) for each potential layer prior to completing the well or zone. Early reservoir evaluation is necessary to provide these answers and because wells are put on rod pumping, which limits the subsequent use of direct evaluation methods.

Our research over the past 2 years in formation evaluation and testing techniques has focused on determining the applicability of new methods that may optimize current evaluation practices. As a result, a methodology based on application of the new-generation wireline MDT tool for evaluating layer productivity before well completion was implemented. In this paper, we present several field cases showing the independent evaluation of a given layer in a multilayer system.

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