Abstract

In recent times, the oil industry has shown increasing awareness towards maintaining optimum well productivity through better HP-HT drilling/completion fluids design. However, the mechanisms of drilling fluid filtration and impact on productivity performance are not well understood, especially in an HP-HT environment.

In open hole completions the productivity losses are critical because the near-wellbore damage is not by-passed by perforations.

Furthermore, a satisfactory model for field applications to simulate the near-wellbore damage in terms of well flow performance from laboratory core test analysis still is not available.

In this paper, the results of in-depth experimental research into rheology, filtration and formation damage phenomena and the relationships between them. The experimental data combined with data analysis of static and dynamic filtration models provided the database for the semi-empirical mechanistic models that were developed. These models have been combined and incorporated into a design and evaluation tool - the productivity tool, for predicting the effect of HP-HT drilling fluid filtration on formation productivity.

A number of results have been presented to illustrate how the new tool can be used to evaluate the damage factor of a given fluid, specify the invaded zone skin as well as the depth of invasion, two key parameters that are useful and relevant to optimum fluid selection and management in addition to well test data interpretation.

Introduction

The most common source of formation damage has proved to be drilling operations1. Permeability is a characteristic of the formation, and can be altered by solids and mud filtrate invasion during drilling operations. Drilling fluids are used to facilitate various drilling processes. Drilling mud filtrate will invade the formation to a greater depth than drilling mud particles. A decrease of the permeability (formation damage), results in a decrease of the well productivity2,3. The formation damage depends upon many parameters such as formation characteristics, type, composition, filtration and rheological characteristics of drilling fluid and operating conditions (overbalance pressure, time, etc.).

A key parameter in quantifying formation damage is the skin factor4. The skin factor estimated from well test data is used in the flow equations to estimate the production rate in wells that are affected by formation damage.

Generally, when rating performance of various drill-in fluid formulations, the permeability damage evaluation is quantified through oil return permeability measurements and flow-initiation pressures performed on core samples damaged during mud filtration tests5,6.

Extensive laboratory studies of formation damage and several modeling efforts for prediction of formation damage have been reported in the literature. Most of the previous studies have focused on formation damage from filtration of WBM and incompressible fluids in LP-LT applications.

Few attempts were made to transfer these laboratory data into a near-wellbore model to evaluate the permeability damage. Liu et al.7 simulated formation damage by fluid injection and mud filtration while Scott Lane8 and Semmelbeck at al.9 simulated filtrate invasion for improving log interpretation, but their impact on well performance was not investigated. Some workers10,11 studied well performance using representative formation damage, but laboratory tests were not integrated in their studies.

The economic impacts of wellbore formation damage justify a thorough study of this problem in order to find ways to minimize its effect on well performance.

This paper presents a productivity tool for screening different HP-HT drilling fluids (WBM and OBM), which specify the invaded zone skin as well as the depth of invasion, and evaluates the damage factor of a given fluid in terms of inflow performance.

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