Laboratory testing of core material to attempt to optimize drilling fluid composition and procedures has been used for many years to attempt to minimize invasive formation damage of a mechanical, chemical or biological nature which can occur during the drilling of horizontal or vertical wells. This paper discusses the deficiencies of past methods, such as the use of non-representative core or fluids, non-preserved or non-restored state core, ambient conditions of temperature and overburden pressure, direct injection of muds/filtrates into samples and unrealistically high drawdown gradients for cleanup. The paper describes the current state-of-the-art technology used to eliminate many of these concerns and also to extend drilling fluid evaluation technology to extremely heterogeneous carbonate and sandstone formations, fractured formations and specific test equipment and procedures used to evaluate the effectiveness and utility of underbalanced drilling programs.


Formation damage occurring during the drilling of horizontal or vertical wells can be a significant mechanism of ultimate reduced productivity in both oil and gas bearing formations. In some cases a combination of petrographic and special core analysis techniques are used to evaluate the potential effectiveness of proposed drilling fluids and procedures, prior to the actual cost and risk of implementation. These tests are conducted to obtain a better assessment of the risk associated with the use of proposed drilling fluids and to optimize the fluid and procedures which will be utilized in a given horizontal or vertical well operation to maximize ultimate productivity of oil or gas. Considerable advances have been made in recent years in both the execution and interpretation of the results of laboratory coreflow tests to obtain representative results for effective field design of horizontal and vertical well drilling programs. This paper highlights test procedures used in the past, describes some potential problems with these test procedures, and outlines the current state of the art in laboratory technology with respect to specialized laboratory testing to evaluate drilling induced damage for vertical and horizontal well applications.

Common Mechanisms of Formation Damage During Overbalanced and Underbalanced Drilling Operations

A number of authors have provided a detailed discussion of potential formation damage mechanisms which may occur during overbalanced and underbalanced drilling operations. A summary of this work is provided in the literature{1–5}. These mechanisms would include:

  1. Mechanically Induced Formation Damage

    • Physical migration of in-situ fines and mobile Particulates

    • The introduction of extraneous solids of either an artificial nature (ie. weighting agents, fluid loss agents, or artificial bridging agents) or naturally occurring drill solids generated by the milling action of the drill bit on the formation.

    • Relative permeability effects associated with the entrainment of extraneous aqueous or hydrocarbon phases within the porous medium

    • Formation damage effects associated with the use of extreme underbalance or overbalance pressures and associated fines migration or spontaneous imbibition phenomena.

    • Direct mechanical glazing phenomena associated with bit-formation interactions. This particular damage mechanism is usually associated with gas drill operations where high bit-rock temperatures commonly occur.

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